8. Interpreting Soil and Tissue Tests

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					                                                           Chapter 8. Interpreting Soil and Tissue Tests




8.              Interpreting Soil and Tissue Tests
                Soil and tissue tests provide valuable information about the properties
                (mostly chemical properties) that affect plant growth. Many field
                experiments have been used to verify the results of laboratory testing of
                soils and of plant tissue. The interpretation of results from soil and tissue
                tests will help you to make more informed, cost-effective fertiliser
                decisions.

                Soil tests are a valuable tool for identifying the macronutrient status of
                paddocks on the farm. However, research has shown that using soil tests
                to indicate trace-element deficiencies can be very inaccurate, especially
                on acid soils. Plant tissue testing is the preferred method for diagnosing
                micronutrient (trace element) toxicities, deficiencies, and imbalances for
                plants.

                Learning outcomes

                At the completion of this chapter, you should be able to:

                • Be aware of the various types of analysis that make up a soil and tissue
                  test.

                • Accurately interpret soil tests.

                • Use the results of soil and tissue tests in your farming operation.



8.1     Interpreting soil tests

                A standard soil test report (Figure 8.1) provides information on:

                •   Soil type.                            • Cation exchange capacity
                •   Organic carbon.                         (CEC).
                •   Soil pH.                              • Aluminium level.
                •   Available phosphorus (P).             • Soil salinity: electrical
                •   Available potassium (K).                conductivity (EC) and salt
                •   Available sulphur (S).                  level (% Na).
                •   Phosphorus buffering                  • Comments on the test results.
                    index (PBI).                          • Recommendations for fertiliser
                                                            application (if requested).




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                                                Figure 8.1 An example soil test




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                Soil test results for nutrients are usually expressed as:

                  mg/kg (milligrams per kilogram), =
                  ppm (parts per million), =
                  µg/g (micrograms per gram).

    8.1.1       Soil type description
                The soil type information describes:

                • The colour of the soil.

                • The soil texture (clay, loam, sand, or some combination of these terms,
                  such as sandy loam or clay loam).

                The colour and texture of a soil are indicators of its properties and are
                always taken into account when interpreting the other results and
                preparing fertiliser recommendations.

                Use the soil texture test (see Section 3.4.1) to check that the laboratory
                analysis is correct in its determination of your soil’s texture, since texture
                can affect the fertiliser recommendation.

                Soil texture is a subjective measure, given it is done by hand; and two
                people can often get differing results. The soil’s cation exchange capacity
                (see Section 8.1.8) also provides an indication of soil type. Soils high in
                clay and organic matter usually have a high CEC, whereas soils with
                sand particles only usually have a low CEC.

                Soil texture shows how we should apply mobile nutrients, such as K, N
                and S. Soils with a light texture (in other words, a low CEC) should have
                nutrients applied in smaller quantities and more frequently, as they are
                prone to leaching.

                Soil texture has been used as a guide to how much phosphorus the soil
                will fix. However, given the same soil texture, phosphorus fixation can
                vary widely. The phosphorus buffering index is a better determinant of
                the effect of soil type on phosphorus fixation (see Section 8.1.7).

    8.1.2       Organic carbon
                Organic carbon is an estimate of the soil organic matter (humus) content.
                Low organic carbon levels in a soil indicate that the soil is low in organic
                matter and can generally hold less nutrients than a soil with a high
                organic carbon. Organic carbon levels will vary according to pasture or
                crop type, as well as the original soil type.

                No test exists to directly determine the exact amount of organic matter in
                soils. The usual procedure is to determine the amount of organic carbon,
                which can be done very accurately; and this is then multiplied by a factor


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                of 1.6 to 1.74 to estimate the amount of organic matter. The factor used
                depends on the source of the organic matter. Check your soil test to see
                whether it is reporting organic carbon or estimated organic matter. If the
                test reports on organic carbon, multiply that number by 1.6 to estimate
                the organic matter for pastures.

                Guidelines for low, normal, and high organic matter percentages
                (calculated from organic carbon percentages) are listed in Table 8.1.

                       Table 8.1 Organic matter percentage over a range of conditions

                                      Crops                                    Crops
                                                          Pastures                                   Pastures
                  Organic         Low Rainfall                             High Rainfall
                                                        Low Rainfall                               High Rainfall
                  Matter            (less than                            (400 to 600 mm
                                                         (less than                                (greater than
                  Levels           300 mm per                              per growing
                                                          400 mm)                                    400 mm)
                                 growing season)                              season)
                Low                   Below 1.5             Below 3             2.5                      5
                ‘Normal’               1.5 to 2.5           3 to 4.5          2.5 to 5                5 to 10
                High                  Above 2.5            Above 4.5         Above 5                 Above 10


      8.1.3     Soil pH
               Soil acidity and alkalinity are described by the term pH. Soil acidity is
               corrected by applying agricultural lime or dolomite. Lime (calcium
               carbonate) is usually applied to dairy pastures to increase the pH and
               neutralise the effects of soil acidity. This subject is discussed extensively in
               Chapter 4, and lime recommendations to increase the soil’s pH are
               described in Section 4.5.8. Three of the most relevant tables are repeated
               here (see Tables 8.2, 8.3 and 8.4), although they are general
               recommendations that don’t take soil texture into account. Therefore,
               rates may be higher or lower, depending on soil type, as a clay soil will
               require more lime than a sandy soil to raise the pH.

                               Table 8.2 The optimum pH range of pasture plants
                              Pasture Species              pH (CaCl2)             pH (water)
                           Sub clover                        4.8 to 6.5               5.5 to 7.0
                           White clover                      5.0 to 6.0               5.8 to 6.5
                           Perennial rye                     4.3 to 6.0               5.0 to 6.5
                           Medic                             5.3 to 8.0               6.0 to 8.5
                           Lucerne                           5.2 to 7.5               5.8 to 8.0
                           Cocksfoot                         4.3 to 6.8               5.0 to 7.5
                           Phalaris                          5.2 to 7.3               6.0 to 8.0
                           Fescue                            4.3 to 6.4               5.0 to 7.0


                                    Table 8.3 Lime recommendations at sowing
                       Existing pH (CaCl2)          Existing pH (water)       Recommendation
                       5.1 and above                5.7 and above          No lime
                       4.5 to 5.0                   5.1 to 5.6             2.5 t/ha
                       4.2 to 4.4                   4.8 to 5               3.75 t/ha to 5 t/ha
                       Less than 4.2                Less than 4.8          5 t/ha to 7.5 t/ha



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                           Table 8.4 Lime recommendations on existing pasture

                   Existing pH (CaCl2)       Existing pH (water)              Recommendation
                   4.7 and above             5.3 and above             No lime
                   4.3 to 4.6                5 to 5.3                  Test strip or 2.5 t/ha*
                   Less than 4.3             Less than 5               2.5 t/ha
                   *Another 2.5 t/ha should be applied after 3 years on soils with a pH (CaCl2) less
                   than 4.3.


                Note: Tables 8.3 and 8.4 are rough guides only. For a more accurate
                estimate of lime application rates, ask your laboratory to do a pH
                buffering test.

                Two laboratory methods are currently used to measure pH: the water
                method and the calcium chloride (CaCl2) method. The results are usually
                reported in one of the following formats:

                • If the water method is used, the results are reported as pHw, pH
                  (water), pH (H2O) or pH 1:5 water.

                • If the calcium chloride method is used, the results are reported as
                  pHCa, pH (CaCl2) or pH 1:5 CaCl2.

                The water method has been the test most commonly used in Victoria for
                over 30 years and more readily reflects current soil conditions than does
                the calcium chloride method. However, the water method is more
                subject to seasonal variations. The pH (water) value may vary by as
                much as 0.6 units over the year. For example, a soil may have a pH
                (water) value of 5.7 before the autumn break, but a pH (water) value of
                5.1 after the break. This variation can affect recommendations for
                fertilisers, particularly lime.

                The calcium chloride test is more useful for long-term monitoring of pH
                and is the one most agronomists tend to use for fertiliser and plant
                recommendations. The calcium chloride method is less subject to
                seasonal variations and better approximates field conditions. The fact
                that the calcium chloride method is less variable throughout the season
                makes it a more appropriate test to use when making management
                decisions regarding soil pH and lime applications.

                When both test methods are used on the same soil sample, the
                pH (CaCl2) value is, on average, about 0.8 units lower than the pH
                (water) value. However, the pH (CaCl2) value can range from 0.5 to 1.1
                units lower than the pH (water) value.

                The pH readings from the two testing methods will be much closer if
                your soil contains high levels of salt. This is typical with soils that have a
                salinity problem or may be seen after a recent application of a fertiliser
                high in salt, such as muriate of potash.



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                Most of the major soil testing laboratories will present pH results for
                both testing methods. It is important to always be aware of which
                testing method is being considered when discussing management
                options with an adviser or agronomist or when interpreting a soil test.

                As soils become more acidic, it is common to see a rise in the plant
                availability of both aluminium (Al) and manganese (Mn), which can both
                be toxic to pasture plants and crops. Aluminium toxicity is particularly
                common in acid soils and restricts root growth in sensitive plant species.

                Most of the major soil testing laboratories will provide a measure of
                available aluminium in the soil, often expressed as percentage of the
                soil’s cation exchange capacity (% CEC). When reading a soil test that
                shows the soil is acidic, it is helpful to check the percentage of
                exchangeable aluminium. The amount of exchangeable aluminium
                should be the lowest of all the cations, and it is desirable that it remains
                less than 5%. See ‘Exchangeable aluminium’ in Section 8.1.8 below for
                more information.

                Soil texture and lime quality also affect lime application rates. A clay soil
                will require more lime to raise the pH than a sand soil will, while a
                better-quality lime will have a greater effect on raising pH (see Section
                4.5 for details).

                Soil alkalinity is uncommon on most dairying soil types. However,
                alkalinity can be reduced by applying elemental sulphur or aluminium
                sulphate at very high rates, which may not be economic.

      8.1.4     Available phosphorus
                Available phosphorus is the amount of phosphorus in milligrams per
                kilogram (mg/kg, or parts per million) extracted from the soil by the
                Olsen phosphorus test method (usually called the Olsen P test). This test
                is used to indicate whether or not phosphorus is required. The Olsen P
                test is a measure of plant-available P. The test has been extensively
                calibrated against pasture production (including the Phosphorus for
                Dairy Farms Project and other trials) over a range of soils and climates in
                Australia and New Zealand.

                The Phosphorus for Dairy Farms Project established that, to maintain a
                vigorous dairy pasture, an Olsen P of 18 to 22 mg/kg is suitable, although
                lower levels would be satisfactory for lower stocked farms (see Table
                8.5). Large gains in pasture production can be seen when low Olsen P
                levels are raised to the recommended level of an Olsen P of 18 to 22
                mg/kg. Although slightly higher pasture responses are achievable at
                higher Olsen P levels, it is unlikely that the returns would be economical.
                The appropriate level of Olsen P for good pasture growth is the same,
                regardless of the soil type being evaluated. The optimal level for Olsen P
                is discussed further in Chapter 11.



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                 Table 8.5 Levels of Olsen P and levels of plant-available phosphorus

                                         Olsen P (mg/kg)               Availability
                                                 Below 12                   Low
                                                 12 to 17                Marginal
                                                 18 to 25                Adequate
                                                 Above 25                   High


                Other available phosphorus tests (for example, Colwell or Bray) are used
                by various laboratories, but field calibration of these tests in Victorian
                conditions are rare. There is far too much variation (1:1 up to 5:1) to be
                able to convert Olsen P to Colwell P (or vice versa) reliably.

    8.1.5       Available potassium
                Available potassium (in mg/kg, or parts per million) is measured by the
                Colwell or Skene methods or is estimated by multiplying the
                exchangeable potassium test result (see ‘Exchangeable potassium’ in
                Section 8.1.8) by 391. The values from either the Colwell or Skene method
                are very similar, except in alkaline or recently limed soils.

                Unlike phosphorus, the appropriate level of available potassium for
                good pasture growth depends on soil type. Clay soils have a higher
                nutrient holding capacity and need higher levels of available K than do
                sandy soils. Table 8.6 shows the generalised ranges for available K for
                different soil types.

                   Table 8.6 Effect of soil type on the nutrient status of potassium*

                                                       Sandy           Clay
                   Nutrient           Sands                                            Clays           Peats†
                                                       Loams          Loams
                    Status
                                                            Available Potassium (mg/kg)
                 Low                 Below 50         Below 80       Below 110       Below 120       Below 250
                 Marginal            50 to 140        80 to 150      110 to 160      120 to 180       250 to 350
                 Adequate           141 to 170       151 to 200      161 to 250      181 to 300       350 to 600
                 High               Above 170        Above 200       Above 250       Above 300       Above 600
                 *This table is not applicable to the Riverina Plains of northern Victoria, where responses
                 to potassium are rare, irrespective of available potassium levels.
                 †In peat soils, plant tissue testing is suggested as a more accurate indicator of available K
                 because few field trials have been done on these soils to verify laboratory analyses.


                When soil potassium levels are high, potassium inputs can be reduced or
                deleted from your fertiliser regime, as a pasture response to additional
                potassium is unlikely.




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      8.1.6     Available sulphur
                This is the amount of available sulphur in milligrams per kilogram
                (mg/kg, or parts per million) as measured by the CPC or the Blair
                sulphur (KCl 40) test method.

                Sulphur is adequate between 9 to 12 mg/kg using the CPC sulphur test
                (also reported as the MCP test), which measures sulphate sulphur only
                and has been calibrated against 72 field trials (Table 8.7). If sulphur levels
                are above 12, then fertilisers containing a low concentration of sulphur
                could be used for one or more years (although monitoring would be
                necessary, especially if in a high rainfall or low organic matter area).

                                      Table 8.7 Sulphur levels and availability

                                                            Sulphur Level (CPC,
                                     Nutrient Level        MCP or KCl 40 Sulphur
                                                               Test) (mg/kg)
                                            Low                   Below 4
                                          Marginal                 4 to 8
                                         Adequate                  9 to 12
                                            High                  13 to 20
                                         Very High               Above 20


                The major laboratories are now also using the Blair sulphur test (also
                called the KCl 40 test), as it provides an improved indicator of sulphur
                status. Although the adequate ranges are similar, the KCl 40 test is more
                accurate because it takes into account some of the sulphur that will
                become available from the breakdown of organic matter. This is relevant
                for dairy pastures, which often have thick root mats and therefore a
                significant potential to supply sulphur via organic matter breakdown.

                Sulphur becomes less available in cold, wet conditions. In those
                conditions, responses to applied nitrogen are sometimes improved if
                some sulphur is applied at the same time.

                If the sulphur level is high to very high, gypsum may have been recently
                applied or the soil may be saline.

      8.1.7     Phosphorus buffering index
                A relatively new figure on the soil test is the phosphorus buffering index
                (PBI). The PBI figure now appears on most soil tests and can help
                improve the efficiency of phosphorus fertiliser use.

                Unfortunately, phosphorus applied as fertiliser reacts with the soil and
                becomes less available for plant uptake. The extent of these reactions
                depends on the phosphorus buffering capacity of the soil. A soil with a
                high phosphorus buffering capacity, for example, will require more
                phosphorus fertiliser (maintenance and capital) than a soil with a low


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                phosphorus buffering capacity. Until recently, measuring a soil’s
                phosphorus buffering capacity has required detailed chemical analysis.
                This is expensive to do; and not surprisingly, soil-testing companies have
                not provided a measure of phosphorus buffering capacity in their normal
                soil testing service.

                In 2001, Cameron Gourley and Murray Hannah from DPI Ellinbank,
                Lucy Burkitt (now working for the Tasmanian Institute of Agricultural
                Research) and Phil Moody from the Department of Natural Resource
                Management, Queensland, worked together to develop a simple and
                inexpensive soil test that would predict the phosphorus buffering
                capacity of a soil. This new PBI test accurately predicts phosphorus
                buffering capacity for all Australian soils measured. The PBI soil test is
                now endorsed as a national standard.

                The PBI test allows you to make more accurate phosphorus fertiliser
                decisions based on your farm’s soil type. Firstly, the PBI is useful when
                looking to boost soil fertility with capital fertiliser applications. Historical
                rules of thumb used in fertiliser recommendations suggest that 10 kg of
                phosphorus per hectare is required (above maintenance) to raise the
                soil’s Olsen P level by one unit. As different soils have different
                phosphorus buffering capacities, they actually require different amounts
                of phosphorus to raise their Olsen P level. The capital application rates
                required to lift Olsen P by one unit, based on the PBI, are shown in Table
                8.8 below.

                    Table 8.8 Capital phosphorus (kg/ha) required (in addition to
                    maintenance phosphorus) to lift Olsen P by one unit (1 mg/kg)

                                                                                  Phosphorus
                                 PBI Class              PBI*                       Required
                                                                                   (kg P/ha)
                     Very low                          0 to 50                          5
                     Low                              50 to 100                         7
                     Moderate                         100 to 200                        9
                     High                             200 to 300                       11
                     Very high                        300 to 600                       13
                     Extremely high                     600 +                          15
                     *Source: Burkitt et al., 2002.



                Table 8.8 shows that soil types with a high PBI number may require up
                to three times more phosphorus fertiliser to raise their Olsen P level by
                one unit. The PBI test allows farmers and advisers to target capital
                phosphorus fertiliser recommendations more accurately.

                Determining maintenance fertiliser requirements also utilises the PBI
                information. Nutrient budgeting to determine what must be replaced
                includes a factor for phosphorus locked up in the soil. A nutrient
                budgeting tool called NutriMatch has been developed to determine farm



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                nutrient requirements based on this nutrient budgeting approach (see
                Chapter 11).

                The PBI also shows which soils will leach phosphorus. Soils that have a
                PBI less than 50 are prone to leaching. On these soils, the required
                amount of capital or maintenance phosphorus fertiliser should be
                applied in small quantities on a regular basis over the year rather than
                applying all of the P fertiliser once a year, especially if the soil is
                irrigated.

      8.1.8     Cation exchange capacity
                Cation exchange capacity (CEC) is a measure of the soil’s capacity to
                adsorb and hold cations (positively charged ions). This measurement
                provides an indication of the amount of nutrients available in the soil,
                their ratios, and the soil’s ability to hold nutrients (the higher the CEC,
                the more nutrients a soil can hold).

                The major cations are magnesium, calcium, potassium and sodium. In
                acid soils, aluminium, hydrogen and manganese are also involved in the
                cation exchange capacity. (See also Section 3.5.1.)

                Generally, your soil test will report on exchangeable calcium,
                exchangeable magnesium, exchangeable sodium, exchangeable
                potassium, and exchangeable aluminium. These will be reported in two
                ways: as milliequivalents per 100 g (meq/100g) and as a percentage of
                the total exchangeable cations.

                On some soil tests, aluminium levels will be assessed by the CaCl2
                (calcium chloride) or KCl (potassium chloride) methods, which are
                reported in milligrams per kilogram (mg/kg) or parts per million (ppm).
                When this happens, exchangeable aluminium is not included in the
                cation exchange capacity.

                The CEC portion of your soil test is most useful for determining soil
                structural problems and high aluminium levels. CEC is also a good
                indicator of soil texture. The CEC depends on the amount and kinds of
                clay and organic matter that are present. A high clay soil can hold more
                nutrients than a low clay soil. Also CEC increases as organic matter
                increases. Therefore, sandy soils with low organic matter have a lower
                CEC than clay soils.

                The unit used to measure cation exchange capacity and the desired
                ranges, relationships or limits for the various cations are discussed
                below.




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                Measuring cation exchange capacity
                A soil’s cation exchange capacity is expressed as equivalents or, more
                commonly, milliequivalents per 100 grams (meq/100 g), which is also
                referred to as milliequivalent per cent (meq %). A milliequivalent is defined
                as one milligram of hydrogen or the amount of any other ion that will
                combine with or displace it.

                After the cations (calcium, magnesium, potassium, sodium and
                sometimes aluminium) have been measured in milliequivalents per 100
                grams, they are totalled (see ‘CEC levels’ below) and their proportional
                relationship to one another is calculated as a percentage of the total.

                There is considerable evidence that the proportions of the exchangeable
                cations are more relevant to plant performance than the actual levels.
                Table 8.9 summarises the desirable range of each cation for many plants.

                Table 8.9 Desirable percentage range of exchangeable cations for soils

                                         Cation                  Range
                                  Calcium                      65% to 80%
                                  Magnesium                    10% to 20%
                                  Potassium                      3% to 8%
                                  Sodium                       less than 6%
                                  Aluminium                    Less than 1%
                                 Source: Andrew Spiers, Hi-Fert (pers. com.).

                CEC levels
                CEC is also referred to as the sum of cations and is the total of the
                five exchangeable cations in meq/100 g. A sum of cations above
                15 meq/100 g means that a soil has a good ability to retain nutrients for
                plants.

                CEC is a good indicator of soil texture and type. The CEC of clay
                minerals is usually in the range of 10 to 150 meq/100 g, while that of
                organic matter may range from 200 to 400 meq/100 g. The CEC of sand
                and sandy soils is usually below 10 meq/100 g. So, the kind and amount
                of clay and organic matter content of a soil can greatly influence its CEC.

                Where soils are highly weathered and the organic matter level is low,
                their CEC is also low. Where there has been less weathering and organic
                matter content is higher, CEC can also be quite high. Clay soils with a
                high CEC can retain large amounts of cations against leaching.

                Sandy soils with a low CEC retain smaller quantities of cations, and this
                has important implications when planning a fertiliser program. In soils
                with a low CEC, consideration should be given to splitting applications
                of K and S fertilisers.




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                Exchangeable calcium
                Exchangeable calcium should make up the largest amount of the cations.
                The desirable range is 65% to 80%.

                The ratio of exchangeable calcium to exchangeable magnesium provides
                a guide to a soil’s structure and any potential problems that might be
                influencing soil drainage, root development and subsequent plant
                growth. Well-structured soils have a calcium-to-magnesium ratio greater
                than 2:1 (in other words, the amount of calcium cations is more than two
                times greater than the amount of magnesium cations). A calcium-to-
                magnesium ratio of 2:1 or less indicates reduced soil stability in heavier
                soil types (clays and clay loams) but is not as important for lighter soils
                (sands and loams).

                However, if the exchangeable sodium is less than 6%, then soil structure
                may not be affected and the addition of gypsum may not be required.

                A calcium-to-magnesium ratio of more than 10:1 indicates a potential
                magnesium deficiency (this can be confirmed with a tissue analysis).

                Exchangeable magnesium
                Exchangeable magnesium should make up the next largest amount of the
                cations. The desirable range is 10% to 20%.

                The ratio of magnesium to potassium should be greater than 1.5:1 (in
                other words, the amount of magnesium should be more than one and a
                half times greater than the amount of potassium). A magnesium-to-
                potassium ratio of less than 1.5 indicates an increased chance of grass
                tetany (although many other factors influence the occurrence of grass
                tetany as well).

                If the exchangeable magnesium is more than 20% of the cations, it may
                cause a potassium deficiency. Conversely, if the exchangeable potassium
                is more than 10% of the cations, it may cause a magnesium deficiency.

                See ‘Exchangeable calcium’ above for the recommended ratio between
                magnesium and calcium.

                Exchangeable potassium
                Exchangeable potassium should make up the third largest amount of the
                cations. The desirable range is 3% to 8%.

                The value of potassium in relationship to magnesium and calcium
                should be less than 0.07. A result of 0.07 or higher indicates a greater
                danger of grass tetany; a result less than 0.07 indicates minimal danger of
                grass tetany. (Note that animal symptoms or blood tests are the most



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                 accurate indicators for grass tetany.) To determine the relationship, use
                 the following formula:

                                                   K ÷ (Ca + Mg)

                 For example, if a soil test showed potassium as 0.47 meq/100 g, calcium
                 as 4.10 meq/100 g, and magnesium as 0.97 meq/100 g, then the
                 calculation would be:
                                           0.47 ÷ (4.10 + 0.97) = 0.059,
                 which is well under the grass tetany danger level of 0.07 or greater.

                 Exchangeable sodium
                 Exchangeable sodium should be the fourth largest amount of the cations.
                 The desirable range is less than 6%.

                 If the sodium cations make up 6% or more of the cation exchange
                 capacity, then the soil may be sodic and susceptible to dispersion (see
                 Section 4.1).

                 Exchangeable aluminium
                 Exchangeable aluminium should be the lowest amount of the cations.
                 The desirable amount is less than 1%.

                 Exchangeable aluminium is used to determine the requirement for lime
                 where aluminium-sensitive species, such as lucerne, white clovers and,
                 to a lesser extent, sub clovers, are concerned. High aluminium levels can
                 be toxic to plants, but aluminium generally falls to harmless levels once
                 the pH (CaCl2) exceeds 5.0.

                 Table 8.10 ranks the aluminium levels, expressed as a percentage of the
                 soil’s cation exchange capacity, by their suitability to most pasture
                 species. Also listed are some crop species, indicating the exchangeable
                 aluminium levels above which yields are reduced.

                    Table 8.10 Aluminium ranges and their suitability to most pasture
                                   species and specific crop species
         Exchangeable                                                     Pasture and Crop Species Whose Yield
          Aluminium                       Comment                        are Reduced if Exchangeable Aluminium
             (%)                                                         is Above the Base of the Range Indicated
                 1 to 3        Most sensitive species not affected                           Lucerne
                 3 to 5              Generally acceptable                             Annual medics, barley
                 5 to 10                   Marginal                          Phalaris seedlings, red clover, some sub
                                                                                          clovers, wheat
                10 to 20                   Moderate                         Maize, white clover, some oats, sub clover,
                                                                              ryegrass, fodder rape, some triticales
                 20 to 30                    High                            Lupins, cocksfoot, some oats, cereal rye
                Above 30                   Very high



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                Lucerne establishment and persistence are particularly susceptible to
                exchangeable aluminium in both the topsoil and the subsoil.

                The desirable aluminium levels in the topsoil for lucerne establishment,
                as measured by the three methods mentioned earlier, are:

                • Less than 1%, if measured as part of the cation exchange capacity.
                • Less than 2 mg/kg (or ppm), if measured by the CaCl2 method.
                • Less than 50 mg/kg (or ppm), if measured by the KCl method.

                Subsoils should also be soil tested if lucerne is to be grown. Lucerne is a
                deep-rooted plant, and it should not be sown if the level of aluminium in
                the subsoil, as measured by the KCl method, is above 50 mg/kg.

      8.1.9     Salinity
                On most soil tests, salinity is determined by measuring the electrical
                conductivity (EC) of a mixture of 1 part soil to 5 parts distilled water. The
                soil and water are continuously mixed for one hour before the electrical
                conductivity is tested. This test is called the EC 1:5 method, and the unit
                of measurement is decisiemens per metre (dS/m).

                Electrical conductivity is an indirect measure of the total soluble salt
                concentration in the soil solution, where high electrical conductivities
                generally correspond to high soluble-salt concentrations in the soil.

                Data on the salt tolerance of plants is usually based on a different test,
                the electrical conductivity of a saturated extract. This is called the ECe
                method and is also measured in decisiemens per metre.

                You will need to check the results of your soil test to see which method
                your figures represent, but they are likely to be EC 1:5. If they are, you
                will need to convert the EC 1:5 figures to ECe figures.

                The conversion of the laboratory result from an EC 1:5 test to the ECe
                figure involves multiplying by a factor that depends on your soil type
                (Table 8.11).

                   Table 8.11 Conversion factor of various soil types for EC 1:5 to ECe

                                                                               Multiplication
                                          Soil Texture Group                      Factor
                          Sands, loamy sands                                         13
                          Sandy loams, fine sandy loams                              11
                          Loams, very fine sandy loams, silty loams, sandy
                          clay loams                                                 10
                          Clay loams, silty clay loams, very fine sandy clay
                          loams, sandy clays, silty clays, light clays               9
                          Light medium clays                                         8
                          Medium clays                                               7
                          Heavy clays                                                6



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                For example, an EC 1:5 of 0.4 dS/m on a clay loam soil (multiplication
                factor of 9) gives an ECe value of 0.4 dS/m x 9 = 3.6 dS/m.

                A term we are more familiar with is parts per million, or ppm. This tells
                us how many parts are salt in every million parts of a soil sample. To
                convert EC 1:5 or ECe results to an approximate ppm measure, we
                multiply the EC 1:5 value by 3000. ECe values cannot be directly
                converted to parts per million. First, the ECe value must be converted to
                an EC 1:5 value. For example, an ECe of 3.6 dS/m on a clay loam soil is
                approximately 0.4 (the EC 1:5 value) x 3000 = 1200 ppm.

                Electrical conductivity of the soil can also be expressed as total soluble
                salts (TSS). This is the amount of total salts dissolved in the soil and is
                measured in mg/kg. The conversion between mg/kg TSS and ppm is a
                direct exchange. For example, 4000 mg/kg TSS is the same as 4000 ppm.

                A conversion table (Appendix C) compares the various measurements of
                soil salinity and water salinity.

                A plant growing in saline conditions will make adjustments to cope with
                the increase in salt levels in the soil solution. The ability of the plant to
                continue this adjustment is a measure of its tolerance to salinity.
                Table 8.12 gives a general guide to plant salinity tolerances based on the
                soil salinity classes discussed in Section 4.4.

                               Table 8.12 Plant tolerance levels to salinity

            Soil Salinity         ECe         Approx. Amount
                                                                         Species That Will Grow
               Class            (dS/m)         of Salt (ppm)
            A+ Very low      Less than 1.8           n.a.          All pastures and clovers
            A Low              1.8 to 3.8      Less than 1800      Most pastures, crops, legumes
            B Moderate         3.8 to 6.5       1800 to 3000       Grass, some legumes
            C High             6.5 to 8.6       3000 to 4000       Grass, not clover
            D Extreme        More than 8.6     More than 4000      Salt-tolerant plants, some barley grass
          Source: Adapted from Norman et al. (1995).


                Plants are more susceptible to salinity in their germination and seedling
                stages than in later stages of growth. Soil texture, salt type present, and
                climatic and management factors can also influence salt tolerance in
                plants.

                Salinity levels are satisfactory for all pasture species if the ECe is under
                1 dS/m.

                If the ECe level is over 2 dS/m, then there may be a need to further
                analyse the type of salts present.

                Clovers are more susceptible than ryegrass to high salinity (Table 8.13).
                Assuming the salt present is sodium chloride (NaCl), white clover suffers
                about a 10% yield reduction at an ECe of 1.5 dS/m, whereas ryegrass will


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                have a 10% yield reduction at an ECe of 6.9 dS/m. In many dairy
                pastures, other salts, such as nitrates, phosphates, etc., will contribute to
                the total salt load.

                        Table 8.13 Susceptibility of some pasture species to salinity

                               Susceptibility                                 Pasture Species
                       Sensitive                            White clover, lucerne
                       Some tolerance                       Sub clover, strawberry clover, balansa clover
                       Highly tolerant                      Phalaris, cocksfoot, ryegrass, tall fescue
                       Extremely tolerant                   Tall wheat grass, puccinellia, salt bush*
                       *All low-producing species.


                Although all salts can be harmful to plants, some salts, such as sodium
                and chloride, are more harmful than others. Care should be taken when
                interpreting high soil salinity levels to be sure that the salts present are
                not due to recent fertiliser or gypsum applications. Further testing of
                chloride salts is preferably done to identify the type of salt present.

      8.1.10    Soil test checklist
                The items in Table 8.14 are the major issues covered in most soil test
                reports. Some tests have different names, so a range of options is also
                given here. Refer to the discussion above for more details on
                interpretation.

                           Table 8.14 Major issues covered in most soil test reports

 What is being            Analytical
                                                                          General Comments
   tested?                 Method
Major soil nutrient
Phosphorus (P)         Olsen                    Target Olsen P Levels: Latest research indicates the optimal level for
                                                most dairy pastures is 18 to 22 mg/kg.
                                                Capital applications using soil texture: The following soil factors are the
                                                amount of nutrient (kg/ha) above maintenance required to increase
                                                the soil fertility by 1 Olsen P unit. It is more accurate to use PBI;
                                                however, soil texture can be used where PBI is not available. Sands
                                                need 5 kg P/ha, sandy loams 8 kg P/ha, clay loams 10 kg P/ha,
                                                clays/red soils 13 kg P/ha and peats need 16 kg P /ha to raise Olsen
                                                P one unit.
                                                Capital applications using PBI values: Following is the amount of
                                                phosphorus needed to raise Olsen P by 1 unit using PBI values: 0 to 50
                                                PBI needs 5 kg P/ha, 50 to 100 needs 7 kg P/ha, 100 to 200 needs 9 kg
                                                P/ha, 200 to 300 needs 11 kg P/ha, 300 to 600 needs 13 kg P/ha, over
                                                600 needs 15 kg P/ha.
Potassium (K)          Colwell/Skene            Maintain a level higher than 180 to 200 mg/kg (soil type dependent).
                       value
                       or as meq K/100g         Multiply result by 391 to get available K value similar to
                                                Colwell/Skene value above.
Sulphur (S)            KCl 40, CPC or           Maintain at or above a level of 12 mg/kg.
                       MCP tests




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                  Table 8.14 Major issues covered in most soil test reports (cont’d)

 What is being         Analytical
                                                             General Comments
   tested?              Method
Other Soil Test Information
pH                  water


                    CaCl2           Maintain a level higher than 5.0 (CaCl2) for productive legume
                                    pastures.
                                    If level is less than 5.0 (CaCl2), consider agricultural lime.
Aluminium           % of cations    A level of less than 1% is desirable; at a level less than 3%, most
                                    pasture species are unaffected.
                    KCl exch.       A level of less than 50 mg/kg is desirable. (Aluminium is toxic to
                                    plants. Increasing the pH will reduce its availability; aluminium levels
                                    are usually okay above pH (CaCl2)5.0.).
ECe                                 A measure of salinity, which takes texture into account:
                                    Less than 1.8        okay for all pastures and clovers.
                                    Less than 3.8        most pasture plants unaffected.
                                    6.5 to 8.6           okay for grass but not clover.
                                    Greater than 8.6     salt tolerant plants only.
Organic matter                      Required for soil protection, structure and nutrient cycling.
                                    10 % is high.
                                    To convert organic carbon to organic matter %, multiply the organic
                                    carbon number x 1.7.
Exchangeable Cations
Soil structure      Ammonium        Ca:Mg ratio greater than 2:1 indicates well structured soil.
issues              chloride
                                    Exchangeable sodium percentage (ESP) or “sodium % of cations” less
                                    than 6% and preferably less than 3 %.
                                    Gypsum may help with structure if soil is dispersive.

                                    Desirable range for cations as % of CEC:
                                    Calcium       65% to 80%            Low Ca - Lime or gypsum.
                                    Magnesium 10% to 20%                Low Mg - Dolomite.
                                    Sodium        Less than 6%          High Na - Gypsum (& drainage).
                                    Potassium     3% to 8%
                                    Aluminium Less than 1%              High Al – Lime.

                                    Cation exchange capacity (CEC) or “sum of cations” will vary with
                                    soil type (usually lower in sands, higher in clays)
                                    A CEC greater than 10 meq/100g is desirable (usually heavy loams or
                                    clay loams).
                                    Less than 10 meq/100g usually indicates a sandy soil, which may be
                                    prone to leaching. Consider split fertiliser applications.

Animal health                       Mg:K ratio: Less than 1.5 indicates possible grass tetany problems.
issues
                                    K ÷ (Ca + Mg): Less than 0.07 desirable or possible grass tetany
                                    problems.




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8.2      Interpreting tissue tests

                Plant tissue testing is the preferred method for diagnosing trace element
                toxicities, deficiencies, and imbalances for plants.

                Tissue tests, also known as plant analysis, determine the chemical
                analysis of the nutrients present in plant tissue. Both the major nutrients
                and the micronutrients (trace elements) are covered on tissue tests (see
                Figure 8.2).




                                       Figure 8.2 An example plant tissue test



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                                                           Chapter 8. Interpreting Soil and Tissue Tests


                Clover samples are usually used to diagnose trace element deficiencies in
                dairy pastures, although ryegrass samples can also be used where only
                ryegrass is available. Tissue tests are also used to confirm that plants are
                accessing the nutrients that have been applied and to confirm a diagnosis
                made by other means.

                Interpretation of the results of a tissue test is complex and depends on a
                number of factors. It is therefore recommended that interpreting the
                results of tissue tests should be done by a trained professional. This is
                because actual adequate levels for any nutrient varies depending on
                species, plant part, time of year and stage of growth. When using tissue
                tests to determine nutrient levels in plants, it is vital to take the sample
                correctly and to provide as much information as possible to aid
                interpretation (as explained in Section 7. 3). Supplying relevant
                information will help the person interpreting the results to correctly
                calculate the level of nutrients in your sample.

                Results from laboratories will be analysed by trained professionals who
                will take into account the information you have provided and will
                interpret the results. The results are usually shown with a number and
                an interpretation of this (for example, deficient, adequate, high). Based
                on this interpretation, you can then make decisions about nutrients
                required in your fertiliser program.

                More information about interpretation of tissue test results can be found
                in Plant Analysis: An Interpretation Manual (Reuter and Robinson, 1986).


8.3     Summary

                • Soil and tissue tests provide valuable information about properties
                  (mostly chemical properties) that affect plant growth.

                • After soil and tissue test results come back from the lab, it is important
                  to determine what they actually mean

                • The interpretation of results from soil and tissue tests will help you to
                  make more informed, cost-effective fertiliser decisions.




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Chapter 8. Interpreting Soil and Tissue Tests




170                                             February 2005

				
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