Nutrient Balance Sheets for Apple Orchards Nitrogen, Phosphorus

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					Nutrient Balance Sheets for Apple Orchards: Nitrogen, Phosphorus and Potassium

Project Name

:

National Landcare Program Project No. NLP074425 “Soil Nutrient Best Management Practices for Apple Growers” Robert Peake, Rural Solutions SA Ben Thomas October 2008

Project Manager : Prepared By Date : :

SCHOLEFIELD ROBINSON HORTICULTURAL SERVICES PTY LTD
118A Glen Osmond Road, Parkside SA 5063 Australia Ph: (08) 8373 2488 Email: srhs@srhs.com.au ACN 008 199 737 ABN 63 008 199 737 Offices in Adelaide and Mildura PO Box 650, Fullarton SA 5063 Fax: (08) 8373 2442 Web Site: www.srhs.com.au

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1

INTRODUCTION

Scholefield Robinson Horticultural Services Pty Ltd (Scholefield Robinson) was engaged by Bob Peake (Rural Solutions SA) to prepare nitrogen, phosphorus and potassium balance sheets for apples as part of National Landcare Program Project No. NLP074425 “Soil Nutrient Best Management Practices for Apple Growers”. The aim of these balance sheets is to allow apple growers within SA to assess their current nitrogen, phosphorus and potassium fertiliser use; compare this with the removal in fruit and soil reserves of each nutrient; and assess whether improvements in fertiliser use efficiency and environmental sustainability are possible.

2

INITIAL WORKSHOPS

Initial workshops were conducted in the Adelaide Hills (Lenswood) and South Eastern South Australia (Kalangadoo) in late 2007. These workshops were conducted to inform the apple growers about the project, highlight the project aims and to establish contacts with growers willing to provide input into the Balance Sheets. Several growers provided management information for their orchard which was used to fine tune the models.

3

DEVELOPMENT OF BALANCE SHEETS

The aim of this project was to develop nutrient balance sheets for apples to allow apple growers to assess the nutrient balance of their orchards. This was limited to the macronutrients nitrogen, phosphorus and potassium. Balance Sheets normally consist of 2 sections – Tree Nutrient Requirements and Inputs. In perennial tree crops, tree nutrient requirements can be estimated by calculating the amount of nutrient removed from the orchard in the form of fruit and/or prunings and the amount of nutrient required for vegetative growth. Inputs include fertilisers, nutrient in water, nutrients in soil and remobilisation of stored nutrients within the plant whereas outputs include product, prunings (if removed from the orchard) and nutrient loss via leaching or volatilisation. The relative importance of each of these components varies for each nutrient. Note that whilst attempts were made to estimate the contribution of the different components to apple nutrition, the Balance Sheets should be used as a general guide only and are not intended to be used as the sole source of information for making nutrient and fertiliser management decisions. Other management tools such as soil and plant analyses and visual assessments of trees should also be used when making nutrition management decisions. Each balance sheet contains modules for growers to insert information on orchard details, soil, water (Nitrogen Calculator only), cover crops and organic supplements (Nitrogen Calculator only), and fertilisers. Data and relationships used in the balance sheets were sourced from published literature.

3.1

Apple Nitrogen Balance Sheet

It has long been recognised that tree fruits have a requirement for nitrogen that may be greater than that supplied by the soil (Neilsen and Neilsen, 2003). Furthermore, nitrogen is required to support the growth of new plant material as well as fruit. As such, simply replacing the nitrogen removed in the form of fruit (estimated by to be 0.32kg of nitrogen per tonne of fresh fruit (Reuter and Judson, 2003)) will not take vegetative growth requirements into consideration and hence may result in the underestimation of the nitrogen requirements of the trees.

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The Apple Nitrogen Balance Sheet consists of a series of modules covering orchard information, soil, water, cover crops and organic supplements and fertilisers. 3.1.1 Orchard Information This module allows growers to enter general information for the block including block name, tree age, tree spacing, irrigation method and yield. Some of this information is used to calculate the following: Number of trees per hectare
100 Tree spacing 100 Row width

Number of trees per ha 1 =

×

Total nitrogen content of trees There is limited data on total nitrogen content of apple trees for the various planting densities used in commercial apple production in Australia. For high density plantings (3333 trees per hectare), Neilsen and Neilsen (2003) estimated total tree nitrogen content by year 6 as probably around 30g actual nitrogen per tree. In comparison, nitrogen content of high density planted (3300 trees per hectare) ‘Golden Delicious’ trees on M.9 rootstock at the end of year 1 was 8.2g nitrogen per tree (Neilsen et al., 2001a) and nitrogen content of high density planted (3300 trees per hectare) ‘Elstar’ trees on M.9 rootstock at the end of year 4 was 19.7g nitrogen per tree (Neilsen et al., 2001b). For lower density plantings, Haynes and Goh (1980) measured total tree nitrogen content of 14 year old ‘Golden Delicious’ trees on Northern Spy rootstocks planted 500 trees per hectare to be 134g nitrogen per tree. Tree densities commonly used in Australian apple production are 1250 trees per hectare (4m row width × 2m tree spacing) or 1667 trees per hectare (4m row width × 1.5m tree spacing). Using the estimated nitrogen content of trees from Haynes and Goh (1980) and Neilsen and Neilsen (2003) and assuming a linear relationship between tree density and nitrogen content per tree, tree nitrogen content can be estimated using the following equation.

Tree nitrogen content (g/tree) = (-0.036 × Number of trees per ha) + 152.3

It is then possible to calculate the total nitrogen content of trees on a per hectare basis
Tree nitrogen content (g/tree) × 1000 Number of trees per ha

Total nitrogen content of trees (kg/ha) =

Tree nitrogen replaced by annual root uptake

1

ha = hectare

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It has been estimated that 40 to 50% of tree nitrogen content in larger trees is replaced annually as a result of current year nitrogen uptake from the soil (Weinbaum et al., 1987). Neilsen et al. (2001b) found a similar relationship in young apple trees. Therefore, it is possible to predict the annual nitrogen requirement of apple trees based on the total nitrogen content of the apple trees.

Tree nitrogen replaced annually by root uptake (kg/ha) = Total nitrogen content of trees (kg/ha) × 0.5

Irrigated area of soil The area of soil that is moist and hence utilised by the trees depends on the irrigation system used. For example, the amount of soil wetted by drippers will be less than that wetted by microsprinklers. Therefore, there needs to be an allowance for the area of soil that is readily accessed by the trees.
Spread of irrigation (m) Row width (m)

Irrigated area of soil (ha/ha) =

3.1.2 Soil Module For this module, growers need to enter the soil type and organic carbon content of the soil for the block. The organic carbon content of the soil is normally provided as part of a standard soil test. The information is used to calculate the organic nitrogen content of the soil and the contribution of soil organic nitrogen to nitrogen uptake by the trees. Soil organic nitrogen content Firstly, the organic matter content of the soil is calculated using the following relationship from Nicholas et al. (2004).

Organic matter content of soil (%) = Organic carbon content of soil (%) × 1.72

The Bulk Density of the soil is then used to estimate the weight of topsoil (0-15cm deep) per hectare. Bulk density is defined as the mass of soil per for a given volume of soil and is measured as g/cm3. It takes into consideration the presence of pores within the soil and generally speaking, sandy soils have higher bulk densities than loams or clays and soils containing more organic matter have lower bulk densities. Guidelines for bulk densities of soils as proposed by Handreck and Black (1994) are shown in Table 1. For the balance sheet, bulk density of soil was estimated using a regression equation derived by JO Skjemstad (CSIRO) based on clay content and using bulk density data for Southeast Australian soils from Forrest et al. (1985) and Geeves et al. (1995). Soil clay contents were “median” values taken from the soil texture triangle published by Marshall (1947).

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Table 1 Guidelines for bulk densities of soils
Bulk density (g/cm3) <1.0 1.0 – 1.2 1.2 – 1.4 1.4 – 1.6 1.6 – 1.8 >1.8 Sandy soils – – Very open Satisfactory Above 1.7, most soils too compact Very compact Loams Seedbed conditions Satisfactory Satisfactory Some soils too compact Very compact Extremely compact Clay soils Satisfactory Satisfactory Some soils too compact Very compact Extremely compact – Handreck and Black (1994)

The bulk density allows the weight of topsoil and organic matter per hectare to be calculated as follows.

Weight of topsoil (t/ha) = Volume of topsoil (m3/ha) × Soil bulk density (t/m3)

Organic matter in topsoil (t/ha) =

Weight of topsoil (t/ha) × Organic matter content of soil (%) 100

Gaskell et al. (2006) estimated that soil organic matter contains 7% nitrogen and so it is possible to calculate the amount of organic nitrogen in the topsoil.

Organic nitrogen in topsoil (kg/ha) = (Organic matter in topsoil (t/ha) × 1000) × 0.07

Available mineralised soil organic nitrogen for uptake by trees Gaskell et al. (2006) state that 2-5% of soil organic matter decomposes annually. Therefore, it is possible to estimate the contribution of mineralisation of soil organic matter to the nitrogen uptake by trees. Allowances are made regarding the area of soil accessible by the trees and it is assumed that the trees will only access 50% of the mineralised soil organic nitrogen. The amount of soil organic matter decomposed annually was set at 5%.

Organic nitrogen mineralised per annum (kg/ha) = Organic nitrogen in topsoil (kg/ha) × 0.05

Available mineralised soil organic nitrogen (kg/ha)

=

Total organic nitrogen mineralised (kg/ha)

×

Irrigated area of soil (ha)

×

0.5

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3.1.3 Water Module Most South Australian irrigation waters contain very little nitrogen. However, a water module has been included for those growers who are using water that do contain significant amounts of nitrogen.

Nitrogen applied in irrigation water (kg/ha)

=

Total Kjehldahl nitrogen content of water (mg/L)

×

Volume of irrigation water applied (ML/annum)

3.1.4 Cover Crop Module Nitrogen fixation by legume based cover crops may make a significant contribution to soil nitrogen reserves. These contributions have been estimated as 25kg nitrogen and 50kg nitrogen per hectare for average and good legume cover crops respectively. These estimates were derived from an Almond Nitrogen Nutrition Calculator devised by Brown (http://ucce.ucdavis.edu/rics/fnric2/almondNKmodel/almond_n_model.htm). 3.1.5 Organic Supplement Module The contribution of organic supplements to apple nitrogen nutrition is difficult to assess. This is due to the large variety of organic products available to growers. Growers now have access to manures, manure composts, plant-based composts and commercial organic fertilisers and examples of organic supplements that may be used in apple orchards are shown in Table 2. These products differ in nitrogen content, level of composting and hence volatility and method of application. Soil type and health (microbiological activity) will also influence the rate of mineralisation of the organic material. Table 2 – Nitrogen content of some common organic materials
Straw, Compost or Manure Type Clover1 Lucerne hay1 Grasses1 Weeds (mixed)1 Aged poultry manure 2 Aged cow manure2 Composted poultry manure2 Composted cow manure2 Pig manure + straw3 Municipal green waste compost 2
1 2 3

Nitrogen Content (% Dry Weight) 2.2 3.1 1.8 2.0 2.8 2.2 2.4 2.1 1.8 1.5

Handreck and Black (1994) Hartz et al. (2000) Unpublished data

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These factors mean that the amount of nitrogen applied and the availability of this nitrogen to the trees varies widely. For example, application of raw manure to an orchard soil may result in significantly more loss of nitrogen via volatilisation compared to the same manure that has been composted whereas some compost may even result in immobilisation of nitrogen (so-called nitrogen drawdown). Hartz et al. (2000) measured nitrogen and carbon mineralisation dynamics of various manures and composts. They used laboratory incubations that involved mixing the manure or compost with soil and measuring the mineral nitrogen concentration of the mix over an incubation period. A change in mineral nitrogen content of the mix represented net nitrogen mineralisation or immobilisation. Hartz et al. (2000) also used a fescue assay which involved growing fescue in soil/compost or soil/manure mixes and measuring the amount of nitrogen recovered in the fescue plants. Overall, Hartz et al. (2000) gave average estimates of plant available nitrogen of 20%, 9%, 6% and 2% for dried chicken manure, other manure, manure composts and plant residue composts respectively and these estimates were incorporated into the balance sheet. 3.1.6 Fertiliser Module A large number of nitrogen fertilisers are available for apple growers with varying nitrogen contents and constituents. As such, growers can enter data for some common nitrogen fertilisers (urea, DAP, MAP and calcium nitrate) or for other products. Method of application and nitrogen use efficiency In the past, fertilisers in apple orchards were commonly applied as single applications, often in late winter. Applying such large amounts of nitrogen fertilisers early in the season when root uptake of nutrients is low had the potential to result in considerable losses of nitrogen via leaching and/or volatilisation and hence low Nitrogen Use Efficiency (NUE). It is assumed that in modern apple orchards, nitrogen fertilisers are applied either as split applications or via fertigation with subsequent improvements in NUE. The use of drip irrigation also resulted in a change in root distribution in apple orchards. Localised application of water cause a concentration of roots in the area wetted by the drippers (Tagliavini et al., 1996). Fertilisers applied outside the area of soil wetted by the drippers are generally much less available for root uptake (Haynes, 1985). In these situations, applying nitrogen fertiliser as surface bands along the soil wetted by the drippers should result in improved NUE (Haynes, 1985; Tagliavini et al., 1996). Many growers are now applying small but frequent amounts of nitrogen fertiliser through the irrigation system (fertigation). This allows growers to apply nutrients to meet specific plant needs and normally results in better NUE than either broadcast or banded fertiliser applications (Haynes, 1985). Levin et al. (1980) suggested that it is appropriate to assume a 50% NUE for fertigated apple trees. For the Apple Nitrogen Balance Sheet, fertigation was deemed to result in 50% NUE whereas split applications of nitrogen fertiliser applied in bands to the soil surface was deemed to result in 25% NUE. This assumes that growers have chosen appropriate fertilisers (i.e. avoiding urea early in the season when soils are cold) and have applied the fertiliser at appropriate times to minimise loss of nitrogen via leaching. As discussed above, it is also assumed that single broadcast applications of nitrogen fertiliser are no longer used in apple production due to the inherent inefficiency of this fertiliser application strategy. 3.1.7 Nitrogen Balance Sheet The information provided and subsequent calculations are summarised in the form of a balance sheet. This consists of 2 sections – tree nitrogen requirements and inputs. Tree nitrogen

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requirements are based on tree nitrogen replaced by root uptake and nitrogen removed in fruit. Inputs are estimated from mineralisation of soil organic nitrogen reserves, nitrogen from legume cover crops, nitrogen from organic supplements, nitrogen fertilisers and nitrogen in irrigation water using relationships shown above. The final balance of nitrogen in the orchard is then calculated as follows.

Nitrogen balance (kg/ha) = Tree nitrogen requirement (kg/ha) – Inputs (kg/ha)

A negative result suggests that more nitrogen is being removed from the orchard or incorporated into the trees than is being applied. In this situation, growers are then advised to check leaf nitrogen analysis values and vigour to determine if additional nitrogen inputs are needed. A positive result suggests that more nitrogen is being applied to the orchard than is being removed in fruit or incorporated into the trees. In this situation, growers are advised to monitor tree nitrogen status using leaf analysis and tree vigour. If leaf nitrogen is high, growers are advised to consider reducing nitrogen fertiliser inputs.

3.2

Apple Phosphorus Balance Sheet

Periods of high phosphorus demand in apples occurs during periods of considerable meristematic tissue of which phosphorus is an important regulator. However, the absolute phosphorus requirements of apple trees are small compared to nitrogen and potassium (Neilsen and Neilsen, 2003). Reuter and Judson (2003) estimated that 0.08kg of phosphorus is removed per tonne of fresh fruit. Like the Apple Nitrogen Balance Sheet, the Apple Phosphorus Balance Sheet consists of a series of modules covering orchard information, soil and fertilisers. 3.2.1 Orchard Information This module allows growers to enter general information for the block including block name, tree age, tree spacing, irrigation method and yield. See Nitrogen Balance Sheet for calculations. 3.2.2 Soil Module Natural reserves of phosphorus are almost universally low in Australian soils (Treeby et al., 2004). As a result, phosphorus fertilisers (particularly superphosphate) were, in the past, applied routinely to most agricultural soils to increase the availability of phosphorus to plants. However, in recent times, costs of phosphorus fertiliser and concern over environmental impacts of phosphorus fertiliser have increased and so most growers are more aware of the need to match phosphorus fertiliser application to requirement rather than applying phosphorus fertiliser as routine. In soil, phosphorus is present in several forms and each differs in availability to plants.
•

Soluble phosphorus is present in the soil solution. This is a small pool but is available for plant uptake. “Fixed” phosphorus. Soluble phosphorus in the soil may be precipitated or “fixed” in an insoluble form with calcium or iron and aluminium oxides. Calcium compounds are most desirable if the soil pH is no higher than 7.5 as the resultant calcium phosphate is soluble enough to supply the plants needs (Leeper and Uren, 1993).

•

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If the soil pH is higher than 7.5 or if calcium carbonate is present in the soil, hydroxyapatite will be formed which is very insoluble and a poor source of phosphorus for plants (Leeper and Uren, 1993). The formation of iron and aluminium compounds with phosphorus is complex. Put simply, phosphorus in soil will form compounds with aluminium and iron oxides and if the saturation of the surface of the oxide with phosphorus is low, this adsorbed form is of little value to plants (Leeper and Uren (1993)
•

Organic phosphorus. Soil organic matter can also be an important source of phosphorus for plants but very little is known about organic phosphorus forms compared to the inorganic phosphorus content of soils (Price, 2006). As a result, organic phosphorus was not considered as part of this balance sheet.

For this module, growers need to enter the soil type and Colwell phosphorus content of the soil for the block. Colwell phosphorus is a measure of the amount of plant available phosphorus in the soil. This information is used to estimate the weight of topsoil (see Section 3.1.2) and amount of available phosphorus in the topsoil per hectare.
(Weight of topsoil (t/ha) × 1000) × Soil Colwell phosphorus concentration (mg/kg) 1000000

Available phosphorus in topsoil (kg/ha) =

Accessible soil phosphorus reserves As discussed for nitrogen, the irrigation method used will influence the area of soil wetted and hence the area of soil that the trees have access to. This is taken into consideration when determining the accessible soil phosphorus reserves.

Accessible soil phosphorus reserves (kg/ha) = Available phosphorus in topsoil (kg/ha) × Irrigated area of soil (ha/ha)

3.2.3 Fertiliser Module Like nitrogen, there are a large number of phosphorus fertilisers available for apple growers with varying phosphorus contents and constituents. As such, growers can enter data for some common phosphorus fertilisers (Superphosphate, DAP, MAP and Triple Super) or for other products. Phosphorus generally stays close to where it is applied and except for very sandy soils, very little phosphorus is lost via leaching (Price, 2006). Moreover, the availability of applied phosphorus depends on factors such as soil type, pH and organic carbon content. As discussed above, phosphorus in soil can become “fixed” by forming compounds with calcium, iron and aluminium. As such, not all the phosphorus applied as fertiliser will be available to plants. Instead, Leeper and Uren (1993) state that on many soils, the recovery of applied phosphorus by plants in the year of application is of the order of 20% or less. The remaining phosphorus is converted into other forms of phosphorus depending on the soil type (see Section 3.2.2). For the balance sheet, it is assumed that 20% of the phosphorus fertiliser applied is recovered by the trees in the year of application. The availability of the phosphorus in subsequent years should be assessed using soil tests.

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Recovery of phosphorus applied as fertiliser (kg/ha) = Phosphorus applied as fertiliser (kg/ha) × 0.2

3.2.4 Phosphorus Balance Sheet Like nitrogen, the information provided and subsequent calculations are summarised in the form of a balance sheet. This consists of recovery of phosphorus applied as fertiliser and removal of phosphorus in fruit. The final balance of phosphorus in the orchard is then calculated as follows.

Phosphorus balance (kg/ha) = Recovery of phosphorus fertiliser applied (kg/ha) – Phosphorus removed in fruit (kg/ha)

A negative result suggests that more phosphorus is being removed from the orchard than is being applied. In this situation, growers are advised to check leaf phosphorus analysis values to determine if phosphorus inputs are needed. A positive result suggests that more phosphorus is being applied to the orchard in fertilisers than is being removed in fruit. In this situation, growers are advised to monitor tree phosphorus status using leaf analysis and if leaf phosphorus is high, growers are advised that no action is needed. 3.2.5 Supply of Phosphorus Remaining in Soil Whilst the Phosphorus Balance Sheet allows growers to compare phosphorus applied in fertilisers and removed in fruit, this does not take the reserves of phosphorus in the soil into account. This is one of the main criticisms of the nutrient balance sheet approach. Assessment of the supply of phosphorus remaining in the soil considers the actual amount of plant accessible phosphorus in the soil, the minimum amount of accessible phosphorus to be maintained in the soil and the amount of phosphorus removed in fruit each season. The minimum concentration of Colwell phosphorus to be maintained in the topsoil is 40mg/kg and it is assumed that no phosphorus fertiliser is applied.
Minimum amount of phosphorus to be maintained in the soil (kg/ha) (Weight of topsoil (t/ha) × 1000) × 40 1000000

=

Minimum amount of accessible phosphorus Minimum amount of phosphorus to = × Irrigated area of soil (ha/ha) to be maintained in the soil (kg/ha) be maintained in the soil (kg/ha)

Supply of phosphorus remaining in soil (years)

=

Actual amount of plant accessible Minimum amount of accessible phosphorus phosphorus in the soil (kg/ha) to be maintained in the soil (kg/ha) Phosphorus removed in fruit (kg/ha)

Growers are provided with an estimate of the number of year’s supply of phosphorus that is available to their trees in the soil. If the value obtained for the supply of phosphorus remaining in the soil is negative, this means that the soil Colwell phosphorus concentration is already below the minimum value of 40mg/kg.

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3.3

Apple Potassium Balance Sheet

Whole tree potassium demands of apple are similar in magnitude to those of nitrogen (Neilsen and Neilsen, 2003). Potassium is mobile within the phloem and the fruit are strong sinks for potassium (Neilsen and Neilsen, 2003). Reuter and Judson (2003) estimated that 1.1kg of potassium is removed per tonne of fresh fruit which is considerably higher than the amount of nitrogen or phosphorus removed. Like the Apple Nitrogen and Phosphorus Balance Sheets, the Apple Potassium Balance Sheet consists of a series of modules covering orchard information, soil and fertilisers. 3.3.1 Orchard Information This module allows growers to enter general information for the block including block name, tree age, tree spacing, irrigation method and yield. See Nitrogen Balance Sheet for calculations. 3.3.2 Soil Module Potassium in soil is generally associated with clay particles (Treeby et al., 2004) and most soils contain thousands of kilograms of potassium (Price, 2006). However, much of this potassium is found in primary minerals or fixed in soil clays and only a small amount of this potassium is available to plants (Price, 2006). In Australia, soils of dry climates commonly have large reserves of high-grade potassium whereas sandy and/or leached soils of coastal and other areas can be deficient in potassium (Leeper and Uren, 1993). In soil, potassium is present in several forms and each differs in availability to plants. The following descriptions are summarised from Price (2006).
•

Soluble potassium is present in the soil solution. This is a small pool but is available for plant uptake. Exchangeable potassium is held in the exchangeable form be the negative charges of soil organic matter and clays. It is readily available to plants. Fixed potassium is fixed or trapped within soil clays and becomes slowly available to plants over a growing season. Structural potassium is held in the lattice of primary minerals and is unavailable for plant growth. This potassium is only released as soil minerals are weathered. Soil organic matter may also be an important source of potassium for plants but very little is known about organic potassium forms. As a result, organic potassium was not considered as part of this balance sheet.

•

•

•

•

For this module, growers need to enter the soil type and Colwell potassium content of the topsoil and subsoil for the block. Colwell potassium is a measure of the amount of plant available potassium in the soil including soluble and exchangeable forms. This information is used to estimate the weight of topsoil and subsoil and amount of available potassium in the topsoil and subsoil per hectare. Subsoil (15-30cm deep) is included as considerable amounts of potassium are common in subsoils and would provide an important source of potassium for apples.
(Weight of topsoil (t/ha) × 1000) × Topsoil Colwell potassium concentration (mg/kg) 1000000

Available potassium in topsoil (kg/ha) =

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Available potassium in subsoil (kg/ha) =

(Weight of subsoil (t/ha) × 1000) × Subsoil Colwell potassium concentration (mg/kg) 1000000

Accessible soil phosphorus reserves Like nitrogen and phosphorus, the irrigation method used will influence the area of soil wetted and hence the area of soil that the trees have access to. This is taken into consideration when determining the accessible soil phosphorus reserves.

Accessible topsoil potassium reserves (kg/ha) = Available potassium in topsoil (kg/ha) × Irrigated area of soil (ha/ha)

Accessible subsoil potassium reserves (kg/ha) = Available potassium in subsoil (kg/ha) × Irrigated area of soil (ha/ha)

3.3.3 Fertiliser Module Like nitrogen and phosphorus, there are a large number of potassium fertilisers available for apple growers with varying potassium contents and constituents. As such, growers can enter data for some common potassium fertilisers (potassium sulfate, potassium nitrate and potassium chloride2) or for other products. Potassium applied as fertiliser is mostly adsorbed on clay or organic matter and very little remains in soil solution and apart from some sandy soils, very little potassium is lost via leaching (Leeper and Uren, 1993). In some soils, however, potassium can be converted into slowly available forms (Price, 2006). For the balance sheet, it is assumed that the majority (75%) of potassium applied as fertiliser is available for tree uptake in the year of application. The availability of the potassium in subsequent years should be assessed using soil tests.

Availability of potassium applied as fertiliser (kg/ha) = Potassium applied as fertiliser (kg/ha) × 0.75

3.3.4 Potassium Balance Sheet Like nitrogen and phosphorus, the information provided and subsequent calculations are summarised in the form of a balance sheet. This consists of availability of potassium applied as fertiliser and removal of potassium in fruit. The final balance of potassium in the orchard is then calculated as follows.

Potassium balance (kg/ha) = Availability of potassium fertiliser applied (kg/ha) – Potassium removed in fruit (kg/ha)

2

Potassium chloride should be avoided by growers with salinity problems or where foliage contact is possible.

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A negative result suggests that more potassium is being removed from the orchard than is being applied. In this situation, growers are advised to check leaf potassium analysis values to determine if potassium inputs are needed. A positive result suggests that more potassium is being applied to the orchard than is being removed in fruit. In this situation, growers are advised to monitor tree potassium status using leaf analysis and if leaf potassium is high, growers are advised that no action is needed. 3.3.5 Supply of Potassium Remaining in Soil Like phosphorus, the Potassium Balance Sheet allows growers to compare potassium applied in fertilisers and removed in fruit, but does not take the reserves of phosphorus in the soil into account. Assessment of the supply of potassium remaining in the soil considers the actual amount of plant accessible potassium in the topsoil and subsoil, the minimum amount of accessible potassium to be maintained in the topsoil and subsoil and the amount of potassium removed in fruit each season. The minimum concentration of Colwell potassium to be maintained in both the topsoil and subsoil is 100mg/kg and it is assumed that no potassium fertiliser is applied.
(Weight of topsoil (t/ha) × 1000) × 100 1000000

Minimum amount of potassium to be maintained in the topsoil (kg/ha)

=

Minimum amount of potassium to be maintained in the subsoil (kg/ha)

=

(Weight of subsoil (t/ha) × 1000) × 100 1000000

Minimum amount of accessible potassium Minimum amount of potassium to be × Irrigated area of soil (ha/ha) = maintained in the topsoil (kg/ha) to be maintained in the topsoil (kg/ha)

Minimum amount of accessible potassium Minimum amount of potassium to be = × Irrigated area of soil (ha/ha) to be maintained in the topsoil (kg/ha) maintained in the topsoil (kg/ha)

Total minimum amount of accessible Minimum amount of accessible Minimum amount of accessible potassium to be maintained in the = potassium to be maintained in the + potassium to be maintained in the topsoil and subsoil (kg/ha) topsoil (kg/ha) subsoil (kg/ha)

Supply of potassium remaining in soil (years)

Actual amount of plant accessible Total minimum amount of accessible - potassium to be maintained in the topsoil potassium in the topsoil and = subsoil (kg/ha) and subsoil (kg/ha) Potassium removed in fruit (kg/ha)

Growers are provided with an estimate of the number of year’s supply of potassium that is available to their trees in the topsoil and subsoil. If the value obtained for the supply of potassium remaining in the soil is negative, this means that the soil Colwell potassium concentration is already below the minimum value of 100mg/kg. Note that considerable amounts of potassium may be present in lower subsoil layers than those considered here and may be accessible by apple trees. Therefore, it is very important that growers use leaf tests to confirm possible potassium deficiency before acting.

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4

PRESENTATION OF BALANCE SHEETS TO GROWERS

The Apple Nitrogen, Phosphorus and Potassium Balance Sheets were presented to apple growers at a meeting held in Kalangadoo (South Australia) and to smaller groups in the Adelaide Hills. Feedback was welcomed and incorporated into the Balance Sheets where appropriate. The Balance Sheets were well received by growers with many indicating they would be likely to use the Balance Sheets to increase their understanding of the nitrogen, phosphorus and potassium nutrition of their orchard.

5

INFORMATION GAPS AND FUTURE WORK

The development of the Apple Nitrogen, Phosphorus and Potassium Balance Sheets highlighted some gaps in information that require attention with future work to improve our understanding of apple nutrition and the accuracy of the Apple Nitrogen, Phosphorus and Potassium Balance Sheets.
•

Total nitrogen content of trees

A key component of the Apple Nitrogen Balance Sheet is the total nitrogen content of the trees which allows the amount of nitrogen to be replaced annually by root uptake to be estimated. However, as discussed in Section 3.1.1, there is limited data on total nitrogen content of apple trees especially for the various planting densities used in commercial apple production in Australia. It is more than likely that a main reason for this lack of data is the high cost of conducting whole tree excavations including roots and subsequent analysis. Ideally, whole tree excavations and analyses would be conducted on apple trees of various ages, rootstocks and planting densities but this would be too expensive. Instead, concentration should be given to generating some up-to-date information and data on total nitrogen (and other nutrient) content of mature trees of a common apple variety and rootstock combination (e.g. Pink Lady on M.9) for several planting densities.
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Estimation of organic nitrogen content and mineralisation of organic matter in apple orchard soils

Nitrogen content of organic matter and mineralisation of organic matter in soils were calculated using estimations of Gaskell et al. (2006). Whilst it is likely that the nitrogen content of organic matter and mineralisation of organic matter in apple orchards varies considerably between orchards, some analyses of these parameters in actual Australian apple orchard soils would increase the confidence in these estimates.
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Contribution of organic supplements to apple nitrogen nutrition

As addressed in Section 3.1.5, assessing the contribution of organic supplements (mulches, manures and composts) to apple nitrogen nutrition is very difficult due to the large variety of organic materials and products available to apple growers as well as the variable soil types and health (microbiological activity) which will influence the rate of mineralisation of the organic material. Whilst it is not possible to assess all organic materials on all orchards, it should be feasible to measure the contribution of some common organic materials (e.g. poultry manure, composted poultry manure, composted green waste etc.) to apple nitrogen nutrition in the major apple growing regions through the use of in-field experimentation.
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Nitrogen use efficiency

Nitrogen use efficiency (NUE) was estimated to be 50% and 25% for fertigated and split nitrogen fertiliser applications respectively which take into account the efficiency of these two
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methods of fertiliser application. Form of fertiliser and timing of application also affects NUE. Therefore, there is clearly a need to assess NUE in modern high density Australian apple orchards using fertigation or split fertiliser applications.
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Contribution of organic forms of phosphorus and potassium apple phosphorus and potassium nutrition

Whilst organic forms of phosphorus and potassium may have a significant impact on apple phosphorus and potassium nutrition, there is limited information on potential contributions from these sources. As such, these sources were not included in the Apple Phosphorus and Potassium Balance Sheets. There is a need to assess the importance of organic forms of phosphorus and potassium in most agricultural soils including apple orchards.
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Longevity of phosphorus and potassium reserves in soil

In the Apple Phosphorus and Potassium Balance Sheets, the number of seasons supply of phosphorus and potassium remaining in the soil were estimated. Soil tests should be used to assess the accuracy of these predictions.

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REFERENCES

Brown, P.H., Zhang, Q., Stevenson, M. and Rosecrance, R.C. Undated. Nitrogen Fertilizer Recommendation for Almond. http://ucce.ucdavis.edu/rics/fnric2/almondNKmodel/almond_n_model.htm Forrest, J.A., Beatty, J., Hignett, C.T., Pickering, J. and Williams, R.G.P. 1985. A Survey of the physical properties of wheatland soils in eastern Australia. CSIRO Division of Soils, Divisional Report No. 78. Geeves, G.W., Creswell, H.C., Murphy, B.W., Gessler, P.E., Chartres, C.J., Little, I.P. and Bowman, G.M. 1995. The physical, chemical and morphological properties of soils in the wheat-belt of southern N.S.W. and northern Victoria. NSW Department of Conservation and Land Management/CSIRO Division of Soils. Gaskell, M., Smith, R., Mitchell, J., Koike, S.T., Fouche, C., Hartz, T., Horwath, W. and Jackson, L. 2006. Soil fertility management for organic crops. University of California, Division of Agriculture and Natural Resources, Publication 7249. Handreck, K.A. and Black, N.D. 1994. Growing Media for Ornamental Plants and Turf. University of New South Wales Press. Hartz, T.K.J., Mitchell, J.P. and Giannini, C. 2000. Nitrogen and carbon mineralization dynamics of manures and composts. Hortscience 35, 209-212. Haynes, R.J. 1985. Principles of fertilizer use for trickle irrigated crops. Fertilizer Research 6, 235-255. Haynes, R.J. and Goh, K.M. 1980. Distribution and budget of nutrients in a commercial apple orchard. Plant and Soil 56, 445-457. Leeper, G.W. and Uren, N.C. 1993. Soil Science: An Introduction. Melbourne University Press. Levin, I., Assaf, R. and Bravdo, B. 1980. Irrigation, water status and nutrient uptake in an apple orchard. In “Mineral Nutrition of Fruit Trees.” Eds. D. Atkins, J.E. Jackson, R.O. Sharpels and W.M. Waller. pp. 255-264. Butterworth. Marshall, T.J. 1947. Mechanical composition of soil in relation to field descriptions of texture. Council for Scientific and Industrial Research, Australia. Bulletin No. 224. Neilsen, D., Millard, P., Herbert, L.C., Neilsen, G.H., Hogue, E.J., Parchomchuk, P. and Zebarth, B.J. 2001a. Remobilization and uptake of N by newly planted apple trees (Malus domestica) in response to irrigation method and timing on N application. Tree Physiology 21, 513-521. Neilsen, D., Millard, P., Neilsen, G.H. and Hogue, E.J. 2001b. Nitrogen uptake, efficiency of use and partitioning for growth in young apple trees. Journal of the American Society for Horticultural Science 126, 144-150. Neilsen, G.H. and Neilsen, D. 2003. Nutrition requirements of apple. In “Apples: Botany, Production and Uses” Eds. D.C. Ferree and I.J. Warrington. pp. 267-302. CAB International.

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Nicholas, P.R., Maschmedt, D.J., Cass, A. and Goldspink, B.H. 2004. Soil chemical properties. In “Soil, Irrigation and Nutrition.” Ed. P.R. Nicholas. pp. 23-30. Winetitles. Price, G. 2006. Australian Soil Fertility Manual. CSIRO Publishing. Reuter, D. and Judson, G. 2003. Nutrient Concentration of Agricultural Produce. info.com/research/nutrientconc/nutrrientconc.htm http://www.potash-

Tagliavini, M., Scudellazi, D., Marangoni, B. and Toselli, M. 1996. Nitrogen fertilization management in orchards to reconcile productivity and environmental aspects. Fertilizer Research 43, 93-102. Treeby, M.T., Goldspink, B.H. and Nicholas, P.R. 2004. Nutrients in the soil. In “Soil, Irrigation and Nutrition.” Ed. P.R. Nicholas. pp. 181-183. Winetitles. Weinbaum, S.A., Klein, I. and Muraoka, T.T. 1987. Use of nitrogen isotopes and a light textured soil to assess annual contributions of nitrogen from soil and storage pools in almond trees. Journal of the American Society for Horticultural Science 112, 526-529.

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