Soil Fertility
Tim Griffin
Sustainable Agriculture
John Jemison
Water Quality
Basic Concepts of Soil Fertility
16 essential elements for plant growth
Carbon (C), Hydrogen (H), Oxygen (O)
Macro / secondary nutrients
N, P, K, - Ca, Mg, S
Micronutrients:
Cl, Mn, Fe, Mo, Zn, B, and Cu
Plant Nutrient Composition
Element Corn
C, H, O 43.6, 6.2, 44.4 %
N 1.5 %
P 0.2 %
K 1.0 %
Ca 0.23 %
Mg 0.18 %
S 0.17%
Micronutrients 0.14 %
Fe, Mn 0.12 %
Forms Available to Plants
Ions are charged molecules
Most plant nutrients are ions (+ / -)
Cations - positively charged ions (Ca++)
Anions - negatively charged ions (NO3-)
Uncharged molecules (H3BO3)
How Plants Absorb Nutrients
High nutrient demand - soil
Low nutrient demand - leaves (foliar) or soil
Micronutrients commonly foliarly applied
Most soil nutrients should come through roots
Plant Nutrient Demand
Uptake patterns for N, P, and K are different
Little uptake early in growing season
Stresses greatly affect nutrient uptake
water stress impact
N needs continue through grain fill
Crop Yield Crop Response Curves
Nutrient Level
Crop Yield Deficiency
Visible Deficiency
Symptoms Seen
Nutrient Level
Adequacy
95%
Crop Yield
Nutrient Level
Crop Yield Excess
Post-mortem tests
* Stalk Nitrate *
Nutrient Level
Potassium Deficiency
Burning along leaf margins
Spotting on leaves
Weaker stalks
Poor winter hardiness
Reduced yields
Phosphorus Deficiency
Purpling on stalks leaves branches
Stunted plants
Poorer fruit/seed set
Delayed maturity
Reduced yields
Micronutrient Form Function
Ion Form Available Deficiency Symptom
Mn Mn ++ Vary among species
Interveinal chlorosis
Necrotic spots common
Zn Zn ++ Generalized chlorosis
Little leaf disease
Cu Cu ++ Generalized chlorosis
or deep green leaves
Leaf margins roll up
Fe Fe++ Chlorosis of young
leaves -
Micronutrient Form Function
Ion Form Available Deficiency Symptom
Chlorine Cl- Very rare - bronzing
leaves
B H3BO3 Heart rot of root crops
B(OH)4- Hard dry tissues
Growing tips die back
Mo MoO4-- Leaves mottled
Entirely stunted plant
Legumes N deficient
Macronutrient Mobility in Plants
N: inverted V in tip of older plant leaf
mobile in plant (robbing N from lower leaf)
Phosphorus: mobile
distributed easily from older to younger parts
dark greenish/ bluish tissue color - stunted plants
Potassium: most mobile of all plant nutrients
deficiency - burning on leaf margins
Secondary Nutrient Mobility
Calcium: Most immobile nutrient in plants
deficiency seen in newest tissue
Magnesium: somewhat mobile in plants
deficiency in newer plant parts
Sulfur: mobile in plants
less redistribution than N - more overall chlorosis
Nitrogen Cycle
Crop Harvest
Volatilization
Denitrification
NH4+ NO3-
nitrification
Soil Organic
Matter Leaching
N Fixation by Legumes
N2 (gas) bacterial cells amino acids
Alfalfa, clovers, beans, soybean
“Free” N input into the production system
Some of this N is available to following crop
Plow Down N Credits
Alfalfa:
50-75% stand - 110 lbs/ac
50% stand - 100 lbs/ac
> N
Going Fast
Conditions Enhancing Immobilization
NO3 NO2 NH4 Organic N
Incorporating high C residues
Manure with lots of bedding
Adding material with C:N ratio > 30
Conditions Increasing Volatilization
NH4 NH3
Warm temperatures
Steady winds
High pH soils
Using an N source that is volatile
Not incorporating manure / fertilizer
Conditions Increasing Denitrification
NO3 N2O NO N2
Warm temperatures
Saturated or near saturated conditions
Lots of microbial activity
Time
Nitrate Leaching
Plant N Sources
Organic Matter
2 - 8% OM in soil
Supplies N, but also P, S, Ca, Mg (5%, 2%, 1%)
Likely supplies 20-30% of crop N uptake
Fertilizers - supply NH4+ or NO3-
Manure / Sludge - NH4+ and organic N
N Fertilizer Forms Available
Urea - 46% N - solid - volatile loss
Ammonium nitrate - 33%N - non-volatile
Ammonium sulfate - 21% N- solid - (NV)
UAN - 28-32% N - liquid - volatile/leaching
Aqueous ammonia - 16-25%N - Liquid
AA = 82% N - vapor - hazardous
Nitrogen Mgmt. and Soil Types
Best management plans:
starter fertilizer on low - med test soils
test manure and apply using nutrient plan
soil test using PSNT
apply N as needed
Sandy or poorly drained soil - split appl.
Volatilization: incorporate or band apply
Plant
Residues/Manures
Soil
Solution Stable
P Soil Organic P
Microbes
phospholipds
Stable
Inorg.
Labile. Labile
P
Inorg. P Org. P
Complexed P HPO4
ATP
Ca/Mg
complexes
Fe/Al
complexes
SF:RPO IV-1a
P Fertilizer Forms Available
Triple super phosphate - 0-46-0
OSP - solid - (0-20 -0) - good S source
MAP - 11-52-0 - Common starter fertilizer
DAP - 18-46-0 - Care with starter
APP - (11-37-0) - Fluid material
Poultry manure - 20 - 80 lbs/ton
Sludge - Rock phosphate
Important P Considerations
Banding reduces P fixation
Immobile in soil under most conditions
High levels - get movement of organics
phospholipids, inositol phosphates, etc.
could tie up Zn, but manure supplies Zn
Erosion loss - greatest concern
Conversions%P2O5 -> %P * 2.29
Fertilizer or
ATP
Plant Residues Manure
Soil
Solution
K
Clay Fixed
Mineral K
K
Exch.
Inorg. K
Important K Considerations
Grain K K
Luxury consumption of K common
excess K in forages for dry cows
Improves winter hardiness
Sandy Soils: some leaching of K
K Fertilizer Forms Available
KCl - 0-0-60 / KNO3 - 0-0-39 solids
Potassium sulfate - solid - (0-0 -50)
Sul-Po-Mag - 0-0-20-11
Dairy Manure - rich K source
Sludge - very low K source Wood Ash - high
% K20 = % K * 1.20
Secondary Nutrient Form
Element Nutrient Form
++
Calcium Ca
++
Magnesium Mg
--
Sulfur SO4
Micronutrients in Plant Nutrition
Calcium: membrane integrity - blossom end rot
Magnesium: photosynthesis - interveinal chlorosis
Sulfur: amino acids - flavor - general yellowing
Zn, Mn, Cu, Fe: photosynthesis
Boron: sugar formation and movement
Molybdenum: Nitrate reductase (legumes)
Chlorine: ionic buffer
Most micronutrients are relatively immobile in plants
Supplying Micronutrients to Plants
Chelated forms
Frits (molten glass formulations
Starter fertilizer
Foliar application in cases of deficiency
Micro-nutrient Availability
pH effect: increased availability with low pH
Exception to the rule: Mo
Micronutrient toxicity - reduced by liming