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Nutrient Availability
Section I
Soil Fertility and Plant Nutrition
Nutrient Availability Defined
• Available nutrients are those that are present in the correct chemical form for uptake by a plant during its life cycle.
• Available nutrients may be derived from the solid portion of the soil or the atmosphere.
• Nutrients must be taken up as ions or molecules from soil solution or through the leaf from the atmosphere.
Nutrient Availability and Soil Fertility
• Much of the science of soil fertility is concerned with the:
Measurement and Adjustment
of Nutrient Availability
SoilMinerals
Soil OrganicMatter
AvailableNutrientPool
Weathering
Precipitation Mineralization
Immobilization
Fertilization, AtmosphericInputs, N fixation
PlantUptake
Leaching, Erosion, Gaseous losses
+ exch. cations
Desorption
Adsorption
Nutrient Availability
• Chemical Availability– Is the nutrient in the proper chemical form and
concentration for plant needs?• Positional Availability
– Is the nutrient accessible by the plant?– Relates to fertilizer placement
• Physiological Availability– Is the plant able to take up the nutrient?– Other growth-limiting factors can intervene
Measuring Nutrient Availability
• What should a soil test measure?– Soluble nutrient only?– Other forms?– What are “available” nutrients anyway?
Chemical Availability
• Available Nutrient forms (macronutrients)N NO3
-, NH4+
P H2PO4-, HPO4
2-
K K+
S SO42-
Ca Ca2+
Mg Mg2+
Chemical Availability
• Available Nutrient forms (micronutrients)Cl Cl-
Mo MoO42-
Fe Fe2+, Fe3+
Mn Mn2+, Mn3+
Zn Zn2+
Cu Cu2+, Cu+
Ni Ni2+
B H3BO3
Minerals and Soil Nutrients (1)
Common Soil Minerals Nutrients provided
Feldspars Ca, Mg, Fe, Na, K
Apatite P
Iron oxides Fe
Carbonates Ca, Mg
Clay Minerals Si, Ca, Mg, Fe, K,Mn, Zn, Cu
Minerals and Soil Nutrients (2)
• Clays are important sources of several nutrients, through the mechanism of cation exchange.
• Exchangeable Ca, K, and Mg are important sources of these nutrients in soils
Organic Matter and Soil Nutrients
• Soil organic matter is an important source of the plant nutrients– N, P,S
• SOM must be ___________________ before these nutrients are available for plant uptake.
decomposed
Nutrient Availability - Positional
• In addition to being in the proper form, nutrients must be in the proper location for plant uptake to occur.
• Depth of plant root zone affects where the plant can extract nutrients
• Fertilization methods, irrigation methods can affect nutrient distribution.
Nutrient Mobility in Soil
Soil volume exploitedfor “mobile” nutrients:N, S
Soil volume exploitedfor “immobile” nutrients:Most others
Consequences of Nutrient Mobility
• Interpretation of soil test values– Mobile
• “total” available nutrient within the root zone is available for uptake
– Immobile• Only a small fraction of available nutrient within
the root zone is available for uptake
• Placement of fertilizers
Soil Tests
Courtesy IAS Laboratories, Phoenix, AZ
Positional Nutrient Availability
Fluid Journal, Fall 2001, used without permission
Apply immobile nutrients here