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