From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Fall 1996, No. 8


Highly productive soils are always fertile. However, fertile soils can differ greatly in amount of soil nutrient reservoir. This is due in large part to a special soil characteristic, the cation exchange capacity. The amounts and types of clays and organic matter collectively establish the size of the soil’s nutrient holding capacity. Thus, cation exchange capacity is a measure of the soil’s potential to store nutrients for later use by the growing crop.

The soil’s nutrient reservoir is similar to a bank checking account. Unless deposits are made, continued withdrawals will eventually deplete the resources. Understanding the sources of nutrient deposits and withdrawals is essential when building a soil/crop nutrient management plan for highly productive soils. Consider the following when balancing deposits and withdrawals.

Nutrient Deposits
Atmosphere --- some nitorgen and sulfur
Organic --- manure, sludge
N fixation --- legumes, algae, bacteria
Mineralization --- of crop residues and soil minerals
Commercial fertilizer use

Nutrient Withdrawals
Crop removal --- grain, forage, animals, etc.

Immobilization --- chemical, physical, biological
Erosion --- loss by wind and water
Leaching --- loss of nitrogen, potassium, magnesium, sulfur, and boron from sandy, low cation exchange capacity soils
Denitrification --- nitrogen loss in wet soils

The primary objective of maintaining a fertile soil is to assure nutrition does not limit plant growth. Research continues to document that healthy plants can better tolerate the stress brought on by drought, insect and disease injury, or temperature extremes. Several tools are available to achieve this objective.

Soil testing --- provides a measure of the nutrient status of the soil reservoir
Research data --- Plant analysis --- provides a measure of nutrient use effectiveness by the crop
Field scouting --- provides in-field evaluation of crop and input use performance
Computers --- accessing more information to make better decisions faster
Yield monitoring --- helps provide site-specific evaluation through computer technology

With today’s knowledge base and technology capability, there is little justification for not providing plant nutritional requirements on a basis that is:


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