AGRI-BRIEFS   AGRONOMIC NEWS ITEMS

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

Winter 1998, No. 4

Base phosphorus and potassium recommendations on soil tests. Test soil at least once every four years, more frequently in build-up situations. It may be most practical to apply build up applications immediately following the soil test, then follow a maintenance program in the years between soil tests, accounting for nutrients removed in the harvested crops. Alternatively, the build up requirement may be distributed among the annual applications.

If soil tests are maintained at high levels, timing within the rotation is less critical, so long as the total nutrients applied across the rotation account for removal by all crops grown. If the rotation changes, or if yields are higher or lower than anticipated, planned applications can be adjusted accordingly. Maintaining high soil tests provides flexibility to skip—or reduce—application in years when financial resources are limited, as long as the deficit is made up in succeeding years.

When livestock manure is used in the program, proper credits must be taken in determining build up and maintenance rates. Nutrient levels in manure vary, so base credits on nutrient analysis of the manure to be applied.

The nutrient management plan should also consider the soil types. If the soil has a high capacity for fixing phosphorus or potassium in unavailable forms, annual applications may be necessary. Timing and placement should also consider the tillage system, the rooting pattern of the crops grown, and any anticipated weather stresses.

Some special varieties of crops are beginning to be grown for specific grain quality or nutritional components which may affect the nutrient uptake and removal by the crop. These varieties may also require adjustment of the nutrient management plan when they are used in the rotation.

Nutrient management plans based on detailed information about the soil, the crops grown, and the climatic factors encountered can improve nutrient efficiency and increase yields and profits. When these factors are considered on a site-specific basis accounting for variability within the field, there are even more potential gains in efficiency. In this case, the variability in all crops in the rotation should be considered in formulating the plan. Newly developed geographic information system (GIS) data management tools make this kind of detailed analysis and planning practical.