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

Fall 2001, No. 8

Determining crop need for supplemental phosphorus has changed little during the era of environmental concern. Sources available and timing and method(s) of application have changed some and are under continued assessment … for their interaction with the environment and for their success in meeting crop needs for growth and quality. The management objective for phosphorus should be to establish a crop’s need and then deliver that need in an agronomic, economic, and environmentally sound manner. It can be done.

A first step is to properly sample and analyze the soil from each site-specific management zone in a field. The soil test becomes the foundation upon which the phosphorus recommendation is built. It provides a measure of phosphorus in the soil reservoir that is normally available to a growing plant. However, availability can be altered by several soil conditions and crop management practices. Following are a few points that deserve attention in the development of site-specific phosphorus recommendation for a crop.

• An increase in soil acidity can reduce the availability of phosphorus to the crop. Incorporate aglime, if possible, well ahead of the planting season to attain desired soil pH level.
• Root growth restrictions, such as soil compaction or injury from soil insects, can reduce root growth, root exploration of the soil, and crop uptake of phosphorus.
• Conservation or reduced tillage is an excellent practice, but it can result in an increase in soil acidity and an accumulation of phosphorus in the upper layer of soil. These conditions develop over time and can render surface soil phosphorus positionally unavailable during periods of crop drought stress.
• Early planting in heavy residue-covered soils often increases the need for supplemental phosphorus. This is due to the cool, moist soil environment around the seed.
• Nutrient requirements change with plant growth stage and with the level of production. These values are known for most crops. Phosphorus must be readily available to the growing crop from the soil reservoir and from supplemental phosphorus applied from fertilizer sources.