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 2001, No. 8

Delineation of each management zone in a field is key to proper analysis of a field and for best determination of crop nutrient needs. These zones begin to take shape with the overlay of basic data sheets associated with soil type, cation exchange capacity, previous crop yield, previous soil analysis data, organic matter, topography, and other such information. Also, a given zone is often resized as new information becomes available that better defines the land’s potential to produce.

With time and experience, management zones can be defined for different variable rate inputs such as lime, nitrogen, phosphorus, and potassium. Other variable rate inputs include seed, crop protection products, irrigation water, etc.

Crop and soil management specialists suggest the following guidelines as management zones are developed for improved fertilizer use effectiveness, crop yield improvement, and profitability.

• Soil analysis data provide a basis for establishing site-specific crop nutrient needs. The limiting factor continues to be how well the soil sample represents the variability in the zone. Research shows that a soil sample should be composed of at least 15 to 20 cores from the soil in a zone.
• Topography of a field can help to identify zones of higher crop response to applied fertilizer. Low nutrient testing, eroded knolls are often less responsive than zones of lower elevation. These higher zones are often characterized as being lower in water holding capacity, lower in organic matter, and having a higher bulk density and other yield limiting factors that prevent crops from best utilizing applied fertilizer.
• One of the main needs for zone management is the ability to locate the specific zones in a field and for the zones to be of sufficient size to justify change in rate of nutrient application.
• Management zones are not static and must be analyzed and adjusted based on experience and growth of the data base.
• Zones with high yield soil characteristics provide opportunities for upward adjustments in nutrients to attain increased crop yield and profitability.
• Zones allow site-specific management of nutrients such as nitrogen, phosphorus and potassium to be applied at rates needed for optimum development of the zone’s potential for crop yield.
• Profitability resulting from fertilization by management zones and the use of variable rate fertilization of specific nutrients improves with experience and fine tuning of the technology and knowledge base.