Agri-Brief: Should You Fertilize Doublecrop Soybeans?

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

Summer 1996, No. 8

SHOULD YOU FERTILIZE DOUBLECROP SOYBEANS?

Each soybean plant functions just like a factory. It requires a minimum of raw materials to generate products, such as grain, at the desired level of yield and quality. Each nutrient...phosphorus, potassium, magnesium, sulfur, boron and others...represents a basic and essential raw material. Few soils are capable of providing these nutrients for an extended period of time without some resupply from fertilizer or other external nutrient sources.

Soils differ in their nutrient storage capacities and their abilities to supply soybean nutrient needs throughout the growing season. Such differences exist in each and every field and they influence the time, rate and method of application of nutrients for highest nutrient use effectiveness.

Soybean nutrient needs are site-specific. The reasons might be due to differences in the soil test level of a nutrient, grain yield goal, grain quality needs, crop nutrient uptake requirements, soil productivity level, rainfall amount and intensity, etc. Each nutrient must become a team player by fulfilling its own functions and then helping other production practices do their best. Optimum soybean yields and profits are most often the product of balanced nutrition in a system of best management practices. The following factors should be considered in the decision to fertilize doublecrop soybeans or to apply soybean nutrient needs to the previous crop, such as wheat.

Soil testing is a recommended practice which provides an estimate of response to an applied nutrient. Many specialists suggest that soil test levels should be in the medium to high range for most profitable yields.
Crop yield goal and research-based nutrient uptake values help to estimate the total quantity of each nutrient which must be available for crop uptake and use. For example, a 50-bushel-per-acre soybean yield will take up nearly 50 pounds of phosphate and over 185 pounds of potash.
Interactions among nutrients and between nutrients and other cropping practices are known. Balanced nutrition helps plants to better cope with stress brought on by disease, insect injury or adverse temperature/moisture relations.
Methods of nutrient application have a strong influence upon nutrient use effectiveness. The classic starter placement two inches below and two inches to the side of soybean seed can give optimum response from fertilizer phosphorus.
Previous crop nutrient use can influence doublecrop soybean nutrient needs. Soybeans, like peanuts, are good users of residual phosphorus and potassium left over from the previous crop. On heavier textured soils where nutrient leaching is unlikely, soybean nutrient needs can be met by applying phosphorus and potassium to the previous crop in rotation, such as wheat...as long as sufficient amounts are applied to meet the needs of both crops in the rotation. This practice elevates the cost of fertilizer for wheat, but it does allow the crop to better respond and benefit from optimum rainfall and other high yield practices and/or conditions.
On sandy, low cation exchange capacity soils of the Southern Coastal Plain, a split application of potassium might help avoid loss during intense rains and better provide for late-season nutrient needs. Late-season foliar application of nutrients such as boron can be beneficial when high yield potential for the crop exists. Fertilizing the previous crop in rotation can provide additional time for planting after wheat harvest.

Proper fertilizer management is essential to soybean production for many reasons. The basic objective is to assure that nutrition is not limiting yield, quality or profit.