Brenda Tubaña, Jim Wang, J Cheston Stevens and Donald J. Boquet

For crops to grow, essential plant nutrients must be available in sufficient and correct proportions in the soil. Nutrients that crops need in the largest amounts, such as nitrogen, phosphorus and potassium, are the most limiting and often deficient in most crop production systems. Soil testing is a tool to obtain information about nutrient-supplying capacity of a soil and to provide a basis for deriving fertilizer rate recommendations.

Applying fertilizer in either insufficient or excessive amounts may result in reduced yields. An insufficient supply of essential nutrients directly inhibits normal growth and development of a crop and reduces yields. On the other hand, applying fertilizer in excessive amounts may cause a deficiency of another nutrient, too much vegetative production or increased incidences of pests or diseases, all of which result in reduced yields. In addition, applying too much fertilizer could reduce net income because of unnecessary costs not only of fertilizer but also weed and insect control.

Adopting effective soil test-based fertilization guidelines is among the best management practices producers can employ to ensure high yields. The existing soil-test-based fertilizer guidelines used by the LSU AgCenter Soil Testing and Plant Analysis Laboratory were established decades ago. Therefore, LSU AgCenter scientists are testing the validity of these fertilizer recommendation guidelines through correlation and calibration studies.

Correlation research establishes soil-critical levels below which a nutrient element is deemed deficient for optimum crop production, whereas calibration research optimizes the rate of fertilizer based on a specific source. For the past several years, LSU AgCenter scientists have focused on improving phosphorus fertilizer recommendations by revising the soil testing protocol.

Research trials were conducted at different sites in Louisiana from 2004-2010. Multiple soybean plots were established with different rates of phosphorus fertilizer. At harvest, yield results and soil samples were collected, processed and analyzed to determine the relationship among fertilizer rates, soil test values and yield. Results showed that for the new protocol, the critical soil test phosphorus level for soybean is 35 ppm (Figure 1). This represents the point beyond which there is no significant increase in soybean yield. In other words, when a soil is testing above 35 ppm, there is little or no chance of increasing yields from applying additional phosphorus fertilizer. On the other hand, applying phosphorus to soil testing less than 35 ppm phosphorus will likely increase yields. The soil testing lab uses a simplified soil rating of very low, low, medium and high based on soil test levels and on the probability soybeans will respond to phosphorus fertilization.
 
Phosphorus response studies conducted in Winnsboro in 2007 and at Ben Hur Farm in Baton Rouge in 2008 demonstrate the benefit of phosphorus fertilization to soybeans grown on soils that tested low and medium for phosphorus (Figure 2). A five-bushel-per-acre increase in yield was recorded with a 50-pound-per-acre application rate in Winnsboro when the soil tested "low." A similar increase was recorded for Ben Hur Farm when the soil tested "medium" with an application rate of 25 pounds per acre.

Several other locations showed no response to phosphorus fertilization even when the soils tested low to medium for phosphorus. This absence of response is attributed to other growth-limiting factors during the critical growth period. Further investigations of the complexities in soil nutrient dynamics may explain other differences in the properties of Louisiana soils. Findings from these correlation/calibration field studies also have provided information to update the recommended phosphorus application rates for soybeans at soil-test levels of very low, low, medium and high.

In Louisiana continuous soil testing and calibration research is necessary to regularly update fertilizer recommendation guidelines for phosphorus and other nutrients. This is necessary to keep pace with today’s high-yielding crop production systems brought about by superior crop varieties, new production technologies and more effective production management practices. Moreover, it is important to develop diversified nutrient management plans to reflect the highly diverse nature of Louisiana soils.