Jim J. Wang, Brenda S. Tubaña, J Stevens and Rodney Henderson

Soil testing is critical to resource management. It provides guidelines for the efficient use of lime and fertilizer materials in crop production. A soil test is developed based on strong correlations between a nutrient’s rapid chemical extraction from soil and its uptake by a plant. Lime and fertilizer recommendations are then based on calibration of this test against crop yields through field trials.

The soil test results report, such as provided by the LSU AgCenter Soil Testing and Plant Analysis Laboratory, gives the levels of the various nutrients found in the soil sample. Interpretation of the soil test requires knowing the relationship between the amount of a nutrient extracted by a given soil test and the expected crop response. Soil test calibration experiments are performed to determine the degree of limitation to crop growth or the probability of getting a growth response to an applied nutrient at a given soil test level.

Even though the exact relationship between a soil test level and yield will vary considerably, the general shape of this relationship is relatively consistent. At low levels of extractable nutrients, the yield is limited by a lack of the nutrient. As the soil test level increases, yield increases until a critical level. For example, for phosphorus the critical level is 35 ppm (Figure 1). Above this level there is no longer a relationship between the extractable amount of the nutrient and yield. At very high soil test levels, the yield may actually decline if additional nutrients are applied.

In the southeast region of the United States, most soil testing interpretations are based on a general rating scale as follows:

Rating         Interpretation     

Very low	Less than 50% of a crop yield potential is expected without the addition of the nutrient. Yield increase to the added nutrient is always expected. A large portion of the crop nutrition must come from fertilization.
Low	50% to 75% of the crop yield potential is expected without the addition of the nutrient. Yield increase to added nutrient is expected. A portion of the crop nutrient requirement must come from fertilization.
Meduim	75% to 100% of the crop yield potential is expected without the addition of the nutrient. Yield increase to added nutrient is expected. A small portion of the crop nutrient requirement must come from fertilization.
High	Yield increase to the added nutrient is not expected. The soil can supply much more than the entire crop nutrient requirement. No additional fertilizer is needed.
Very High	Yield increase to the added nutrient is not expected. The soil can supply much more than the entire crop nutrient requirement. Additional fertilizer should not be added to avoid nutritional problems and adverse environmental consequences.

These ratings have traditionally worked well in most situations. However, as new crop varieties are developed, improvements in soil testing methods and calibrations are necessary. LSU AgCenter scientists have been working to improve efficiency efficiency and accuracy of soil testing through updating testing methods and recommendations based on in-season calibrations. In addition, great efforts have been made to boost soil testing for home consumers by simplifying interpretations for their landscapes and gardens.

While soil tests are successful in most cases, sometimes they fail. For these instances, soil test levels indicate the deficiency of a nutrient, but application of a fertilizer amendment shows no crop response in improving the yields. This has happened in soils for sugarcane production, especially with potassium. Research at the LSU AgCenter has identified the cause for the inability of a rapid soil test in quantifying the fraction of potassium that is able to be slowly released by soil constituents during the growing season. Through the use of a procedure called quantity-intensity relationship, LSU AgCenter scientists have determined the nonexchanging fraction of potassium in certain Louisiana soils. Norwood silt loam soil releases 1.17 units of nonexchangeable potassium per change of plant-available exchangeable potassium as compared to 0.71 unit for Crowley silt loam soil. The low release of nonexchangeable potassium in the Crowley soil suggests its strong requirement of nutrient intensity for crop growth and the likelihood of plant response to potassium. This information improves the accuracy of soil testing interpretation and is important for managing the efficient use of fertilizers for sugarcane production.

Jim J. Wang is a professor, Brenda S. Tubaña is an associate professor, and Rodney Henderson is an instructor in the School of Plant, Environmental & Soil Sciences. J Stevens is an extension specialist at the Dean Lee Research and Extension Center in Alexandria.

(This article was published in the spring 2013 issue of Louisiana Agriculture magazine.)