Linda Benedict, Henderson, Rodney E., Tubana, Brenda S., Wang, Jim Jian
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:
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.
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.
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.
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.
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.