Highlights

This fact sheet provides nitrogen (N), phosphorus (P), potassium (K) and sulfur (S) recommendations for the following crops: corn, cotton, oat, rice, rye, sorghum, soybean, sugarcane and wheat.

No increase in yield is expected when fertilizer is applied to soil with nutrient concentrations above the critical level (optimum range).

The highest benefit of fertilizer application in crop yield is likely to occur in soils exhibiting low to very low nutrient levels.

In older soil test reports from the LSU AgCenter Soil Testing and Plant Analysis Laboratory, high nutrient levels indicate optimum levels and very high nutrient levels indicate above optimum levels according to recommendations developed for Louisiana.

Nutrient Critical Levels

Critical levels of plant nutrients are determined to assess the extent to which adding a nutrient will increase crop yield. In Louisiana, the critical level of a nutrient is defined as its concentration in the soil being in the optimal range (previously named as high). When a soil test indicates optimum levels of a nutrient, applying more of that nutrient will not increase yield in 95% of the cases. In other words, if a soil already has an optimum level of a nutrient, there is only a 5% chance that additional application will boost yield. Therefore, for soils with nutrient concentrations in the optimum and above optimum ranges, no application of fertilizer containing that nutrient is recommended.

Conversely, soils with low to very low levels of a nutrient will almost certainly show an increase in crop yield when that nutrient is applied. The probability of increasing crop yield is approximately 100%, 80%, 50%, 5% and 0% for very low, low, medium, optimum (previously named as high), and above optimum (previously named as very high) nutrient levels, respectively (Figure 1).

Soil tests are calibrated through a series of experiments. This process typically begins with greenhouse studies and is further refined through multiple field experiments conducted on the most common soil types in the state. The recommendations in this publication come from the nutrient management publications published by LSU AgCenter scientists and the Soil Testing and Plant Analysis Laboratory Database.

Figure 1. Illustration of expected crop response to fertilization (%) as a function of soil test nutrient levels. Illustration by Leandro Vieira

Nitrogen

Nitrogen is likely the most important and yield-limiting macronutrient for field crops. This nutrient is essential to plants because it is a key component of amino acids, nucleic acids, chlorophyll molecules and energy transfer compounds. As a result, plants require a higher quantity of N than any other nutrient.

In Louisiana, N recommendations are based on the specific needs of the crop and yield potential rather than its concentration in the soil (Table 1). Nitrogen can be lost through various processes including leaching, volatilization and denitrification. Therefore, to maximize yield potential, splitting N applications between different timepoints is highly recommended.

Phosphorus

Phosphorus (P) is likely the second most crucial and yield-limiting macronutrient for field crops. It is a component of ATP (adenosine triphosphate), which is the energy currency for cellular processes. In Louisiana, P recommendations are based on P concentration in the soil extracted through the Mehlich-3 methodology (Mehlich, 1984). For most field crops, the critical P level is 35 ppm (Table 2). In other words, P should be applied to the soil when the soil test report shows values lower than 35 ppm. The only exception is sugarcane, in which no increase in yield is expected from additional P application when soil test report shows values higher than 10 ppm.

The soil test reports present the Mehlich-3 P concentration in the empirical form of phosphorus pentoxide (P₂O₅). Phosphorus pentoxide contains 44% of P. However, there is no need for conversion, as fertilizer bags report percentage of P₂O₅ in their labels, rather than the percentage of P.

Phosphorus is taken up by the roots through diffusion and the nutrient can exhibit relatively limited mobility. Therefore, all P should be added in the spring before planting, then ripped or tilled into the soil. The exceptions to this are crops like oat, rye and wheat which are planted in the winter.

Potassium

Potassium (K) is typically regarded as the third most important and most yield-limiting macronutrient for field crops. It regulates water, nutrients and carbohydrates in the plant tissue. The regulations of these functions play a significant role in improving plant resiliency to abiotic stresses, such as water deficit and low temperatures. In Louisiana, K recommendations are based on K concentration in the soil which is extracted through the Mehlich-3 methodology (Mehlich, 1984). Unlike P recommendations, K recommendations are more complex as the K critical level depends on the crop of interest and soil texture (Table 3). Recent research conducted by Rasel Parvej, former LSU AgCenter soil fertility specialist, has shown that fertilizer recommendations for corn, cotton and soybeans could be simplified. For this reason, a K critical level of 150 ppm was established for corn, cotton and soybean, regardless of soil texture and soil type.

Soil test reports present the Mehlich-3 K concentration in the form of potassium oxide (K₂O). Potassium oxide is composed of 83% K. However, there is no need for conversion, as fertilizer bags report the percentage of K₂O in their labels, rather than the percentage of K.

Potassium is also taken up by the roots through diffusion and can exhibit relatively limited mobility, especially in heavier soils. Thus, all K fertilization should be performed in the spring before planting, then ripped or tilled into the soil. The exceptions to this are crops like oat, rye and wheat which are planted in the winter.

Sulfur

Sulfur is also one of the most important and yield-limiting macronutrients for field crops. Sulfur is essential for the synthesis of some amino acids, and it is required for the synthesis of chlorophyll, despite not being a constituent of the molecule. In Louisiana, S recommendations are based on S concentration in the soil extracted through the Mehlich-3 methodology (Mehlich, 1984). The critical S level ranges from 12 ppm to 20 ppm, depending on the crop of interest (Table 4). Therefore, sulfur should be applied to the soil when the soil test report shows values below the critical level for these specific crops.

Sulfur is taken up by the roots through mass flow and diffusion but due to its increased mobility in the soil, it can be lost through leaching and runoff. Thus, S fertilization should be performed 100% preplanting and incorporated. In the case of wheat, it is advised to split the S application, applying at preplant and with the first spring N application. Except for the crops of oat, rye and wheat, S fertilization should be done preferably in the spring.

For detailed tables and more information, please see PDF.