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Leandro Vieira (Leo), Pereira Neto, Jose, Twidwell, Edward K.
Download Publication ID: 3950-E
Critical levels of plant nutrients are established to determine the possibility that applying a specific nutrient will enhance forage yield. In Louisiana, a nutrient reaches its critical level when its concentration in the soil is classified as the high range. When a soil test indicates nutrient levels in this range, adding more of that nutrient is unlikely to improve yield; in fact, 95% of the time, no yield increase is observed. This means only a 5% chance that additional application will have a positive effect. Therefore, when soil nutrient levels are already high, further fertilizer application is not recommended.
On the other hand, soils testing low to very low in nutrient concentration are much more likely to respond positively to fertilization. The probability of a yield increase is approximately 100% for very low levels, 80% for low, 10%–50% for medium, 5% for high, and 0% for very high nutrient levels (Figure 1). These critical levels are determined through a process known as soil test calibration, which begins with greenhouse trials and is validated through a series of field experiments across the state’s most prevalent soil types.
When it comes to K fertilization in hayfields, there is an exception. There will be recommendations for potassium application for soils at the high nutrient level. Unlike grazed pastures, hayfields experience a greater removal of potassium due to the multiple complete removals of the aboveground biomass of the forage from the field. Consequently, soils that already exhibit high levels of K2O still require additional potassium fertilization to maintain optimal nutrient balance and support healthy crop growth.
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.
CritNitrogen is normally the most important and yield-limiting macronutrient for forages. It plays a vital role in plant growth, as it is a fundamental building block of all amino acids and nucleic acids. Nitrogen is also a key element in chlorophyll and energy transfer molecules, making it essential for photosynthesis and overall plant metabolism. Due to its critical functions, plants demand more N than any other nutrient.
In Louisiana, N recommendations are determined based on the plant-specific requirements and its yield potential, rather than the soil’s existing N concentration (Table 1). Because nitrogen is susceptible to losses through leaching, volatilization, and denitrification, split applications are strongly recommended to optimize nitrogen use efficiency and maximize yield potential.
Refer to PDF to access Table 1.
Phosphorus is considered the second most important and yield-limiting macronutrient for forage crops. It plays a vital role in plant energy metabolism as a key component of ATP (adenosine triphosphate), the primary energy carrier in cellular processes. In Louisiana, P fertilizer recommendations are based on soil P concentrations measured using the Mehlich-3 extraction method (Mehlich, 1984). For all forage crops, the critical soil P level is set at 35 ppm (Table 2). This means that P should be applied when soil test results show levels below this threshold. Soil test reports express P levels in the form of phosphorus pentoxide (P₂O₅), which contains 44% elemental P. However, no conversion is necessary when calculating application rates, as fertilizer labels list P content as a percentage of P₂O₅.
Phosphorus is absorbed by plant roots primarily through diffusion, and its movement in the soil is relatively limited, which can affect its availability to plants. Therefore, P fertilization should be performed 100% preplanting and incorporated when establishing a forage (except for hayfields). In the case of hayfields, P rates should be split into two or three applications. If splitting into two applications, apply one-third of the recommended P₂O₅ at the beginning of the growing season, one-third after the first cutting and one-third after the second cutting. Alternatively, apply one-half of the recommended P₂O₅ at the beginning of the growing season and one-half after the first cutting. A single application of a high rate of P₂O₅ is not recommended because it can cause excessive nutrient losses.
For established pastures, it is not recommended to incorporate fertilizers, as it will damage the stand and favor the emergence of weeds. Therefore, for established pastures, the fertilizer can be broadcasted only. For cool-season forage crops (e.g., tall fescue and ryegrass), P fertilization should be done preferably in the fall. For warm-season forage crops (e.g., millet and bermudagrass), P fertilization should be done preferably in the spring.
Refer to PDF to access Table 2.
Potassium is generally considered the third most important and frequently yield-limiting macronutrient for forage crops. It plays a crucial role in regulating the movement of water, nutrients and carbohydrates within plant tissues, and is key to enhancing plants’ tolerance to abiotic stresses such as drought and cold temperatures. In Louisiana, K fertilizer recommendations are based on soil K levels determined using the Mehlich-3 extraction method (Mehlich, 1984). Unlike P, K recommendations are more nuanced, as the critical K level varies depending on the specific crop and soil texture (Table 3). Soil test reports express K levels in the form of potassium oxide (K₂O), which contains 83% elemental K. Similar to P, no conversion is necessary since fertilizer labels list K content as a percentage of K₂O.
Plant roots primarily absorb potassium through diffusion, which, like P, has limited mobility in the soil, especially in heavier, fine-textured soils. Thus, K fertilization should be performed 100% preplanting and incorporated when establishing a forage (except for hayfields). In the case of hayfields, K rates should be split into two or three applications. If splitting into two applications, apply one-third of the recommended K2O at the beginning of the growing season, one-third after the first cutting and one-third after the second cutting. Alternatively, apply one-half of the recommended K2O at the beginning of the growing season and one-half after the first cutting. A single application of a high rate of K₂O is not recommended because it can cause excessive nutrient losses and/or plant injury due to excess salt in the soil. In areas where substantial quantities of hay are harvested and soil tests are not conducted yearly, it is advised to apply around 40 pounds of K₂O for every ton of hay removed in the previous season.
For established pastures, it is not recommended to incorporate fertilizers, as it will damage the stand and favor the emergence of weeds. Therefore, for established pastures, the fertilizer can be broadcasted only at the beginning of the growing season. For cool-season forage crops (e.g., tall fescue and ryegrass), K fertilization should be done preferably in the fall. For warm-season forage crops (e.g., millet and bermudagrass), K fertilization should be done preferably in the spring.
Refer to PDF to access Table 3.
Sulfur is also among the most important and potentially yield-limiting macronutrients for forage crops. It is vital for the synthesis of certain amino acids and is required for chlorophyll production, even though it is not a direct component of the chlorophyll molecule. In Louisiana, S fertilizer recommendations are based on soil S concentrations determined using the Mehlich-3 extraction method (Mehlich, 1984). For all forage crops, the critical soil S level is 12 ppm (Table 4), meaning sulfur should be applied when soil test results fall below this threshold. The only exception is the case of high-yielding hay, where S should be applied at a rate of 6 to 12 pounds per acre per cut, regardless of the soil test level.
Sulfur is absorbed by plant roots through both mass flow and diffusion. However, due to its relatively high mobility in the soil, it is susceptible to losses via leaching and surface runoff. Except for already established pastures, S fertilization should be performed 100% preplanting and incorporated. For established pastures, it is not recommended to incorporate fertilizers as it will damage the stand and favor the emergence of weeds. Therefore, for established pastures, the fertilizer can be broadcasted only. For cool-season forage crops (e.g., tall fescue and ryegrass), S fertilization should be done preferably in the fall. For the warm-season forage crops (e.g., millet and bermudagrass), S fertilization should be done preferably in the spring.
Refer to PDF to access Table 3.
P3950E_FertilizerRecommendationsforPasturesinLouisiana_AG0625pdf / 0.60MB