Alfalfa fertilizer guidelines

An assessment of the need for lime is usually the first consideration for alfalfa production. When soils are acid (pH less than 7.0), optimum alfalfa yields are usually associated with a soil pH in the range of 6.5 to 7.0. There are no management practices that are economic that will decrease soil pH values in excess of 7.4. When soil pH values are calcareous (7.4 and higher), the best strategy is to concentrate on appropriate management of fertilizer. 

The soil test report form may include two pH. We determine soil pH by suspending soil in water and taking a reading. If the soil pH is less than 6.0, the sample is placed in a buffer solution and a reading is taken. This buffer pH value determines the rate of lime to apply.

Tables 1 and 2 list liming suggestions. Use Figure 1 to determine location of Area I and Area II. We analyze Minnesota liming for Effective Neutralizing Power (ENP) and report results as lb. ENP per ton of material. 

The soil test report form lists suggestions as lb. of ENP per acre. You can calculate rate of application (tons per acre) from these two pieces of information. Analysis of byproduct liming materials is not required; it is voluntary.

Table 1: Lime guidelines when the soil pH is less than 6.0

Guidelines are for mineral soils when the soil pH is less than 6.0. The rates suggested should raise the pH to 6.5. Lime guidelines are approximate guidelines based on the average ENP value of ag lime. An ENP of 1,000 pounds per ton is an average value for ag lime (crushed limestone) in Minnesota.

Table 2: Lime guidelines when the buffer test isn’t used

Guidelines for mineral soils when the buffer test is not used (Soil pH is 6.0 or higher). The rates suggested should raise the pH to 6.5. These are approximate guidelines based on the average ENP value of ag lime. An ENP of 1,000 pounds per ton is an average value for ag lime (crushed limestone) in Minnesota.

We do not recommend the use of nitrogen (N) when alfalfa is seeded in medium or fine-textured soils because it might reduce nodulation.  Small amounts of N fertilizer supplying about 25 lb. N per acre may enhance establishment when alfalfa is seeded in a coarse-textured soil. You can apply a small amount of N when alfalfa is seeded with a nurse or companion crop. This is especially true when soils are sandy. The suggested N rate for this nurse or companion crop situation is 30 lb. per acre.

We adjust suggestions for phosphate and potash for expected yield and the soil test values for phosphorus (P) and potassium (K). Tables 3 and 4 list the guidelines for phosphate and potash. These fertilizers can be top-dressed to established stands or broadcast and incorporated before planting. See FO-03814, Fertilizing Alfalfa in Minnesota for more detailed information about the management of phosphate and potash fertilizers.

Table 3: Phosphate fertilizer guidelines for alfalfa production in Minnesota

Use the following equations to calculate phosphate fertilizer guideline for specific expected yields and specific soil test values for P:

• Recommend P₂O₅ = [18.57 - (0.93) (Bray p), ppm] (Expected yield)
• Recommend P₂O₅ = [18.57 - (1.16) (Olsen P), ppm] (Expected yield)

Table 4: Potash fertilizer guidelines for alfalfa production in Minnesota

Use the following equation to calculate potash fertilizer guideline for specific expected yields and specific soil test values for K:

Recommended K₂O = [55.7 - (0.38) (K soil test), ppm] (Expected yield)

You may need sulfur (S) and boron (B) in a fertilizer program for alfalfa. Use of sulfur will probably increase alfalfa production if soils are sandy. However, use of S on fine-textured soils will probably not increase alfalfa yield unless soil organic matter in the top 6-8 inches is 3.0% or less. The soil test for sulfur is not accurate for fine-textured soils and is not recommended. We suggest an annual application of 25 lb. S per acre for sandy soils and 15-25 lb. S per acre in fine textured soils when soil organic matter is 3.0% or less.

When needed, boron can be top dressed to established stands (Table 5). Because of the low rates of B needed, you should blend the B fertilizer with phosphate and/or potash fertilizers and broadcast for best results. Soils in Minnesota contain adequate amounts of copper (Cu), manganese (Mn), iron (Fe), and zinc (Zn) for optimum alfalfa production and those four micronutrients should not be needed in a fertilizer program.