Fertilizing alfalfa in Minnesota

A pH of 6.5 or higher is desired for optimum alfalfa yields. When lime is used to raise the soil pH to this level and above, alfalfa growth is improved because there is a more favorable environment for the growth and development of rhizobia bacteria. These bacteria allow the alfalfa crop to manufacture the nitrogen it needs from the nitrogen in the atmosphere.

The availability of phosphorus (P) is also affected by soil pH. Liming to a pH of 6.5 increases the availability of both soil and fertilizer P to plants.

Soils in Minnesota contain ample calcium (Ca) for crop growth. Liming materials are not used to supply Ca.

Most soils have optimum amounts of magnesium (Mg) for alfalfa production. If Mg is needed, the use of dolomitic lime will provide enough Mg for production.

The need for lime is not uniform across Minnesota and recommendations will vary. Analyzing a soil sample for pH and buffer pH is the only way to arrive at an accurate lime recommendation. Soils should be sampled to 6-8 inches. The recommendations will not be accurate if other sampling depths are used.

Consider the location of the field within the state when using the liming recommendations from the tables below.

In Minnesota, lime recommendations are given in pounds of ENP (Effective Neutralizing Power) per acre. The effectiveness of liming materials is reported as pounds of ENP per ton. To calculate the tons per acre of a liming material needed to raise the soil pH to 6.5, divide the recommended pounds of ENP per acre by the pounds of ENP per ton of material.

The approximate recommendations for the use of Ag lime (crushed limestone) can be used when soil testing laboratories report lime recommendations in terms of tons per acre instead of pounds of ENP per acre.

Lime should be applied several months before seeding for the best effect.

*Suggestions are approximate based on the average ENP value of Ag lime. An ENP of 1,000 lb. per ton is an average value for Ag lime (crushed limestone) in Minnesota.

*Suggestions are approximate based on the average ENP value of Ag lime. An ENP of 1,000 lb. per ton is an average value for Ag lime (crushed limestone) in Minnesota.

When needed, phosphate fertilizers can produce substantial increases in alfalfa yield. Phosphate fertilizer recommendations are based on a yield goal and the results of the analysis of a soil sample for phosphorus (P).

*Pounds of P₂O₅ suggested to apply per acre for alfalfa production based on either the Bray-P1 or Olsen soil methods test reported in parts per million (ppm). 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)

No phosphate fertilizer is suggested if the soil test for P is greater than 20 ppm (Bray) or 15 ppm (Olsen).

Potassium (K) may be the most limiting nutrient for alfalfa production in central, east-central, and southeastern Minnesota.

Potash fertilizer recommendations should be based on a realistic yield goal and the results of the analysis of a soil sample for K.

*Pounds K₂O/acre for alfalfa production in Minnesota based on K soil test reported in parts per million. 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)

Apply fertilizer annually, based on the results of a soil test, to produce high-yielding alfalfa.

In the year of establishment, broadcast and incorporate the suggested rates of phosphate and potash before seeding. These suggested rates should be adequate for the seeding year. Repeat the application that was used for the seeding year for the first full year of production.

Collect soil samples again in the fall of the first full year of production. The amounts of phosphate or potash needed for the second and third production years can be based on the results of this test.

Split applications

• Apply fertilizer in either spring or fall if soils are not sandy. Apply in spring  for sandy soil.
• Split applications for alfalfa are a good management practice. This is especially true if high rates of phosphate or potash fertilizer are needed.
• When using split applications, apply fertilizer in early spring and repeat after the first cutting.
• Split applications of K can also reduce the incidence of high K concentrations in forage, which can cause reduced forage quality for some classes of livestock.

Spreading small amounts of phosphate and potash

Some of the recommended rates for phosphate and potash are small. Most fertilizer spreaders cannot be adjusted to apply these low rates. In some situations, the recommended rate of phosphate can be blended with the recommended rate of potash and the mixture can then be spread with available equipment.

In other situations, broadcast applications of low rates of only phosphate or potash may be suggested. For these fields, it may be more practical to double the suggested broadcast rate and apply on alternate years.

Don't use of nitrogen (N) fertilizer when alfalfa is seeded in medium or fine-textured soils. In these situations, application of N fertilizer may reduce nodulation.

Small amounts of a N fertilizer may enhance establishment when alfalfa is seeded in a coarse-textured soil. The N rate should be held to 25 pounds per acre or less.

A small amount of N may be applied when alfalfa is seeded with a nurse or companion crop. This is especially true when soils are sandy. The suggested N rate for this planting situation is 30 pounds per acre. 

There is usually no benefit from topdressing fertilizer N to established stands unless there is firm evidence that nodulation is not present.

Many times, weeds and grasses appear as the alfalfa stand ages. The application of fertilizer N or manure will stimulate the growth of both. This could accelerate the disappearance of alfalfa from the stand.

Several research trials have clearly demonstrated that the use of sulfur (S) in a fertilizer program will increase the production of alfalfa grown on sandy soils. Recent research in Northeast Iowa has shown positive yield responses to S on soils with organic matter concentrations of less than 3.0% in the top 6-8 inches. These soils can be heavier textured and found on side slopes and eroded spots in fields.

In Minnesota, the soil test for S is only reliable for sandy soils. This soil test has no value for medium and fine textured soils. It is suggested that 15-25 pounds of sulfur per acre should be applied to medium and fine textured soils with organic matter concentrations less than 3.0% if the producer suspects a sulfur deficiency.

An application of 10-15 pounds of sulfur per acre may be necessary under high yielding alfalfa production systems when organic matter concentration is greater than 3.0%.

Sulfur is mobile in soils, especially sandy soils. When needed, this essential nutrient should be applied each year in early spring. The annual applications of S fit easily with annual applications of phosphate or potash.

In Minnesota, boron (B) is the only micronutrient that might be needed in a fertilizer program for alfalfa. Soils in Minnesota contain adequate amounts of copper, manganese, iron and zinc for optimum alfalfa production.

Soils that have either marginal or deficient levels of B are limited to the state's east-central and northeastern regions. A soil test for B is available, but this test is recommended for use only in the two areas just mentioned.

When needed, B fertilizers can be top-dressed to established stands. Because of the low rates of B needed, this nutrient should be broadcast with phosphate or potash fertilizers for best results.

Boron is also mobile in soils and should be applied each year. This nutrient should not be applied directly to actively growing green tissue because some serious plant injury could occur. Boron fertilizers should never be applied to germinating seed.

Table ALF-6.