Fertilizing alfalfa in Minnesota
George Rehm, Michael Schmitt, and Robert Munter
Copyright © 2000 Regents of the University of Minnesota. All rights reserved.
Alfalfa is grown on approximately 2 million acres in Minnesota. It is a key component of farm enterprises that include dairy and/or beef animals. Alfalfa is also frequently grown as a cash crop being sold to a variety of users. A well managed fertilizer program is a key ingredient in the efficient and profitable production of this crop.
When alfalfa production is considered, major emphasis should be devoted to 1) the proper use of lime and 2) application of appropriate rates of phosphate, potash, sulfur, and boron.
pH and liming
Profitable alfalfa production starts with a consideration of soil pH and lime needs. 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 (N) that 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.
Determining the Need for Lime- 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 inches. The recommendations will not be accurate if other sampling depths are used.
Lime recommendations for alfalfa production are summarized in Tables 1 and 2. The location of the field within the state must also be considered when the recommendations in Tables 1 and 2 are used (see Figure 1).
Figure 1. Reference map for lime recommendations.
In Minnesota, lime recommendations are given in terms of pounds of ENP (Effective Neutralizing Power) per acre. Liming materials are analyzed and the results are reported as pounds of ENP per ton. With this information, it's easy to calculate the tons per acre of a liming material needed to raise the soil pH to 6.5. A more detailed discussion of ENP and the variety of liming materials available is provided in Extension Fact Sheet FS-5957.
The approximate recommendations for the use of ag lime (crushed limestone) are also listed in Tables 1 and 2. These suggestions can be used when soil testing laboratories report lime recommendations in terms of tons per acre instead of lb. ENP per acre.
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). These suggestions are summarized in Table 3.
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. The potash suggestions for alfalfa production in Minnesota are summarized in Table 4.
Phosphate and potash management
Annual applications of fertilizer, based on the results of a soil test, are suggested for the production of high-yielding alfalfa. In the year of establishment, the suggested rates of phosphate and/or potash should be broadcast and incorporated before seeding. These suggested rates should be adequate for the seeding year. For the first full year of production, repeat the application that was used for the seeding year.
Soil samples should be collected again in the fall of the first full year of production. The amounts of phosphate and/or potash needed for the second and third production years can be based on the results of this test.
Needed fertilizer can be applied in either spring or fall if soils are not sandy. Spring applications are suggested when soils are sandy. Sulfur (S) may be needed when soils are sandy, is mobile, and should not be applied in the fall. A soil test for S is suggested if soils are sandy. Split applications can be used for alfalfa and are considered to be a good management practice. This is especially true if high rates of phosphate and/or potash fertilizer are needed. If split applications are used, the fertilizer should be applied in early spring and repeated after the 1st cutting.
Some of the rates for phosphate and potash use listed in Tables 3 and 4 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.
The use of nitrogen (N) fertilizer is not recommended when alfalfa is seeded in medium or fine-textured soils. In these situations, application of a 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 lb./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 lb./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. Table 5 lists sulfur suggestions based on the results of a soil test for S.
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 and/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 (Cu), manganese (Mn), iron (Fe), and zinc (Zn) 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. The suggestions for use of B fertilizer are listed in Table 6.
When needed, B fertilizers can be topdressed to established stands. Because of the low rates of B needed, this nutrient should be broadcast with phosphate and/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.
Check your local county extension office for these publications:
FO-0792--Phosphorus for Minnesota Soils
FO-0794--Sulfur for Minnesota Soils
FS-5957--Liming Materials for Minnesota Soils
FO-5956--Lime Needs in Minnesota