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Extension > Agriculture > Nutrient Management > Nutrient/Lime Guidelines > Fertilizing Alfalfa in Minnesota

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Fertilizing Alfalfa in Minnesota

Daniel E. Kaiser, John A. Lamb and Roger Eliason

Alfalfa is grown on approximately 1.1 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.

pH and liming

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.

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

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 6-8 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).Minnesota lime reference map

Figure 1. Reference map for lime suggestions

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 AG-FS-5957.

 

Table 1. Lime suggestions for mineral soils when soil pH is less than 6.0. The rates suggested should raise the pH to 6.5.
Buffer
Index
Area 1 Area 1
ENP AG Lime* ENP AG Lime*
  lb/acre ton/acre lb/acre ton/acre
6.8 3000 3.0 2000 2000
6.7 3500 3.5 2000 2000
6.6 4000 4.0 2000 2000
6.5 4500 4.5 2000 2000
6.4 5000 5.0 2500 2500
6.3 5500 5.5 2500 2500
6.2 6000 6.0 3000 3000
6.1 6500 6.5 3000 3000
6.0 7000 7.0 3500 3500
5.9 7500 7.5 3500 3500
5.8 8000 8.0 4000 4000
5.7 8500 8.5 4000 4000
5.6 9000 9.0 4000 4500

*These are approximate suggestions 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.

Table Table 2. Lime suggestions for mineral soils when the SIKORA BUFFER TEST IS NOT USED (soil pH is 6.0 or higher). The rates suggested should raise the pH to 6.5.
Soil-Water PH Area 1 Area 1
ENP AG Lime* ENP AG Lime*
  lb/acre ton/acre lb/acre ton/acre
6.5 0 0 0 0
6.4 2000 2.0 0 0
6.3 2000 2.0 0 0
6.2 3000 3.0 0 0
6.1 3000 3.0 0 0
6.0 3000 3.0 2000 2.0

*These are approximate suggestions 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.

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

Phosphate use

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.

Table 3. Phosphate suggestions for alfalfa production in Minnesota*.

YIELD
GOAL

BRAY:
OLSEN:
Phosphorus (P) Soil Test (ppm)
0-5 6-10 11-15 16-20 21+
0-3 4-7 8-11 12-15 16+
— ton/ac — — lb. P2O5 / acre to apply* —
3 or less 40 35 20 5 0
4 65 45 25 10 0
5 80 55 30 15 0
6 95 65 40 15 0
7 110 80 45 20 0
More than 7 125 90 55 25 0

*Use the following equations to calculate phosphate fertilizer suggestions for specific yield and specific soil test values for P:
P2O5 Suggested = [18.5 — (0.93)(Bray P Test, ppm](Yield Goal)
P2O5 Suggested = [18.57 — (1.16)(Olsen P Test, ppm](Yield Goal)
No phosphate fertilizer is suggested if the soil test for P is greater than 25 ppm (Bray) or 20 ppm (Olsen).

Potash suggestions

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.

Table 4. Potash suggestions for alfalfa production in Minnesota*.

YIELD
GOAL

Potassium (K) Soil Test (ppm)
0-40 41-80 81-120 121-160 161+
— ton/ac — — lb. K2O/ acre to apply* —
3 or less 145 100 55 10 0
4 190 130 70 10 0
5 240 165 90 15 0
6 290 195 105 15 0
7 335 230 125 20 0
More than 7 380 265 145 20 0

*Use the following equation to calculate potash fertilizer suggestions for specific yield goals and specific soil test values for K:
K2O Suggested = [55.7 – (0.38)(Soil Test K, ppm)](Yield Goal)

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. 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. The split applications of K can also reduce the incidence of high K concentrations in the forage that can cause reduced forage quality for some classes of livestock.

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.

Nitrogen use

The use of nitrogen (N) fertilizer is not recommended 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 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.

Sulfur needs

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 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 lb. S 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.

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.

Micronutrient needs

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 top-dressed 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.

Table 5. Sulfur suggestions for alfalfa production on sandy soils in Minnesota.

Soil test for S

Sulfur Recommendation
— ppm S — — S to apply (lb/ac —
0-6 25 (annual)
7/12 20-25 (annual)
More than 12 0

 

Table 6. Boron suggestions for alfalfa production in Minnesota.

B Soil Test

Relative Level Boron to Apply
— ppm B —   — lb./ac —
Less than 1.0 Low 2-4
1.1-5.0 Adequate 0
More than 5.0 High 0

See also Understanding Phosphorus in Minnesota Soils, Sulfur in Minnesota Soils, Liming materials for Minnesota soils, and Lime Needs in Minnesota.

Revised 2011

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