Forage Facts

March 2003

In This Issue . . .

     - Fertilizing Alfalfa

     - Visual symptoms of nutrient deficiencies

     - Pounds of nutrients removed/ton alfalfa

    - Links to alfalfa variety results

    - Guidelines for annual topdress application

A well managed fertilizer program is a key ingredient in the efficient and profitable production of alfalfa. With high production costs and low profit margins, proper fertilizer application is more important than ever. 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.

Alfalfa has long been noted as a soil builder. Its ability to fix atmospheric nitrogen and improve soil structure is well known. However, alfalfa can rapidly deplete the soil of other nutrients, as shown below.

There are three ways to determine whether or not the soil has sufficient nutrients for the plant: soil testing, interpretation of visual symptoms of the plant, and plant tissue analysis.

Soil testing is the most convenient and economical method of evaluating the fertility level of the soil. It is the only diagnostic method that predicts nutrient needs before the crop is planted. Soil should be sampled to a depth of 6 inches for the most accurate readings.

Visual deficiency symptoms will reveal nutrient problems that occur during the growing season. By the time symptoms appear, however, the problem may be severe, and significant yield losses may have been experienced. 

The visual symptoms of nutrient deficiencies are detailed below.

Tissue analysis can determine the nutritional status of your crop possibly before any visual symptoms appear. For some nutrients such as sulfur and many of the micronutrients, this is the best method of determining fertilization needs. For a more comprehensive nutrient management interpretation, include a soil sample with the tissue sample.

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.

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.

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).

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.

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.

In some areas, the rates for phosphate and potash use 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. 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.

When needed, boron 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.

Guidelines for annual topdress applications: 

1. Fertilize immediately after harvest and before regrowth occurs. Avoid fertilizer contact with wet foliage. 

2. Topdress following first cutting to stimulate second and third cutting regrowth or topdress in early September to increase winter hardiness. 

3. Avoid topdressing plants when soils are soft (such as early spring) as physical damage to the alfalfa crown is likely. 

4. Split the application to avoid salt damage if using more than 500 lb/a of fertilizer material (irrespective of grade) in any year. 

5. Base fertilizer purchases on cost per unit of plant food provided and need for all nutrients contained in the fertilizer. For example, since there is not difference in nutrient availability with red versus white potash or with ortho- versus poly- phosphate on most soils, the best choice is the least expensive product per unit of plant food provided. 

6. Do not use foliar treatments to apply moderate to high rates of macronutrients as salt damage may occur and nutrient uptake is not enhanced. Foliar treatments are, however, an excellent method for applying micronutrients.