Fertilizing alfalfa can be uniquely challenging because it is a perennial crop. In addition, high-yielding alfalfa removes a tremendous amount of soil nutrients when compared to other crops grown. A thorough understanding of alfalfa’s growth habits, nutrient requirements and the soil nutrient supply mechanisms for alfalfa is necessary to effectively manage fertilizer inputs and maximize profitability.

Fertilizing perennial crops
The goal of any fertilizer management program should be to maximize the profitability of the crop. Growers should be aware, however, maximum yield and maximum profit are seldom the same. Often, additional yield can be obtained with additional inputs, but the cost of these inputs may exceed the value of the additional yield. Consider the example of an alfalfa producer who could increase his alfalfa yield five additional bales per acre by adding 50 pounds of potassium fertilizer. The additional fertilizer would only be a wise decision if the value of the five bales exceeds the cost of the 50 pounds of fertilizer.

Perennial crops like alfalfa present an added challenge: one year’s productivity is not the only topic of concern, but also the overall longevity of the crop. Often, early management decisions will determine the number of years the stand will remain productive. Fertilizer decisions prior to planting are particularly of importance, since this is the only opportunity the producer will have to incorporate immobile soil nutrients like P and K. After planting, annual fertilizer applications can only be broadcast to the soil surface.

Alfalfa establishment
One of the most common mistakes I see producers making is not properly preparing for alfalfa establishment. Soil samples should be collected from prospective alfalfa fields at least the fall prior to planting. Actually it is much better to sample a year and a half (two falls) before establishing. Early sampling gives the producer time to correct nutrient deficiencies and make adjustments in soil pH. Soil pH is probably the most important soil test measurement; the solubility of all other plant nutrients is a function of pH. In addition to nutrient solubility, the survival of rhizobium (nitrogen-fixing) bacteria is dependent on soil pH.

If the soil pH is below 6.4, lime should be applied according to the soil test recommendation. Soil test recommendations may need to be adjusted depending on the quality of limestone that is available and the depth of mixing into the soil. Your local county Extension agent can help make the needed adjustments. Added lime should always be incorporated into the soil. If more than 4 tons of lime are required, the lime should be mixed into the soil by applying half of the recommended rate before plowing and the other half after plowing, followed by disking.

The reaction of limestone is not immediate, so if the soil is extremely acidic (pH 5.3 or lower), it is advisable to collect new soil samples in the spring to reassess pH and the feasibility of planting. If the spring pH is below 6.2, the producer has two options:

1. delay planting for one year to allow the limestone more time to react
2. apply 1 pound sodium molybdate (6.4 ounces of molybdenum) per acre

Uniform distribution of sodium molybdate can only be achieved by mixing it in 20 to 40 gallons of water, and spraying uniformly per acre followed by disking it into the soil. It is important not to add more than 2 pounds of sodium molybdate per acre during a given five-year period of time.

A productive stand of alfalfa removes many pounds of phosphorus (P) and potassium (K) per acre per year. The goal of your P and K fertilizer program should be to maintain soil test P at or above 60 pounds per acre and soil test K at or above 300 pounds per acre. Because of this goal, it is imperative to have soil test values at or above these levels at planting. If fall soil analysis shows low or very low levels of P or K, the producer should apply the recommended fertilizer in the fall, then resample in the spring and apply additional fertilizer if recommended.

Phosphorus and K are very immobile in the soil, so incorporation of these elements is also beneficial. Once the alfalfa is established, rhizobium bacteria living on the roots will supply all of the nitrogen (N) required by the crop. However, research has shown that 30 pounds N per acre applied at planting is needed for crop establishment.

Fertilizing established stands Adequately fertilized alfalfa removes approximately 14 pounds P2O5 and 55 pounds K2O per harvested ton. This is a much higher nutrient removal rate than many grain crops or pastures. However, nutrient removal rates alone cannot be used as the basis for fertilizer recommendations. Soils contain a large amount of nutrients in primary and secondary minerals. The nutrients in these minerals are not extracted by the soil test solution but are slowly available to the plant.

Additional nutrients can be taken up from the subsoil and are also not measured in a routine 6-inch soil sample. For these reasons, soil test results are not an actual measurement of plant-available nutrients, but only give an indication of the amount of fertilizer that is required for maximum productivity. For these reasons, soil tests have been calibrated based on alfalfa growth and yield.

When soil test levels are above 50 pounds P per acre and 300 pounds K per acre, research studies show yield is not limited by these nutrients. In order to assure that yield is not limiting, we continue to recommend some fertilizer until soil test values (for the composite field sample) are 60 pounds P per acre and 450 pounds K per acre. Unfortunately, not all soils have the same ability to supply nutrients (especially K) to the crop.

In order to accurately monitor soil test levels for crops like alfalfa, I believe it is important for alfalfa producers to collect soil samples annually. With good annual sampling procedures, the producer will be able to make changes (if necessary) to the fertilizer recommendations. These changes should be made when a clear downward or upward trend is observed.

Avoid luxury K consumption Luxury consumption of K is a phenomenon that should concern all forage producers. Simply put, luxury consumption occurs when a plant is supplied with more than adequate amounts of K. When an alfalfa plant is adequately fertilized, the K concentration in the tissue is usually between 2.5 and 3 percent. If the soil supply of K exceeds the needs of the crop, higher tissue K concentrations can be as high as 4.5 percent.

For our grain crops like corn or wheat, luxury consumption is not a problem since excessive K in the tissue is returned to the soil with the fodder. For alfalfa (and any other crop where the entire plant is harvested), the excessive K is removed and is not recycled in the soil. Luxury consumption can increase K removal rates to 90 pounds K2O per ton (55 pounds is normal removal).

Probably the main drawback to luxury consumption is that fertilizer applied to increase soil test K can be immediately removed with the first harvest, leaving low soil test K and possibly causing future K deficiency.

There are several ways to limit the risk of luxury consumption. First, try to maintain soil test K near 300 pounds per acre, thus limiting the supply of K to the plant. At this soil test level, only small rates of K fertilizer will be recommended. Second, avoid applying any K fertilizer between the last fall harvest and the first cutting the following spring. The freezing and thawing of the soil through winter months usually releases enough K to supply crop needs for the first cutting. The first cutting is also the highest yielding cutting, so more hay harvested with a higher K concentration equals more pounds of K removed from the soil.

If soil test K is allowed to slip below the medium range, then split applications during the summer months should also reduce the effects of luxury consumption.

Conclusions
Properly managed alfalfa stands can provide profitable yields for six or more years, while poorly managed fields may not last more than three years. Understanding and managing the fertilizer input is one of the keys to alfalfa stand longevity.

It is critically important to have the appropriate soil pH, P and K levels prior to planting to obtain maximum plant density. Additionally, delaying nutrient application until after the crop has been established usually results in high fertilizer application rates, and increases the need for split application as well as the risk of luxury K consumption.

Because of the valuable nature of alfalfa and the high rate of nutrient removal, I recommend soil samples be collected in the fall of each year after establishment. Lime and phosphorus fertilizer can be added anytime, but K fertilizer should not be applied in the period after the last fall cutting to the first cutting in the spring. Taking these steps will maximize nutrient use efficiency and help to minimize the effects of rising fertilizer prices.  FG

—From 2006 Kentucky Alfalfa Conference Proceedings

Greg Schwab, Extension Soil Management Specialist, University of Kentucky