Alfalfa is a highly valued perennial forage crop, known for its high protein content and yield potential. Achieving the best results requires a precise nutrient management strategy tailored to specific field conditions. Because alfalfa harvests remove large quantities of nutrients, strategic fertilization is necessary to maintain stand health, longevity, and high productivity.
Soil Testing and pH Management
The foundation of any successful alfalfa fertilization program is a comprehensive soil test, conducted annually or biennially to accurately determine existing nutrient levels and soil acidity. Sampling the soil to a depth of six to eight inches provides the necessary data for informed application decisions.
Alfalfa is particularly sensitive to soil acidity, making pH management a crucial initial step. The crop performs optimally in a slightly acidic to neutral environment, targeting a pH range between 6.5 and 7.0. Low pH reduces the availability of nutrients, including phosphorus, and significantly impairs the activity of beneficial soil bacteria.
Correcting low soil pH involves applying agricultural lime, ideally six to twelve months prior to seeding. Liming raises the soil pH, creating a more favorable environment for the alfalfa plant and the Rhizobium bacteria responsible for nitrogen fixation. Dolomitic lime is recommended if the soil also tests low in magnesium.
Essential Macronutrients: Phosphorus and Potassium
Once the soil pH is corrected, the focus shifts to phosphorus (P) and potassium (K), the two macronutrients alfalfa demands in the largest quantities. An average ton of hay harvested removes approximately 15 pounds of phosphate (\(\text{P}_2\text{O}_5\)) and 60 pounds of potash (\(\text{K}_2\text{O}\)). This high removal rate necessitates annual maintenance applications to prevent nutrient depletion and yield decline.
Potassium is frequently the most limiting nutrient in established alfalfa fields. It plays a significant role in water use efficiency, photosynthesis, and regrowth rate. Adequate potassium levels are also linked to improved winter hardiness and stand longevity. Common sources include potassium chloride (muriate of potash, 0-0-60) and potassium sulfate (0-0-50).
Phosphorus is primarily involved in energy transfer, root development, and seedling establishment. Maintaining sufficient phosphorus promotes vigorous root systems and helps hasten plant maturity. While deficiencies can be subtle, appearing as stunted growth or dark green foliage, a lack of the nutrient severely limits productivity. Applying P and K based on soil test recommendations helps ensure the stand remains productive for several years.
Addressing Micronutrient Needs and the Role of Nitrogen
Beyond phosphorus and potassium, the two most commonly required secondary and micronutrients are sulfur (S) and boron (B). Sulfur is a component of amino acids and proteins, making it necessary for high-quality forage with high crude protein content. Alfalfa removes about five pounds of sulfur per ton of harvested hay, and deficiencies are becoming more common due to reduced atmospheric deposition.
Boron is involved in cell wall formation, sugar transport, and successful flowering and seed set. It should be monitored, especially on sandy soils or in high-pH environments. Boron is highly mobile, meaning it may need to be applied annually, often mixed with potassium fertilizer, at a rate typically ranging from one to three pounds per acre.
Alfalfa is a legume and, when properly inoculated with Rhizobium bacteria, fixes atmospheric nitrogen (\(\text{N}_2\)) into a usable form. This symbiotic relationship provides the plant with all the nitrogen it needs, making supplemental nitrogen fertilizer unnecessary on established stands. Applying nitrogen can inhibit the nitrogen-fixing bacteria and encourage competing weeds. A small amount of nitrogen (20 to 30 pounds per acre) may be beneficial only during the establishment year before root nodules are fully functional.
Timing and Methods of Fertilizer Application
The most effective strategy for applying maintenance fertilizer to established alfalfa is through annual broadcast application. Since phosphorus and potassium are relatively immobile, topdressing the fertilizer on the surface is an appropriate method. Deep placement or tillage is typically avoided to prevent damage to the plant crowns and deep taproots.
Optimal timing is usually after the first or second cutting during the growing season, or in the late fall or very early spring before growth begins. Split applications are often recommended, particularly for fields requiring large amounts of potassium. Applying half the recommended potassium after the first cutting and the remainder after a later cutting minimizes the risk of “luxury consumption.” This split timing ensures a continuous supply of nutrients, supporting rapid regrowth and maximizing annual yield.