Growing Alfalfa crop nutrition advice

What soil type, pH and climate does alfalfa prefer?

Alfalfa prefers well-drained, slightly alkaline soils and thrives in semi-arid climates with warm summers, cool winters, and moderate temperatures during the growing season. Adequate moisture, either from rainfall or irrigation, is essential for alfalfa cultivation.

• Soil type: Alfalfa grows best in well-drained soils with good moisture-holding capacity. Sandy loam, loam, and silt loam soils are ideal for alfalfa cultivation. These soil types provide good aeration and drainage, which are crucial for alfalfa root development and overall plant health. Good soil drainage is essential to grow alfalfa. Farm management, especially mowing and other machinery operations, need to considered to reduce soil compaction.
• pH: Alfalfa thrives in slightly alkaline soils with a pH range between 6.5 and 7.5. Soil pH influences nutrient availability to the plant, and maintaining the pH within this range ensures optimal nutrient uptake by alfalfa. Soil pH outside this range can lead to nutrient deficiencies or toxicities, affecting alfalfa growth and productivity.
• Climate: Alfalfa thrives in semi-arid climates with warm summers, cool winters, and moderate temperatures during the growing season.
• Moisture: Adequate rainfall or irrigation is necessary to support alfalfa growth, but due to its deep root system, it also tolerates drought conditions well.

What nutrients are most important for alfalfa yield and quality?

The most limiting nutrients include nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), and certain micronutrients like boron (B), iron (Fe), manganese (Mn), zinc (Zn), and molybdenum (Mo).  Yield and quality are affected by deficiencies in essential nutrients, with deficiencies leading to stunted growth, reduced vigor, lower yields, and poor forage quality.  Keep in mind that alfalfa fertilization planning and management should consider both nutrient uptake and removal by the alfalfa crop and the nutrition required to promote optimal N fixation due to Rhizobia.

• Nitrogen (N): Essential for alfalfa growth, protein production, and chlorophyll synthesis. Deficiency leads to poor growth and yield, but alfalfa can fix atmospheric nitrogen with root bacteria, reducing the need for external fertilizers.
• Phosphorus (P): Critical for root development, energy transfer, and overall growth. Deficiency causes stunted growth, delayed maturity, and yield reduction, particularly during establishment.
• Potassium (K): Regulates water uptake, enhances stress tolerance, and improves disease resistance. Deficiency results in reduced vigor, increased susceptibility to pests and diseases, and poor yield and quality.
• Calcium (Ca) and Magnesium (Mg): Essential for cell wall formation, structure, and chlorophyll synthesis. Deficiency leads to poor growth, reduced vigor, and lower yields. Calcium improves nodulation of Rhizobium and sulfur supply is essential for effective N fixation.
• Sulfur (S): Alfalfa has a high sulfur requirement for protein synthesis, and sulfur deficiency can lead to reduced forage quality.  Required for protein synthesis, deficiency lowers protein levels and forage quality. Proper sulfur fertilization and monitoring are essential, especially in regions with low sulfur availability in soils. Sulfur fertility can promote increases in quality parameters like crude protein, relative feed quality, and milk per ton of hay.
• Micronutrients: While micronutrient deficiencies are less common in alfalfa, deficiencies in boron (B), iron (Fe), manganese (Mn), zinc (Zn), and molybdenum (Mo) can occur in certain soil conditions and affect alfalfa growth and yield. Regular soil testing and foliar application of micronutrients may be necessary to address deficiencies effectively.

Lime amendments in acidic soil should be considered as preparation before sowing alfalfa.

Nutrient requirements

Estimated nutrient uptake (lbs/ton):

Source: IPNI

Dynamic of nutrient uptake over a crop season in alfalfa

Crop nutrient requirements and the dynamics need to be considered when setting up the fertilization plan. Nutrient uptake in alfalfa occurs during the growing season, in several waves during between cuts. Fertilizers with prolonged availability such as Polysulphate, are of high value in this case.

N uptake does not need to be considered in the fertilization, plan, as alfalfa uptakes its N from the atmosphere, through the symbiosis with Rhizobium at the root zone. However, nutrient availability will determine the effective nodulation and N fixation by the Rhizobium and, hence, it needs to be considered. Depending on the environmental and soil conditions, nutrients may be limited at different moments of the growing season. For example, sulphur may be limited early in the season, due to leaching of sulphate beyond the rooting zone during the winter period.

Other nutrients, such as potassium, may be more limiting later in the season as the crop uptakes the soil reserves.

The total quantity of nutrient taken up by the crop depends on the number of cuts that the farming system allows and its productivity. Productivity is higher during the second and third year, with over 20 tons of dry matter in irrigated alfalfa of southern locations.

Role of nutrients

+ = improving; – = decreasing; +/- = different results, depending on the rate of nutrient applied

Source: Different sources and references in the literature

Nutrient deficiencies

Alfalfa plant on the right shows shorter leaves and reduced branching due to phosphorous deficient in comparison to a well-nourished plant. Source, Lissbrant et al., Purdue University Extension

Alfalfa crop showing some early symptoms of combined nutrient deficiencies, of potassium, sulphur and magnesium (irrigated alfalfa in north-east Spain)

What are the main nutrient deficiency symptoms in alfalfa?

1. Nitrogen (N):
   • Yellowing of older leaves (chlorosis) starting from the tips and progressing towards the base.
   • Reduced plant vigor and stunted growth.
   • Decreased leaf size and overall biomass.
   • Delayed flowering and reduced seed production.
2. Phosphorus (P):
   • Stunted growth, particularly noticeable during early growth stages.
   • Dark green leaves with reddish or purplish discoloration, especially on the undersides.
   • Reduced root development and poor root branching.
   • Delayed maturity and decreased seed yield.
3. Potassium (K):
   • Marginal scorching or burning of leaf edges (necrosis).
   • Leaf yellowing (chlorosis) starting from leaf margins and progressing inward.
   • Weak stems and lodging susceptibility.
   • Increased susceptibility to drought stress and diseases.
4. Calcium (Ca) and Magnesium (Mg):
   • Leaf margin necrosis or browning, particularly on younger leaves.
   • Interveinal chlorosis (yellowing between leaf veins) progressing to necrosis.
   • Poor stem and root development.
   • Reduced overall plant vigor and yield.
5. Sulfur (S):
   • Pale green or yellowing of young leaves.
   • Stunted growth and reduced leaf size.
   • Delayed flowering and poor seed development.
   • Decreased protein content and forage quality.
6. Micronutrients:
   • Specific symptoms vary depending on the nutrient deficiency:
     • Boron (B): Brittle or distorted leaves, death of growing points.
     • Iron (Fe): Interveinal chlorosis on younger leaves.
     • Manganese (Mn): Interveinal chlorosis similar to iron deficiency.
     • Zinc (Zn): Reduced leaf size, short internodes, leaf bronzing.
     • Molybdenum (Mo): Yellowing or rolling of leaf margins, reduced nitrogen fixation.

Recognizing these symptoms can help diagnose nutrient deficiencies in alfalfa and guide appropriate corrective measures, such as soil amendments or fertilization adjustments.

Common fertilization strategies for alfalfa

Alfalfa is typically harvested multiple times per growing season, often at early bloom or pre-bloom stages, to optimize forage quality. Nutrient management plays a critical role in achieving the highest yields on multiple cuts for alfalfa without depleting nutrient resourcse. With good fertilization practices, we can sustain growth and regrowth, promote forage quality, enhance stand persistence, and manage environmental stress.

Manure Application: Organic sources of nutrients, such as manure or compost, are applied to alfalfa fields as soil amendments. Manure can be spread or incorporated into the soil to supply nutrients and improve soil organic matter content. Our research has found that applying OMRI-approved Polysulphate to compost and manure allows growers to apply more nutrients in just one pass and provide more nutrient value in a gradual-release form.

Balanced Fertilization: Providing adequate soil fertility is crucial for maintaining alfalfa productivity and quality. Based on soil test results and crop requirements, farmers apply fertilizers containing nitrogen (N), phosphorus (P), potassium (K), sulfur (S), and micronutrients such as boron (B), iron (Fe), manganese (Mn), zinc (Zn), and molybdenum (Mo). Polysulphate in combination with MOP is an excellent source of K and S for season-long release of nutrients to replace those removed with each cutting. Foliar application of micronutrients to proactively address deficiencies or supplement soil-applied fertilizers is recommended.

Split Applications: split applications of nitrogen (N), phosphorus (P), potassium (K), sulfur (S), and micronutrient fertilizers may also be beneficial, depending on soil conditions, crop requirements, and management practices. Farmers can improve nutrient efficiency, reduce nutrient losses, and optimize alfalfa yield and quality by dividing nutrient applications into multiple doses. This is especially important to improve yield in 3rd and 4th cuts. In our research, split applications of S and K resulted in the greatest 4th-cutting yields for alfalfa.

pH Management: Liming is a common practice used to raise soil pH levels in acidic soils. Agricultural lime, typically in the form of calcium carbonate (CaCO3) or dolomitic lime (which also contains magnesium), is applied to alfalfa fields based on soil test recommendations. Since commonly used N and S sources like ammonium sulfate can acidify the soil, we recommend Polysulphate (0-0-14-12.2Ca-3.6Mg-19.2S) as an alternative. This mineral-based fertilizer delivers a sustained release of sulfate-S that does not acidify the soil. In high-pH environments, acidifying amendments may be applied to lower soil pH levels and increase nutrient availability for alfalfa. Agrolution pHLow and Nova PeKacid are examples of fertilizers that supply nutrition while also acidifying soil and water to promote compatibility and plant availability of Ca and P as well as other nutrients like micronutrients that benefit from a more acidic environment.

Nitrogen Management: While alfalfa has the ability to fix atmospheric nitrogen through symbiotic relationships with bacteria, supplemental nitrogen fertilization is sometimes used judiciously to enhance yield and quality. Split applications of nitrogen or use of legume-friendly nitrogen sources may be employed to avoid nitrogen leaching and excessive vegetative growth. Nova Pulse is a water-soluble foliar fertilizer originally designed for nitrogen-fixing legumes and pulse crops to support the biological N fixation (BNF) process.

What are common nutrient management challenges for alfalfa growers in the United States (USA)?