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Fertilizing Alfalfa (05/07/20)

The updated Alfalfa Soil Fertility Requirements circular has been published on-line for about a year.

The updated Alfalfa Soil Fertility Requirements circular has been published on-line for about a year. You can find it at: or on my webpages under ‘Extension Publications’. The highlights are as follows:

Nitrogen is not necessary for alfalfa for establishment nor annually. Nitrogen application might result in nitrate accumulation in the forage, so even manure or compost should be applied moderately, with a forage nitrate test performed if areas of the field were over-applied. At establishment, the seed should be inoculated with Rhizobium meliloti. The inoculant bacteria will persist over the life of the stand.

Phosphorus and Potassium- North Dakota P rates are based on the Olson soil test. Establishment broadcast rates of P are critical, since removal rates of P per ton of hay can add up over years. Potassium recommendations for establishment are based on soil test and clay chemistry, which is estimated with the map available in the circular. A high smectite-illite clay ratio soil will require a greater K soil test than a lower ratio soil. For both P and K, fertilization in years after establishment should be applied after 1st cutting. The availability of P and K are higher early in the season, reducing the chance of a P and/or K response, and in some years frost heaving will expose alfalfa crowns above the soil surface, and a fertilizer application may damage stand when applied in early spring. After 1st cutting, crowns are protected and the alfalfa response to P and K is greater.

Sulfur and micronutrients- The sulfur (S) soil test is not diagnostic for sufficiency or deficiency in the region, so a S fertilization decision based on soil test will not be helpful. In years with high fall rainfall, normal to above average snowmelt and/or significant spring rains (like this year), S may be deficient. The problem would be greater in establishment years, because alfalfa roots very deep, and established stands have roots in soil layers that might contain gypsum or other S-containing minerals and overcome any early-season top-yellowing, whereas year-of-establishment alfalfa in sandier soil will not have that advantage. Other micronutrients, including zinc (Zn) and boron (B) have not been a problem in North Dakota, even in soils testing very low. Alfalfa can evidently extract what it requires from our soil regardless of what the soil test shows. There have been B responses in Minnesota on deep sandy, irrigated soils, but not in ND.

Soil pH- Alfalfa, more than any other major ND crop is sensitive to acid pH. The pH for maximum alfalfa production is 6.7, and a greater pH does not reduce production. Estimates for reduction with acid pH are about 7% reduction at pH 6, 28% reduction at pH 5.5, and 42% reduction at pH 5. It would be wise to zone-sample alfalfa fields for pH, and in long-term no-till fields, sample the 0-2 inch depth and the 2-6 inch depth separately. The amendment for increasing pH is some form of liming material, such as mined limestone/dolomite (there are no native sources in ND), municipal waste water lime, and sugar beet waste lime. The field should be limed, if necessary, before establishment. If the acid pH is at the 0-2 inch depth, surface application would be effective based on research to date and ongoing in North Dakota.


Dave Franzen

Extension Soil Specialist


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