As we move into fall, alfalfa harvest is beginning to wind down for the year. Whether another cutting is still planned or the final crop is already off, this is a good time to take stock of stand fertility and start thinking about what that field needs for next year.
Alfalfa is a heavy user of nutrients. Nebraska Extension estimates that each ton of alfalfa hay removes about 10 pounds of phosphate, 60 pounds of potash, or K₂O, and 8 pounds of sulfur. Even at four tons of production per acre, that adds up quickly.
That doesn’t mean we should automatically replace every pound removed with fertilizer. UNL recommendations are based on correcting deficiencies, so soil tests, soil type, irrigation water, and other nutrient sources should guide applications. Let’s look at four things worth watching: phosphorus, potassium, sulfur, and soil pH.
Phosphorus is important for root growth, stand vigor, and maintaining production. For established stands, phosphorus can be broadcast in early spring or after the last cutting in fall when soil tests indicate a need. For example, an irrigated field testing in the low range, 6 to 15 ppm Bray-1 or Mehlich-3 phosphorus, would call for about 40 pounds of phosphate per acre annually. Once soil tests are above 25 ppm, no phosphorus is recommended.
Potassium also deserves attention. Alfalfa removes around 60 pounds of K₂O per ton of hay, but many Nebraska soils naturally supply enough potassium that fertilizer may not be needed. When deficiencies do occur, they are more common on coarse-textured or sandy soils.
Adequate potassium supports plant vigor, stress tolerance, and stand persistence. Fall after the final harvest is a good application window when soil tests show a deficiency. Don’t view potassium simply as a fall “winterizer,” though. Adding potassium where levels are already adequate may simply add cost without improving winter survival.
Sulfur is another nutrient worth watching, especially on sandy, low-organic-matter soils. When needed, UNL recommends about 30 to 40 pounds of sulfur per acre annually for irrigated alfalfa. However, irrigation water may already be supplying part or even all of that need.
A water test can tell you how much sulfate-sulfur is present. Each ppm of sulfate-S applied in one inch of irrigation water supplies about 0.23 pounds of sulfur per acre. So, water containing 10 ppm sulfate-S applied at 10 inches during the season would supply roughly 23 pounds of sulfur per acre. That amount should be credited before deciding how much additional sulfur fertilizer is needed.
Finally, pay attention to soil pH. Alfalfa performs best around neutral soil conditions, and low pH can limit nodulation, nutrient availability, and stand productivity. Lime can be surface-applied to an established stand, but it moves slowly and mainly affects the upper soil surface. The best opportunity to correct a major pH problem is still before the next alfalfa stand is planted, when lime can be incorporated.
As harvest winds down, think of fall fertility as a chance to evaluate the field rather than an automatic fertilizer application. UNL recommends soil sampling established alfalfa every three to five years. If you’re due for a test, this fall is a good opportunity to find out what the stand actually needs and put fertilizer dollars where they have the best chance of paying back.
Ben Beckman is a livestock systems Extension Educator based out of the Cedar County Extension office in Hartington. You can reach him by phone: (402) 254-6821 or email: ben.beckman@unl.edu.
