Diagnosing the Cause of Yellow Alfalfa

Recently, Nebraska extension offices have been receiving yellowish alfalfa from growers fields. When agents and growers discussed the growing conditions and cultural practices between each case, the common factors were discovered within the field.

Common Factors

  • The alfalfa was planted last year in the spring. When planted in the spring, it usually does not grow very vigorously.
  • In a lot of the cases, alfalfa had not been planted in the fields for many year and before it was planted, there was no soil test performed.
  • The yellowing of the alfalfa was spread across the whole field. Occasionally it was associated topographically, but not always.

So what caused the alfalfa to turn yellow?

If the alfalfa was only yellow in the lower area, the problem could be Phytophthora root rot, but most of the time is was a nitrogen deficiency.

You may be wondering how can alfalfa have a nitrogen deficiency when it makes its own nitrogen.

This is happening because the alfalfa is not forming the nodules that make nitrogen because the soil acid or the seed was not inoculated. In order for alfalfa to form nodules, the spoil has to have a pH above 6.2. Since tillage and nitrogen fertilizer increase the soils acidity, surface soils are becoming more acidic. To neutralize the soil acidity, lime is needed. If your soil pH is acceptable, the correct type of Rhzobium bacteria is needed. If a field has not grown alfalfa in the past two or three years, more bacteria is often needed when the seed is planted.

If your alfalfa fields are turning yellow, get your soil tested, if acidic add lime, and inoculates may be needed to improve your fields.