OKLAHOMA ALFALFA FIELDS NEED FERTILIZER  

Observations from the Alfalfa Production Evaluation Project indicate that many more alfalfa producers should soil test and fertilize their alfalfa. When looking at alfalfa soil test data, three numbers are critical. Soils that do not limit yield have --   

• pH = 6.5 or more.  

• Phosphorus (P) index = 65 or more.  

• Potassium (K) index = 350 or more.  

* % Adequate: pH (>6.5), P (>65), and K (>350). (Considered 100% sufficient)   

** % "Almost Adequate": pH >6, P >60, and K>250. (Roughly 30% decrease in yield)   

Only 10 to 24% of the samples indicated that the soil was not limiting alfalfa yield (shaded column). Conversely, 74 to 90 % of the soil samples were limiting yield to some degree.   

We lower our standards and looked at the same groups of soil analysis data (right column). We found 26%, 27%, and 35% of the soil samples were better than pH=6, P=60, and K=250. In other words, 65 to 74% of the fields, depending on the group studied, lost at least 30% of their yield because of acidity or insufficient P or K.   

Infertile and acid soils generally limit alfalfa yields. Infertility and acidity also decrease stand persistence and allow broadleaf and grassy weeds to infest stands, lowering quality.   

Many of our alfalfa fields did not need to be fertilized when they were first cultivated; however, alfalfa removes large amounts of potassium, phosphorus, and calcium. Five tons of alfalfa hay contain about 70 lb P₂O₅ and 300 lb K₂O. Each year this is removed with hay and must be replaced by fertilizer or soil weathering. The soil can supply P₂O₅ and K₂O for just so long and then they must be added.   

If a soil is deficient in P and K, it pays to fertilize alfalfa. Cost/benefit ratios of about 1:3 are normal. For $1 invested in correct alfalfa fertilization, $3 are returned as increased yield. Longer stand life is an added bonus.