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Importance of Phosphorus and Potassium Fertilization in Grass-Legume Mixtures

by Wyoming Livestock Roundup

By Anowar Islam, UW Extension Forage Specialist

Some time ago, I wrote an article about the importance of phosphorus and potassium in grass-legume mixtures and livestock production. Today, I will share some results from a recent study.

Livestock production in Wyoming relies heavily on good quality hay. On an average of the last few years, Wyoming produced about 1.3 million tons of alfalfa. Its value is about $2.7 million – far exceeding corn grain, dry beans, barley and winter wheat. Alfalfa is an important crop in Wyoming. 

However, bloat problems associated with grazing alfalfa with livestock are a major concern. One of the best options in reducing this risk is to use a mixture of alfalfa and grass. Grass-legume mixtures can produce not only high quantity and quality of forage but also can reduce the need of commercial nitrogen fertilization for monoculture grass stands. As a legume, alfalfa can fix atmospheric nitrogen and provide enough usable forms of nitrogen to plants.

There is direct benefit establishing grass-legume mixtures, which has the potential of increasing farm profits. Unfortunately, poor persistence of alfalfa in mixtures can result in an unproductive stand. Loss of alfalfa in grass-legume mixtures can enhance weed infestations and reduce forage yield and quality. In addition, it may increase the production costs significantly.

There are many reasons for disappearance of legumes from grass-legume mixtures. 

Carbohydrate content of roots is important for winter survival of alfalfa. Low levels of phosphorus and potassium in the soil can be harmful for alfalfa causing poor persistence of alfalfa in the mixed stands. 

On the other hand, grasses can absorb much phosphorus and potassium even in situations when their availability is low in the soils. Grasses have fibrous and extensive root systems that help them to extract more phosphorus and potassium from soils. As a result, grasses can make alfalfa deficient in phosphorus and potassium.

Fertilizing soils low in phosphorus and potassium may be necessary for improving productivity. Phosphorus and potassium have the potential to increase the longevity of alfalfa in mixed stands, thus increasing the productivity and quality of the grass-legume stands. 

A recent study in the Department of Plant Sciences at the University of Wyoming suggests that a combination of phosphorus and potassium can enhance plant growth in both alfalfa and grass and improve persistence of the mixed stands. It is therefore recommended to have a routine soil test to determine phosphorus and potassium levels in the soils.

Anowar Islam is an associate professor and the University of Wyoming Extension Forage Specialist in the Department of Plant Sciences in the College of Agriculture and Natural Resources. He can be reached at 307-766-4151 or

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