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Phosphorus Levels Can Affect Nitrogen's Impact on Forage Production

Posted Apr. 30, 2005

Why is a forage specialist writing about a soil nutrient? Simple lack of this nutrient makes a forage specialist's job harder, because it directly influences forage production and management. Phosphorus (P) in soil solution is generally low, making P second only to nitrogen (N) as the most-limiting nutrient for plant growth in many of our soils. A survey of soil samples from Noble Research Institute cooperators showed 70 percent of soils needed additional P and 40 percent were severely deficient.
P has many roles in the plant:

  • it is used during photosynthesis for the conversion of light energy into chemical energy which is then used by the plant for growth;
  • N fixation in legumes;
  • root growth and development;
  • essential for the transfer of carbohydrates;
  • crucial during a plant's establishment phase because of its roles in photosynthesis, root growth, and energy development and transfer.


Consider this: On average, each ton of bermudagrass hay will remove 43 pounds of N, 10 pounds of P2O5 and 48 pounds of K2O (potassium) from the soil. N is applied yearly to introduced grasses to obtain a yield goal, and it should be obvious that if the P and K removed in hay is not replaced, it can turn into a nutrient-mining operation. Many times this results in reduced yields and the appearance of undesirable plants such as broomsedge.

Reduced yield is not what you want to hear at today's fertilizer prices, you want to get as much as you can out of your fertilizer dollar. Taliaferro, et al., (1975) found that on a soil with a soil test P level of 13 lbs./ac., Midland bermudagrass receiving 80 lbs./ac. P2O5 had a 31 percent increase in average forage yield over plots that received no P. In this same study, when 80 lbs./ac. P2O5 was applied with N at either 120 or 240 lbs./ac., N use efficiency increased as well as forage yield. Winter pasture results are similar. Altom and Rogers (1996) reported on a Renfrow silt loam soil testing 7 lbs./ac. of soil test P that an application of 46 pounds of P2O5 increased dry matter yield almost 1 ton per acre, with most of the yield difference occurring in the fall and winter. In this study, N applied at 100 lbs./ac. in the fall without P produced only 129 lbs./ac. more dry matter than plots receiving zero N. This could probably be attributed to enhanced root development of plants with adequate P. Ludwick and Rumburg (1976), working with perennial cool-season grasses on a soil testing 6 parts per million P, found that yield response to N was restricted in the absence of applied P.

The research is pretty clear. If P is limiting, you will not get the bang you could out of N, especially at higher rates of application. If you have not soil tested for a while, and you are beginning to see broomsedge encroachment or reduced yields, check your P levels. It would be well worth your time and money to do so, especially at today's nitrogen prices.

Altom, W. and J. Rogers. 1996. Effects of Phosphorus Source and Placement on Small Grain Forage Production. Great Plains Soil Fertility Conference Proceedings.

Ludwick, A.E., and C.B. Rumburg. 1976. Grass Hay Production as Influenced by N-P Topdressing and Residual P. Agronomy Journal 68:933-937.

Taliaferro, C.M., F.P. Horn, B.B. Tucker, R. Totusek, and R.D. Morrison. 1975. Performance of Three Warm-season Perennial Grasses and a Native Range Mixture as Influenced by N and P Fertilization. Agronomy Journal 67:289-292.