Phosphorus: An Essential Nutrient for Soybean Production

When fertilizing soybeans the emphasis is usually placed on potassium (K). Soybeans have a reputation of not responding to phosphorus (P) fertilizer. This idea comes in part from the fact that beans remove large amounts of K and much less P. There is now a growing body of evidence that phosphorus is vital to achieving high yielding soybeans. When soil test levels are low for P, soybeans can show a substantial yield response to added P2O5.

Why is P important?

Phosphorus along with nitrogen and potassium is a primary nutrient required by plants to complete their life cycle. It's especially important during the early stages of growth and development. One important role of P in plants is to store and transfer energy produced by photosynthesis which is then used for growth and reproduction. If P is limiting, plants cannot grow adequately, which limits their ability to cope with stress. Slow root and shoot development results in delayed maturity and reduced yields. Phosphorus is also a component of cell membranes and is part of the structure of DNA. A 50 bu/ac soybean crop will take up as much as 50 lb/ac of P2O5. Relatively large amounts of P are required by plants compared to most other nutrients.

Soybean yield Response to P

Traditional thinking has been that soybeans do not show a significant yield response to P fertilizer unless soil test values are very low. Recent trials have demonstrated surprising yield responses to P in soybeans. Visual P deficiency symptoms are rare and difficult to identify even when present. In extreme cases the plants are slow to grow, spindly, and the leaves remain smaller and lighter in colour. Most of the time these symptoms are subtle and usually overlooked unless there is a good side by side comparison. Soil compaction limiting root growth will also cause weather induced deficiency. Ontario trials conducted over the last 5 years by the University of Guelph and OMAFRA have shown that when soil tests are less than 20 ppm for P and less than 120 ppm for K the application of potash by itself only raised yields by 1 bu/ac. When both P and K were applied yields increased by 4 bu/ac. When P soil test levels were low but soil test levels for K were adequate the application of P increased yields by 3 bu/ac. This is strong evidence that phosphorus is a critical component to high yielding soybeans. If soil tests were adequate for P and K additional fertilizer did not increase yields.

One of the most important findings of the study to date has been that applying fertilizer to low testing soils produces lower yield than a soil that was built in P and K. Building soil test levels to reasonable soil test values (20 ppm for P and 120 ppm for K) appears to be a good long term strategy to maximize soybean yields.

What about Environmental Concerns and P?

Soil erosion is a major concern for phosphorus loss from land to water. However, P can also leave fields in the form of soluble P, either in surface runoff or through tile drains. Best management practices that limit soil erosion will go a long way to reduce field losses, but should also be paired with fertilizer placement and timing practices that minimize risk of loss. Summer (after wheat harvest) is an excellent time to apply fertilizer since the risk of loss is lower at that time. If applying phosphorus in the non-growing season (e.g. October), it should not be left unincorporated on the soil surface.

There is no reason to believe that excessively high soil test values through over fertilization will lead to economic returns. Very high soil test levels can lead to increased environmental risks. On the other hand, not building soil test values to reasonable soil levels will lead to lower yields in the long run. So, clearly a balanced approach is necessary. This includes the right amount of P2O5 for soybeans at the right time, which appears to include building low soil testing fields over a number of years. Building soil test levels to moderate values does not necessarily increase the risk of loss to the environment, but it is critical that best management practices are followed.

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