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  AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Summer 1998, No. 1

FOLIAR PHOSPHORUS AND POTASSIUM FOR SOYBEANS

Foliar fertilizers for soybeans are receiving a lot of attention. At times, dramatic yield responses have been seen, but the conditions for a good response are difficult to predict.

There are good reasons why foliar fertilizers can work for soybeans. The leaves have an outer waxy layer called the cuticle which helps keep water in the leaf. The waxy cuticle sheds water and stops the rain from leaching the nutrients out of the leaf. There are billions of tiny pores through this cuticle on every square inch of leaf. These pores can help the plant to absorb small quantities of nutrients through the leaf. Once the canopy has closed, the total leaf surface area in crops can be larger than the total root surface. So there is ample opportunity for nutrients applied to the leaf to move into the plant directly, avoiding some of the losses that occur when nutrients are added to soils.

Once the pods have started growing, the soybean plant devotes much of its resources to grain-filling. When the grain-filling process uses up all the material and energy produced by photosynthesis in the leaf, the root system suffers and may be unable to continue supplying nutrients from the soil. The mineral nutrients needed in the developing seeds are then moved in from the leaves. Loss of nutrients from the leaves reduces photosynthesis and can start the "self-destruct" process, hastening maturity. The extent of this process depends on many factors, including the variety and weather conditions. In some situations, growth can be prolonged by adding foliar nutrients to prevent nutrient depletion and keep the leaves more active.

The risk of leaf burn due to excessive salts is an issue with foliar feeding. With the exception of urea, foliar nutrients can only be applied in very small amounts. A foliar application supplies the plant at best for only a few days following the application. Imbalances can also be a risk, because the nutrient goes directly to the leaf, as opposed to the soil where there is some buffering. Therefore, many of the positive responses to foliar nutrition have been seen when nutrients are applied together, for example, nitrogen-phosphorus-potassium-sulfur applied to soybeans during pod fill. The "catch" is, the more nutrients applied, the greater the salt load and risk of leaf injury.

Despite good reasons why foliar nutrition can work, responses in the field are less than frequent. A recent study in Iowa found responses to a 3-18-18 foliar product at eight out of 38 sites over a two year period. The responses at these eight sites averaged about 5 bushels per acre. So far, it has not proved possible to predict where and when these responses would occur.

In plants that have adequate nutrition, movement of phosphorus and potassium into the leaf is limited. The reason is that normal concentrations of these two nutrients inside the leaf are quite high. This makes it difficult for a nutrient applied outside the leaf to diffuse inward. Foliar-applied phosphorus and potassium move in more easily when the leaf is deficient. Unfortunately, a plant that becomes deficient at any stage loses some of its yield potential.

There is no replacement for adequate fertility levels in the soil. Building soil phosphorus and potassium to the optimum range and maintaining it has a solid economic foundation. Efforts to improve on that foundation will continue. Foliar applications may someday form part of the management package that takes us beyond current yield limitations. Until we can better identify the plant's daily needs, we can't assure response.

For more information, contact Dr. Tom W. Bruulsema, Director, Eastern Canada and Northeast U.S. PPI, 18 Maplewood Drive, Guelph, Ontario N1G 1L8, Canada. Phone (519) 821-5519. E-mail: tom.bruulsema@ppi-far.org
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