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Fluid Journal : Fluid Journal 2005-2007
Early Spring 2005 What About Foliar K On Soybeans? Dr. P. Motavalli, Dr. K. Nelson, Dr. G. Stevens, and S. Phurahong Summary: Among the conclusions reached by this research is that foliar potassium (K) fertilization may be a supplemental practice to longterm K fertilization practices that build up and maintain soil test K levels. However, foliar K fertilization may become a more useful management tool if further research determines what soil and environmental conditions promote soybean crop response to foliar K fertilization. In addition, methods must be developed to assist growers in making more rapid assessment of soil K availability during the growing season to decide when foliar K fertilization may be profi table. Our ongoing research also has been examining mixing foliar K sources with glyphosate, since combining foliar fertilizer with post-emergence weed control may make foliar fertilization more cost-effective. Despite a relatively inconsistent soybean response to foliar K, studies show an opportunity may exist to provide growers with a cost-effective method of applying foliar K. Fluid Journal 1 Soybean response to foliar fertilization applied several times during the growing season has been extensively examined by researchers, starting in the 1970s. Most of the reported responses to foliar fertilization have been variable and inconsistent, especially when tested over a wide range of locations. However, recent changes in agricultural management practices and other developments justify additional research into use of foliar K applications for improved soybean production. The incidence of K deficiency in agronomic crops has increased in recent years in Missouri and other Midwestern states due to 1) the effects of drought conditions and soil compaction resulting in decreased K availability, 2) reduced amounts of applied K fertilizer, 3) lower frequency of soil testing by producers due to low commodity prices, and 4) higher K fertilizer requirements because of increasing crop yields and larger soybean acreage. Moreover, 83 percent of soybean varieties produced on over five million acres in Missouri were Roundup Ready® or contained another form of transgenic herbicide resistance in 2003. Widespread use of glyphosate for post-emergent weed control in soybeans opens the possibility of making foliar K fertilization more cost- effective by combining foliar fertilization with post-emergent herbicide applications. Finally, the goal of recent developments in soil fertility management practices, such as the practices being developed for site-specific nitrogen (N) management of corn, has been to provide growers with tools that allow them the flexibility of assessing and responding to changes in and spatial variation of soil nutrient availability over a longer portion of the growing season. If effective, post-emergent application of foliar K would have the advantage of increased flexibility for growers to more rapidly respond to observed K deficiency owing to the effects of variable soil properties, management practices, or climatic conditions. Objectives of our 2001 to 2002 experiments conducted on a farmer's field, with low to medium soil test K, were to 1) compare how soybeans respond to foliar K applied at several growth stages versus a preplant
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