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Will high soil test K levels protect against SDS or other diseases?  More is not always better. 

With nutrient deficiencies becoming more commonplace, we are often reminded of the need to keep adequate levels, as well as the correct application timing, of macronutrients and micronutrients to supply the proper nutrition needed for optimal performance of today’s higher yielding soybean varieties.

Dan Davidson recently reviewed soybean potassium (K) deficiency. But what happens when your soil test reveals that you have high levels of K available in your soil? This nutrient is known for promoting stem and root growth as well as playing a role in metabolism, protein synthesis, chlorophyll development, enzyme activation, cell division, formation of carbohydrates, translocation of sugars and water regulation in plants. So, would more K in your soil be better?

Recently, I was asked: “Do I need to worry about diseases like sudden death syndrome (SDS) if I have high levels of K in my soil?” I assume this question was asked because K has been proven to increase the resistance of plants to certain diseases. But, the involvement and influence of K nutrition on plant disease is complicated. 

Consider these facts based on research:

• The rate and form of K needs to be in balance with other nutrients to help manage crop disease.
• K+ (When K is in the plant, it is in its ionic form (K+)) has been found to directly affect different stages of pathogen establishment and development within a plant host.
• K can promote wound healing, which can indirectly affect the infection of certain pathogens.
• If the K level in the soybean becomes too high, it can delay maturity and senescence. In turn, this can give some pathogens a longer timeframe to attack.
• Overall, K has been shown to reduce disease severity of some plant diseases. However, if there is too much K, disease severity can sometimes increase, especially with soybean root rot and stem rot caused by Phytophthora sojae.
• Soybeans grown in soils rich in N, P and K can be more prone to disease, because fertilizer applications can cause nutrient-induced changes in both the host and pathogen. Increased levels of K can either increase or decrease diseases, depending on the host (plant) or specific pathogen. 

SDS development is influenced by many factors such as soil temperature, moisture, SCN pressure, planting date, soybean variety, maturity date, tillage and soil fertility. SDS often can be associated with high-yielding production environments or increased soil fertility, which means soil chemical factors could influence incidence and severity of this disease. 

Research has shown that increased K concentrations can increase the severity of SDS foliar symptoms.  Other studies suggested increasing fertilizer application ratios of potassium chloride (KCl) at planting for SDS control, but it was the chloride (Cl) responsible for the decrease in SDS symptoms, not K! 

Lastly, it has been reported that SDS fungal mycelia growth or disease severity was significantly increased by applications of calcium phosphate, potassium phosphate, potassium sulfate, sodium phosphate and potassium nitrate.  Research continues, but thus far, more K does not appear to be better for controlling SDS.

Resource:  http://opensiuc.lib.siu.edu/cgi/viewcontent.cgi?article=3014&context=theses