Do you Have Potassium Deficiency?

Several of our maturing crops are now exhibiting deficiency symptoms that are too late to correct, but important to address for next year.
Potassium (K) is often overlooked in much of Manitoba due to our naturally high K levels in clay and clay loam soils.  But deficiencies on lighter textured soils are increasing – particularly with soybeans. 

 

Picture 1: Mild potassium deficiency symptoms on upper leaves in August

Potassium deficiency often shows up during pod and seed fill, since soybeans remove 1.4 lb K2O/ bu of grain, the heaviest rate of removal of any grain crops.  As K is translocated out of leaves to fill seeds, the deficiency shows up as yellowing and later necrosis of the leaf margins.
Sometimes odd strips occur of alternating deficient and normal soybeans occur in fields.  These are often related to a previous canola or cereal swath that has had the K leach out of the swath into the soil beneath, and hence marginally increasing K supply in that strip.
If either of these symptoms are observed, a K deficiency can be readily identified with a traditional soil K test and a recommendation will be made for future K fertilization.

Picture 2: Alternating strips of varying potassium deficiency in maturing soybeans due to previous canola swaths.
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Do I Need to Apply Zinc to my Soybeans?

Question: When we grew edible beans in the 90’s, we were advised to use zinc. My notes show it was not so much a yield factor, it was for maturity.

We have generally high organic matter (8% and up), most pH is over 7.8 , clay soils, and our phosphorus levels are reasonbe to high at 28ppm -all factors that my notes say tie up Zinc. Some fields are ok for zinc at 4 to 6 ppm but some are lower at or less then 1 ppm. So should I be putting on a zinc product as a safe guard or are soybeans totally different from edible beans?

Answer (supplied by John Heard, MAFRI Crop Nutrition Specialist):

Zinc deficiency is rare in soybeans as compared to edible beans.  Manitoba studies have documented edible beans respond well to applied zinc fertilizer when soil test levels are below 0.5 ppm DTPA extractable zinc.  Soybeans are much less sensitive to low soil zinc levels deficiencies and have  not responded to zinc fertilizer applications in North Dakota and Minnesota studies.  At 11 Minnesota sites in 2011-2012, there was no soybean response to applications of either zinc, sulphur, manganese, molybdenum or boron (see attached reference http://www.smallgrains.org/2013SGU/KaiserSoybean.pdf ).

The main nutrients to watch in soybeans are nitrogen (through proper rhizobium inoculation) and phosphorus and potassium.  Based on your high phosphorus soil tests (26 ppm P) and the high K potential associated with high clay content, your soybeans may do quite well on residual fertility.

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