Understanding Potassium and Water StressPosted in: Farming By Elston Solberg March 28 2016
Learning the Language of your Crops!
We have all spent a considerable amount of time “talking” to crops in the field. And if we are listening crops will talk back to us. Often crops (particularly cereals) tell us that they are short of potash. Crops speak to us not in words, but in a “plant language” that we need to understand. Nutrients exhibit certain traits or symptoms when they are deficient in a crop. These signs are key indicators as to what is going on inside a crop. Crops that lodge – crops that have a high disease incidence – crops that do not withstand drought well – crops with poor bushel weight, all might be crops that are showing signs of a shortfall in potash.
The most commonly used potassium fertilizer is muriate of potash (0–0–60), which chemically is potassium chloride (KCl). While chloride required in small amounts, it works in tandem with potassium;, the chloride anion (Cl-) balancing the potassium ion (K+) concentration in the guard cells during the opening and closing of stomata. During dry periods, K is pumped out of the guard cells and the stomata close tightly, conserving water, minimizing drought stress.
However, if K in the plant is in short supply, the stomata are slow to respond and may take hours, rather than minutes, to close. As a result, plants become much more susceptible to drought.
As you walk into a wheat or barley crop in July and notice that the bottom leaves are “burning” on the outside edges while some are dying off completely. Disease? Well there certainly may be some infestation occurring. Heat stress? It’s definitely been warm in some areas and perhaps the bottom leaves have done their job. In both cases you may be correct, but what you may be diagnosing are the symptoms, not the cause of the problem. Often we see diseases infecting crops after deficiencies have manifested themselves. These deficiencies may weaken the plant to such an extent that it no longer has defense against disease or environmental stress.
Wherever moisture is often a limiting factor, potassium is of great importance. Potash regulates water transpiration rates by helping to control the opening and closing of the “guard cells”. These guard cells are located on either side of the stomatal openings on the leaf surface. It is through these openings that water passes or “transpires” to the atmosphere. Potassium must be present in plentiful supply to ensure that we are able to maximize the results from the water we are given. Even though your soil may “appear” to have enough available potassium, the real question is, “How rapidly can that soil release soluble potassium when the plant needs it during its’ peak demand period? Potassium exists in three states in the soil, “fixed”, “exchangeable” and “soluble”. What we need to understand is how much extractable potassium is there in the soil along with how much “soluble” potassium is readily available to a crop on a “daily basis”.
During its peak demand period, high yielding wheat (60–80 bu/ac) may require 4–6 or even 8 pounds of potash per acre per day! We see plenty of soils that should contain adequate potassium to support the crop and yet the plants “consistently” show signs of K deficiency. Results from previous research done near Red Deer, AB looking at crop response to K application to a marginally deficient K soil showed that yield, bushel weight and return per acre increased consistently with higher K rates. We were able to obtain an additional 23.2 bushels per acre through potash application. Bushel weight climbed from 48.5 to a high of 52 lbs/bu. Once covering the cost of the potash, our best economic response netted an additional $32.11/ac! (old fertilizer and crop prices). This was important as it showed significant response at rates much higher than conventional recommendations. The other important thing to note is that these increases were all obtained with no additional input of nitrogen!
Folks say, “We have huge potash reserves in Western Canada, we export potash all over the world. There’s no way we need potash to grow our crops.” Well, our plant roots do not go down 3500 feet and there are many soils where additional potash is necessary. Behind nitrogen, there is no fertilizer nutrient required in higher amounts by plants than potassium. To grow a crop, spring wheat requires 1.6 lb K2O/bu as compared to 1.9 lb N/ac. Unlike N and P, very little of the accumulated K is used for grain fill, less than 20%. Yet, K deficiency early in the growing season limits N uptake and compromising yield and protein.
After K deficiency show or tissue tests indicate low K, it is difficult to get additional K into the plant. Often we recommend the use of root driving nutrients; foliar P and B to grow the roots allowing the plant to explore more soil mass and access more K from the soil. However, only a small fraction of the plant K requirement is attained by root interception. The bulk of K has to be transported to the growing roots by mass-flow and diffusion; this means available water is critical. As my wise US colleague, Senior Agri-Coach Markus Braaten says, “plant sip, they don’t chip”.
Maximum K accumulation occurs by about mid-heading. Potassium is mobile in the plant; meaning that the plant is able to repurpose it during times of need. It is partly for this reason we want to see very high K in early tissues, so the crop can become its own fertilizer plant should the soil get dry or crop needs outstrip the soils ability to supply K during peak uptake.
This may be hindsight but could have adequate K levels earlier this spring helped your drought/water stressed crops hang on just a little bit longer.
Take a good look at your soil test potassium levels.
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