Potassium deficiency problems showing up in corn
A large number of corn fields across Kansas, especially non-irrigated corn in the eastern part of the state, are showing up with potassium (K) deficiency symptoms. The symptoms are showing up in a wide variety of field patterns, and on fields with all kinds of K soil test levels. What is going on? Basically, the problems are mostly the result of the stage of growth the corn is in (V-4 to V-6) combined with dry soil conditions and root development issues. There are many combinations of these factors this year, resulting in many different scenarios.
The basic issue, of course, is that potassium has to reach the plant roots before it can be absorbed by the plant. This becomes a physical and chemical problem. There has to be enough potassium in the soil, it has to get dissolved into soil water, the dissolved potassium in the soil solution has to move easily through the soil pores to the plant roots, and there has to be an extensive root system present to come into contact with the potassium dissolved in soil water.
Nutrient movement to the plant root
Water -- and the uptake of water -- is critical to the movement of nutrients to the root surface. Some nutrients, such as nitrate-nitrogen (N), are very soluble in water and move to the root as the plant takes up water. The process of dissolved nutrients moving toward plant roots in soil water is called mass flow. Nutrients such as nitrate-N, calcium, magnesium, and sulfur move to the root surface for uptake in this way.
Some nutrients are needed in large quantities, such as phosphorus (P) and K. These nutrients are not very soluble in water, however. As a result, concentration in the soil water is low and the P and K content in the soil water at the root surface is quickly depleted by nutrient uptake. This creates a diffusion gradient between the bulk soil water a short distance from the root and the water at the root surface. P and/or K then start to diffuse, or move to the root surface. Essentially all of the P, and 80 to 90% of the K, taken up by plants moves to the root by diffusion. The rate of diffusion of nutrients through soil is influenced by several factors, but the most important are: (1) the buffering capacity of the soil, or its ability to add P or K to the solution (soil test level); (2) the size of the gradient, or difference in concentration of nutrient between the bulk soil water and the water at the root surface; and (3) the soil water content.
At high soil moisture, the pathway for diffusion is shorter, and the process runs more quickly because all the pores are full of water and the K ions can move in basically a straight line. But at low soil moisture the soil water in many of pores in the soil is depleted and the pathway for diffusion becomes more tortuous as dead ends develop along the path. As a result, the diffusion distance becomes longer as the K goes one way and then another to avoid the dead ends or blockages created by dry pores. So, in dry weather, soil moisture becomes depleted, the pathway for diffusion becomes longer, and K availability to the plant is reduced by the slower rate of delivery.
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