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.

The nutrient uptake process

Once the nutrients are delivered to the root surface, nutrient uptake can occur. Three factors determine the amount of nutrients that will move into the root:

  • The concentration of nutrients at the root surface.  
  • The amount of root surface area (number of roots); and
  • The amount of nutrient inside the plant, and the feedback that provides to the uptake process.  

If the amount of water moving to the roots is limited, nutrient uptake is limited through the limitation of the delivery of nutrients to the root surface that creates.  If the root system or root growth is limited -- by compaction or soil density and limited pore space (as in no-till) -- uptake is limited since each unit of roots has a finite maximum uptake rate. If the concentration of the nutrient in the soil is limited, either because you didn’t apply enough fertilizer or you put it on and it was lost, nutrient uptake is limited.

When nutrients dissolved in the soil water reach the surface of the root, the root selects the ions it needs and moves them across the cell membranes. The ions it doesn’t need remain at the root surface and accumulate. This process is highly selective and requires energy. In some special cases such as might occur with young wheat shortly after green-up, during a prolonged period of cool cloudy days which would limit photosynthesis, nutrient uptake could be limited by a lack of available energy.

How growth stage of the corn impacts nutrient uptake

One additional important issue impacting nutrient uptake is the growth stage of the corn, especially early in the season. When corn seed germinates, the radicle or seed root grows down and begins to expand and take up water along with a limited amount of nutrients. This seminal root system supports the corn seedling for the first 20-30 days, but it doesn’t grow deep, nor does it become very dense. Its job is primarily to take up water and anchor the plant until the permanent nodal root system develops.  

About the 4-leaf stage, the nodal or crown root system will begin to develop, and the seminal roots will reach their maximum size. Around V-6 the nodal root system will begin to take over and the seminal system will begin to die. Between V-4 and V-6 or V-7, it’s not unusual to see plants exhibit pale green color, purple color, striping, and so forth as nutrients can be limiting. This is not because of a real deficiency of nutrients in the soil per se, but rather it is because of a limit in the amount of active roots available to take nutrients up as this root system transition occurs. 

Any condition that will limit the delivery of nutrients to the root surface will aggravate this situation. This is why many people use starter fertilizer. Providing a high concentration of nutrients close to the plant helps overcome some of these problems created by limited root systems. While we can often see significant differences in growth at the V-4 to V-6 growth stage with starter, many times these differences seem to vanish as the nodal roots develop, and little or no yield benefit is seen.

How tillage systems impact potassium movement in the soil

In no-till soils, bulk density increases, and pore size distribution changes. There are commonly fewer, but bigger pores. With fewer pores, the nutrient diffusion route becomes longer. So if the soil becomes dry, and the big pores begin to empty out, the diffusion issue becomes even more of a problem in no-till than in conventional till.

This year’s problems

This year, plants that got off to a quick start are now at V-6 or V-7. The nodal roots are established and growing well, and the plants look pretty good. But there was a lot of uneven emergence. Where seeds were planted a little shallowly and the ground dried out, those seed may have come up a week later and are a leaf or two behind seed planted into moist soil. These later-emerging plants are smaller, pale yellow, and are showing K deficiency due to the dry conditions. The nodal roots on these plants are just getting started and they haven’t rooted as deeply yet as more developed plants.   

In one case this year in Atchison County, small plants in spots of a field were found showing K deficiency and yellowing. These spots were surrounded by bigger corn in most of the field, which was greener and one or two leaves further advanced. The big corn had good roots. On the small corn, the nodal roots were only 2 or 3 inches long and just getting started. This is likely an issue of uneven emergence, along with some soil compaction. The soil test K level was good at 200-300 ppm. But all the reasons mentioned above were interacting to cause trouble. The big corn had a greater root density, so it could take up more nutrients under adverse conditions. With more moisture, the small plants will perk up as the root system develops more fully. There may not be a big yield difference in the end, but there will probably continue to be differences in growth stage and eventual maturity as the season goes on.

We have done some plant analysis on some of these problem fields this spring and most of them are showing the “bad” plants as being deficient in K. The problems are worse where the soil test K levels are low. But there are also problems even where soil test levels are high, caused by problems in delivery of available nutrients to the root surface for uptake.

Hybrid differences

Hybrid differences are also something to consider. We know that different hybrids will respond differently to nutrient deficiencies (particularly to phosphorus). This is likely related to differences in root growth early in the season. Because of all the conditions mentioned above, root growth habits of different hybrids will be particularly important this year, and relevant for K uptake. However, these differences in early growth do not always translate into yield differences at the end of the season. Certain hybrids may look “bad” early in the season, but provide a big advantage later.