Dry soil conditions can be challenging for crops to take up nutrients.  Elements such as phosphorus and potassium move to the plant roots via diffusion.  Diffusion is a process by which things move from areas of high concentration to areas of low concentration.  If you can think back to high school science if you open a bottle of ammonia at one end of the room over time you will smell the ammonia as it diffuses through the air.  That is a simplistic explanation of a more complex process in the soil.

When we look at the structure of the soil it is made up of a solids, liquids, and gasses.  In soils the area occupied by liquids and gasses are very important when it comes to nutrient uptake.  Diffusion only occurs in the liquid phase in the soil.  Soil water is attracted to the surface of the soil particles as a film of water on the surface and as the soil dries the films become smaller.  As the films become smaller the path to which a potassium ion need to take to reach the root of the plant increases.  Since diffusion only tends to work over short distances then uptake of potassium can be very limited causing deficiencies to occur.

Some reports have been made on small grain crops that appear to have visual K deficiencies.  There are a few things that can be done to determine whether there are K deficiencies in a field.
–First, start with a soil test.  If the soil test is above 160 ppm crop yield should not be limited and the deficiencies that are seen in the field are likely related to soil moisture or some other factor.  If the test does come back low (80 ppm or less), the question is whether any fertilizer can still be applied?
–Secondly, a nutrient analysis of the plant is very helpful to diagnose the problem if there are areas of the field that look normal to compare to.  Comparing three areas of the field with normal, less than normal, and poor growth will greatly help you interpret the results.  When the sample comes back to the lab compare the nutrient concentration numbers and if there is a definite pattern in the numbers then you may be able to determine what is going on.  It is still recommended to take soil samples as backing data when plant sampling is utilized.

One thing to note is that there can be some issues with K soil tests if the soils are dry.  Potassium can be held on clay surfaces but also can be held between clay layers.  We commonly refer to the K held between layers as “fixed”.  If soils are dry or become dry then there can be significant differences between soil test values.  These differences are magnified in high clay soils.  Current research has found little difference between air-dried and moist soil samples for sandy soils.  The other thing to note is that crop residues are a large store house for K.  Over time the K can be leached out with rainfall.  If this has not occurred the soil test may come back low.

If you identify that K levels are low there still may be some time to apply fertilizer.  However, any top-dressed fertilizer would need some form of rainfall for incorporation.  In addition, K does not move that far so if soils remain dry, the roots still may not have access to the K if it is near the surface.  Movement of K is very limited even in sandy soils.  Therefore, in-season top dress applications of K should only be viewed as a rescue treatment if needed and not as a standard management practice.

We do not have set guidelines for rates to apply.   Currently, a project funded by the MN corn growers is examining side-dress application of K for irrigated corn.  We have seen that K applied at early growth stages as a side-dress application can be taken up by the plant.  However, pre-plant application still appeared to be better when comparing similar rates.  One advantage for this work is that irrigation is used so the soil surface does not stay dry for an extended period of time.

If a problem field is identified as being K deficient, a small rate of K applied over the top in fields with sandy soil textures may help to alleviate problems.  Since we have not had much precedence for this there are many unknowns such as the burning potential from a top-dress application and whether it will be effective in non-irrigated situations.   This type of treatment should only be viewed as a rescue treatment and may not be successful if there is not enough rainfall to incorporate the fertilizer.  If field areas are going to be cropped in future years taking any soil samples taken will be beneficial in aiding in applying fertilizer for future years.