Harris – The Two Most Important Results on Your Soil Test : pH and Potassium

Soil testing is the backbone or “cornerstone” of any good soil fertility program.  If you want to produce high-yield and high-quality cotton you have to take a good soil sample, have it analyzed correctly and get reliable liming and fertilizer recommendations in return. But there can be a lot of information on a soil sample report. Not only that, but depending on which lab you use, you may get different results in terms of what they analyzed for and what is recommended. What is “CEC” and “base saturation” ? Why are the lime and potassium recommendations different between labs ? How much phosphorous or micronutrients do I really need?  These are all good questions and if you’re not careful it’s easy to get bogged down and confused.

By now you have probably heard of “Liebig’s Law of the Minimum”, which states that (plant) growth is dictated not by total resources available but by the scarcest resource (limiting factor). This is the Wikipedia definition by the way. There are many variations of this definition. You should see some of the answers that students who take my soils course at UGA-Tifton come up with. So instead of the “leaking barrel” that everyone loves to show, I like to point to one of the alternative definitions (also mentioned in Wikipedia) which is “a chain is only as strong as its weakest link”. So what is the weakest link on your soil test report form? The factor most likely to limit your cotton growth and yields ? I would argue that the weakest links are soil pH and potassium.

Let’s tackle soil pH first. Over the course of my 26 years as a UGA Extension Specialist for soils and fertilizers, I have probably gotten more calls about soil pH than any other topic. This is not that surprising when you think about  all the factors that can affect soil pH in the field — and also how it is measured in the lab.  When I took a soils class in college, the professor said “ I dare you to take 20 soil cores from a hundred acre field two days in a row and see if you come up with the exact same soil pH”.  His point, of course was “spatial” variability or how soil pH can change even within 3 or 4 feet down the row. This is an obvious challenge for us on our Coastal Plain soils in the Southeastern US and begs the questions “should I grid sample and variable rate my lime applications?” Spoiler alert: yes,  I think you should !  Another sampling variable is depth. In general, it is recommended to sample to “plow depth” or shallower with less tillage. But another source of variation is “temporal” or with time. I am not sure how many people realize that due to the nature of our soils (sandy and poorly buffered), the way most labs measure soil pH, and the weather (basically the rainfall situation), soil pH can vary “naturally” over the course of a calendar year on Coastal Plain soils.  This “natural variation” can be as much as 0.5 points in pH — a fairly big difference considering the pH scale is “logarithmic” i.e a 5 is 10 times lower than a 6 but a 4 is 100 times lower than a 6 !  Soil pH is usually lowest in the Fall (think November around cotton harvest) but goes up naturally in the “winter” (think January and February).  This gets tricky to explain but basically, soil pH is measured using a meter based on electrical conductivity which can be affected by how much salt is in solution (more salt, more conductivity). Salts have a tendency to leach out of our sandy soils during the winter time and can give a “false high” pH reading, i.e. you think you are better off than you really are.  This is not a widespread problem in my opinion but seems to be getting more common every year. The only way to avoid this problem is to sample in the Fall when it is “dryer” or have the soil sample tested for pH in salt rather than water. To my knowledge, the only soil lab that currently measures soil pH in a salt solution rather than water is the UGA Soil Lab in Athens.

Potassium is the other “weakest link” on your soil test report form, in my opinion.  Across the board, we seem to have more problems with potassium deficiency than any other nutrient in Georgia. Unlike soil pH though, I don’t think there are any issues on how potassium is measured in the lab.  Like pH, spatial variation and sampling depth can play a role when taking soil samples. Again, I would recommend looking into precision ag soil sampling and variable rate potassium fertilizer applications to tackle ‘weak spots” in your fields.  Unfortunately, there doesn’t seem to be any new, “magic bullets” to solve our current potassium issues.  Split applications are getting a lot of attention, but recent research hasn’t shown any benefits in terms of cotton yield between putting all your potash out at planting verses splitting it up between planting and sidedress and even late applications.  The amount or rate of potassium fertilizer recommended can vary between soils labs. But even the rates recommended by UGA , which some consider too low, have been field tested in recent years and have proven not to be a yield limiting (by yield goal). Another possible cause of potassium issues may be high soil test calcium levels. These are usually seen with high soil pHs. Low soil pH is usually our issue on our sandy soil but in recent years I have seen quite a few high soil pHs and subsequent high soil calcium levels.  Calcium (and magnesium too) can compete with potassium for sites on the cation exchange capacity (CEC) of soil. Basically, if you “flood” our low CEC soils with calcium, potassium doesn’t have a seat at the table or an open chair in musical chairs which makes it more susceptible to leaching out of the roots zone and unavailable for uptake by cotton roots. And finally, there are other factors that can limit the uptake of potassium even when you have plenty in the soil between soil test potassium and fertilizer potassium applied.  Nematodes are a good example.  It can be very frustrating when you do a good job with potassium fertilization but then find out you have a nematode problem that leads to potassium deficiency and reduced yields. Unfortunately you cannot “fertilize” out of a nematode problem, you have to take care of the nematodes first.

Obviously there are other nutrients on your soil test report form that you have to pay attention to and make sure aren’t the weakest link.  Nitrogen would probably be next line for me, but overall I don’t see near as many nitrogen problems as pH and potassium on Georgia cotton. When I first started doing Extension meetings in 1994, I would show a “fertilization strategy” for each row crop. They all start with soil testing and include soil pH and “N-P-K” management as key components.  I am going to miss seeing cotton producers in person this winter due to Covid, but I hope you tune into the “Zoom” meetings sponsored by the Georgia Cotton Commission. Don’t be afraid to ask questions after our Zoom talks (me or other specialists). Or contact your local county agent if you need to follow up on something that will prevent pH and K from being a “weakest link”!