By Brian Hefty
Most soil tests I look at have parts per million for N, P, and K. That’s a start, but that doesn’t tell you the whole story. To figure out if you actually have enough potassium, the test you NEED is called base saturation.
The base saturation test is simply the ratio of five nutrients (calcium, magnesium, potassium, hydrogen, and sodium) to each other. Each element is represented by a percentage figure, and the total of the five elements equals 100 percent. With potassium, unless your level is in the 4 to 8 percent range, the odds are extremely high that at some point during the growing season, plant tissue analysis will reveal your crop is low to deficient in potassium. Here’s why.
We often talk about how nutrients must be balanced in the soil, but how do you know the right “balance” of nutrients? That’s basically what base saturation is telling you. Let’s say your soil is ridiculously high in both calcium and magnesium (by the way, we have LOTS of those soils in North America). If your potassium is even at a moderate level, it isn’t in balance, and it can’t get into the plant at a quick enough pace to satisfy the plant’s demand.
We first discovered this issue on our farm about 12 or 15 years ago. We could walk out in fields that had “enough” parts per million according to most, yet I was seeing potassium deficiency on the leaves. Like we always say about SEEING a visual nutrient deficiency, there was obviously a huge lack of available potassium in the soil and tremendous yield loss. That corn not only yielded terribly, it was also low test weight, didn’t dry down well, and had lodging issues. These are all symptoms of low K, and low K levels hurt any crop you raise.
As soon as we then started tissue sampling, we realized the issue and started addressing it. The problem was, we still didn’t know about how high the base saturation test needed to be. Some people will claim that having 2 percent base saturation K is enough; but if it is, I haven’t seen that yet with any farmer we’ve ever worked with.
If you want to get your soil to 4 percent base saturation K, and it’s at 2 percent right now, that means you need to double the parts per million in your soil. We have done that slowly in some of our fields, and this fall we are finally fixing the last of our ground, but it’s not always cheap. In a couple cases this fall, we are investing $50 to $100 per acre to get our base saturation K up to 4 percent. The good news is once we’re at 4 percent, it’s easy to keep it there.
When you are soil sampling and fertilizing this fall and next spring, take a close look at your base saturation K. If it is below 4 percent, your odds are extremely high that applying additional K will give you a good return on investment until you reach that 4 percent level.