Neal Kinsey has heard some people say that the purpose of a soil test is just to point you in the right direction.
“I don’t think that’s good enough,” says the Charleston, Mo., agronomist and fertility expert. “You don’t want to be pointed in the right direction, you want to know what to do.”
That starts with taking a good soil sample, which means taking it “in such a way that it truly represents the area in question.” If a field has a lot of variation — enough to justify fertilizing different areas separately — then those areas need to be sampled separately.
“If you don’t take the sample right, the answer you get is going to be wrong.”
Think of fertilizing like buying a car, Kinsey says. You can buy a cheap, simple car, or you can buy an expensive fancy sports car. Both will get you where you need to go, but you might end up spending a lot more on features you don’t actually need or skimping on features you should have spent more on.
“You can spend a bunch of money on fertility,” he says. “But what you want to do is put the money where it makes the most difference. Until you have a soil test that will give you that kind of guidance, how do you know what to do?”
Highlighting specific lines from two real-life soil tests, Kinsey explains how an accurate soil test can provide guidance on exactly what is needed to achieve optimum crop health and yield and what his recommendations are for fertilizing those soils. The featured tests are from different geographical locations and represent different soil types.
An Exchange Capacity of 22.24 indicates a relatively
heavy clay soil.
At 1 ppm, there’s at most 2 pounds of sulfur per acre. Absolute minimum should be 20 ppm, but for a yield of 175 bushels per acre of corn, consider at least 50 ppm. Kinsey recommends supplying 80 pounds of sulfur for 225 bushels per acre or more. This could be achieved with 250 pounds of ammonium sulfate, which would supply 60 pounds, along with another 20 pounds of elemental sulfur.
Potassium on this soil is likely the most limiting factor for corn aside from nitrogen. Kinsey recommends 200 pounds of potassium chloride (0-0-60) as a minimum and for yields over 200 bushels per acre, apply 300 pounds per acre until K levels stay above 4%.
- Boron needs 2 pounds applied to bring it from 0.79 ppm to the minimum 0.8 ppm necessary. If you don’t keep boron above 0.8 ppm, Kinsey says, it won’t fill out the kernels at the top of
the corn cob.
- Some believe kernels not filling out is due to low phosphorus. But if you have good, straight corn rows and your soil test indicates you have enough phosphorus, then it’s likely your boron is below 0.8. However, you need sufficient phosphorus or boron won’t work.
- Ideally, boron should be 1.5-2 ppm to help prevent rust and fungus diseases. In a heavy clay soil like this, you can build boron levels. But in light sandy soils, you may never get there, Kinsey says.
Copper needs to be above 2 ppm or you’ll get rust in your wheat. Here Kinsey would apply 20 pounds per acre of 23% copper sulfate. That amount won’t show much of a difference on the soil test in the first year, but that’s what’s needed to start correcting it.
Exchange capacity of 8.98 indicates a sandy loam soil.
Good pH of 6.2.
Humus is low at 2.1. Anything below 2.5 puts the soil microbes on a starvation diet.
Line 14: Potassium
At 6.95%, potassium is great. Kinsey says he likes to see potassium above 5% on heavy soils and close to 7% for sandy soils.
Line 14: Magnesium
- At 18%, magnesium is too high. If you go above 12%, the soil will begin to tighten because magnesium disperses the clay particles in the soil, and when they spread out, it tightens the pore space.
- Magnesium is also key to nitrogen utilization; it’s the center of the chlorophyll the nitrogen attaches around. High magnesium will result in less response from nitrogen because it’s antagonistic to it.
- Even if you have excess magnesium in the soil, your plants may show a deficiency. Foliar magnesium is likely needed on this crop — about 5-5.5 pounds of magnesium sulfate sprayed three or four times at 3-4 week intervals. But once you get it below 12%, it won’t be a problem.
- Here we want to get boron up to 1.5 ppm. Every pound of boron applied will raise the level by 0.1 ppm, at best, so this soil needs 3.5 pounds of boron.
- You don’t want to apply that much boron at once, though, unless your calcium is in good shape — above 60% (see line 14). The lower your percentage of calcium, the harder boron is on the crop.