Crop Comments: Autumn is the best time to soil test

This title is the opinion of my friend, Certified Crop Advisor Tom Kilcer. I’ll explain the logic behind his statement.

He said with the current geopolitical situation, the supply of available fertilizer has been squeezed. Consequently, the price of fertilizer ingredients has increased, threatening growers’ profits. But there are ways to offset this dilemma, the first of which is a soil test. If you’re putting fertilizer on a field, you need to know what the limiting factors are for the nutrients required by this crop.

Tom and I are both stunned at the number of acres planted where the farmer or the fertilizer salesperson guessed at what they need. Quoting Kilcer directly (from his October 2026 Crop Soil News, at advancedagsys.com), “Yes, it takes time, but the result is that you know what fertilizer nutrient, and how much of it, is needed for economic return.”

He talks mostly about phosphorus (P). He stressed with “very low soil test levels of P, there is a 95% chance that you will get back what you paid to apply that fertilizer nutrient, netting a profit afterwards.”

At a very high P soil test, the opposite occurs, and there is a 95% chance that you won’t even get the money you spent on the crop back the next year. Soil tests will tell you the difference. Tom said, “Fall is the best time to test, as you are also measuring the results of all the nutrients the crops of that season removed. Here’s a classic example that Cornell and Extension in New York found with statewide research: When soils have enough P to reach high or very high levels, there is no response in corn from putting additional P on at planting time. Many of our dairy farms are adding nearly 150 lbs. of P/acre each year from a typical 30 tons of manure applied per acre.”

This accumulates in the soil, giving the high soil test levels. The plants have all they need for maximum growth. The recommendation was also given that a starter is still needed. The difference is that the starter can be just nitrogen (N), applying 30 lbs. of commercial N in the band.

What about cold, wet, early-planted fields? A number of years ago Kilcer and his crew had a May that started warm and dropped temperature-wise as the month progressed. Tom said, “The deluge of cold rain on Mother’s Day started the slide that culminated in five to six inches of snow at May’s end. Research on cold, wet, low-medium P-testing soils showed a response for applying this nutrient. In 2002, corn silage field trials were conducted by Quirine Ketterings, Ph.D. (Cornell), in nine New York counties. She was able to document a significant response to a starter fertilizer containing N. With these field trials, cooperating farmers enjoyed increased yield and made money with P banded on low-medium P-testing fields.”

Due to cold, wet weather, in this experiment there was no response to P on these soils testing high in this nutrient. This lack of response occurred in both silage and grain corn crops. The bottom line is to buy what will give you a return, not just because “this is what we always did.” The best way to know is to soil test. Most states have university soil test labs that can give you unbiased recommendations for your fields.

A frequently underestimated source of fertilizer is livestock manure. Daily spreading on top maximizes its loss, due to denitrification or volatilization of N. Erosion over the four to five months of colder weather moves this top-spread fertilizer to nearby streams or hedgerows. Simply injecting it a couple inches into the soil profile allows the ammonia and other nutrients to react and stick to the soil biomass.

If the soil temperature is below 55º F, this physical introduction will hold the N until spring rewarms the soil. Kilcer showed that with winter triticale forage, injecting 10,000 – 15,000 gallons of liquid manure in November or December, with rolling coulter injectors, can meet all the N, P and potassium (K) needs of a four-ton dry matter crop the next spring without buying any fertilizer.

Quoting Tom again: “We have farms that inject manure in the spring that also meets all the N, P and K needs of the crop. On several farms, we have calculated that they paid for the manure injectors in one year of fertilizer savings!”

Fields that have had multiple years of manure but were only harvested for grain have nearly all the K returned to the soil. Fertilizer nutrients applied after manure applications without a soil sample will be off-target relative to the response growers hoped for.

Another cost for guessing at soil fertility is caused by the unknown, but very real, high P levels tying up zinc, which often is marginal to start with. Thus a zinc deficiency commonly starts showing up due to too much fertilizer P.

We can’t balance the soil without knowing what nutrients are already available. We need to look at all field factors to increase our yields. Response to purchased fertilizer applied to fields with high nutrient levels will be slim to none. Kilcer recommended buying what soil tests say will give you an economic response.

Finally, for dairy/livestock farms, simple crop rotation goes a long way in reducing the fertilizer bill. Injecting manure at the end of the last sod season will ensure P and K remain at high to very high levels. Killing the stand in October sets the field up for early no-till planting the next spring; then growers only need starter N.

The bonus is that first-year cornfields enjoy the rotation benefit of 15% – 20% higher yields. Autumn-killed and then no-tilled in spring has shown 20% – 30% yield gains. Following a comprehensive rotation for every field will move manure nutrients around and increase yields while saving on fertilizer.

My suggestion is for growers to sample as many fields as possible this autumn, specifically sites that haven’t been tested in the last three years. The Dairy One Lab in Ithaca gives three years’ worth of recommendations.

by Paris Reidhead