Guessing on Fertilizer Rates Costs Real Money, Soil Tests Show

At very low soil-test levels, a farmer has a 95% chance of profiting from applied fertilizer; at very high levels, the odds invert to a 95% chance the application will not pay for itself the following year. That probability spread, drawn from soil-test response data, frames the core argument of an October 27 advisory directed at growers who apply fertilizer — or sell it — without sampling first.

The advisory takes direct aim at rate-setting by guesswork, a practice it describes as widespread among both farmers and fertilizer salespeople. Soil testing costs time, but it identifies which nutrient limits yield and at what rate, which is the difference between a purchased input generating a return and one that does not.

What does the Cornell research show?

Cornell University researchers in New York documented that once soil phosphorus reaches high or very high test levels, corn shows no response to additional phosphorus applied at planting. Many New York dairy farms apply roughly 150 pounds of phosphorus per acre annually through a typical 30 tons of manure. That phosphorus accumulates in the soil, and the crop already has what it needs for maximum growth. Starter fertilizer remains necessary in these situations, but nitrogen alone suffices.

Cold, wet conditions shift the economics on lower-testing ground. Trials coordinated by Quine Ketterings of Cornell — 16 farm trials in 2002 across nine New York counties, conducted in a season where a warm early May slid into cold rain and 5 to 6 inches of snow by month's end — recorded high corn silage yields. On low-to-medium phosphorus soils, banded starter containing nitrogen increased yield and returned a profit. On high-phosphorus soils, the response to phosphorus was absent in both silage and grain, despite the cold weather.

How does manure management change the fertilizer bill?

Daily surface spreading of manure maximizes nutrient loss through denitrification (nitrogen escaping as gas) and volatilization of nitrogen, with winter erosion carrying fertilizer into streams and hedgerows. Injecting manure a couple of inches into the soil profile lets ammonia and other nutrients bind to the soil exchange sites. When soil temperature sits below 55 degrees F, that reaction holds the nitrogen until spring warming releases it.

Trials with winter triticale forage showed that injecting 10,000 to 15,000 gallons of manure in November or December, using rolling coulter injectors, met all nitrogen, phosphorus and potassium needs of a 4-ton dry matter crop the following spring without any purchased fertilizer. Spring injection has achieved the same full NPK replacement on several farms. On several operations, one year of fertilizer savings paid for the manure injector. Fields with multiple manure years harvested only for grain returned nearly all potassium to the soil.

What happens when fields go untested?

Applying fertilizer after manure without a soil sample produces results far from the expected response. Two managed fields sampled in 2012 and again in 2015, with manure applied in between, illustrate the drift. Field 13, previously neglected, moved from medium-low to high phosphorus. Field 9, already relatively high, reached a very high level and showed no response to additional phosphorus. The excess phosphorus also tied up zinc — marginal to begin with — and zinc deficiency appeared. The farm now samples yearly.

Can rotation cut the bill further?

For dairy and livestock farms, injecting manure at the end of the final sod season ensures phosphorus and potassium at high-to-very-high levels. Killing the stand in fall enables early no-till planting the following spring with only starter nitrogen required. First-year corn after sod carries a rotation effect worth 15% to 20% higher yields; fall-killed sod followed by spring no-till has shown 20% to 30% gains. A written rotation plan for every field moves manure nutrients across the operation while raising yields and cutting purchased fertilizer.