January 12, 2021 - 8:00am -- lehman.488@osu.edu

In order to grow most crops, fertilizer must be applied to produce the most profitable yield. Often, we see manure and manufactured fertilizers used together. In 2020, Rory Lewandowski (retired Ag Educator) investigated the possibility of reducing purchased fertilizer by accounting for available nutrients in the soil. The goal was to reduce fertilizer costs without reducing yield, thus increasing profits.

In decades past, fertilizer application was often pushed to produce the maximum yield. However, there is always a point at which an extra pound of fertilizer will not generate enough extra yield (if any) to pay for that fertilizer. In the new 2020 Tri-State Fertilizer Guide released by Michigan State, Purdue, and Ohio State Universities, the recommendations all focus on maximizing profitability.

Beyond unnecessary expense, another reason to avoid overapplying fertilizer is tied to waterway pollution. Unfortunately, some nutrients will escape our soils regardless of the level of management. Some slowly leach down through the soil until they are out of reach of plant roots. Others may get washed away in runoff after heavy rain events. Nutrients that leave our fields are dollars washed down the drain.

Here are a few options to help reduce nutrient loss. Split fertilizer applications, manure incorporation directly or soon after application, and maintaining a growing crop year-round all help to prevent nutrient loss. Taking and then using soil test results are also key in reducing nutrient losses.

Now, let’s discuss Rory’s field research plots, which only focused on nitrogen levels in the soil (but the principle also applies to phosphorus and potassium). The maximum return to total nitrogen (MRTN) describes the amount of needed nitrogen fertilizer to produce a yield with the greatest return over costs, or maximum profit. Online calculators are available to generate application rates and are based on nutrient costs and projected grain prices.

Easy enough, right? So, if the calculator says I need 175 lbs. of nitrogen per acre I just calibrate the planter and apply 175 pounds. Hold on one second, though. Do you know what is already available in the soil? Consider the following common scenarios:

You applied manure last year after harvest. Those nutrients, especially if you incorporated them into the soil, will be there this spring at planting. Or, you are planting a cereal grain crop into a field that has been in alfalfa or pasture for a number of years? Again, nitrogen credits may be there.

So, before planting in 2020, Rory took soil samples and had a pre-sidedress nitrogen test (PSNT) run at the lab to determine the available nitrogen in the soil. The analysis showed that there were 125 pounds of available nitrogen per acre due to previous years of manure application. The MRTN was calculated at 175lbs.

Part of factoring in the PSNT as part of the MRTN was to investigate the value of the nitrogen credits as a starter fertilizer- so in several plots, no nitrogen was applied at planting.

This experiment included 5 different treatments. Three of them ignored the available nitrogen credits (as if no soil test was taken). One applied the full MRTN (175lbs.) of purchased nitrogen at plant, another applied the full rate as a sidedress several weeks later at V7, and the third split the MRTN between plant and V7.

The remaining 2 factored in the PSNT and only 50 pounds of nitrogen was applied (125 PSNT + 50 = 175MRTN). The first applied the 50 pounds at plant and the second at V7. Accounting for the nitrogen credit reduced fertilizer costs by $45 ($0.36/lb.) per acre compared to the other 3 treatments. Corn prices were estimated at $3.70/bushel.

The Results

When the PSNT was ignored and the full 175lbs. of nitrogen were applied at planting, return over cost of nitrogen was lowest. When PSNT was accounted for and the remaining MRTN of 50 pounds was applied at V7, yields were slightly less than the highest (196 bu/acre compared to 203), but the savings on purchased nitrogen still netted an additional $19 more revenue per acre than the highest yielding treatments.

In short, when PSNT is relatively high (in this case over half of the needed nitrogen) applying a starter fertilizer, at plant, negatively affected profitability. Next, accounting for PSNT can save money when purchasing fertilizer while still producing yields that are equally or more profitable than purchasing full rates of fertilizer. Last, the highest yield does not always produce the fattest wallet.


Matthew Nussbaum is an OSU Extension Agriculture and Natural Resources Program Assistant and may be reached at 330-264-8722.

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