Improved nitrogen testing may be around the corner

Ontario’s corn crop is past the halfway point at this time and is left to Mother Nature’s will to help mature the corn and give us a decent harvest. For the most part, that means we can look back at what 2022 brought us and how we can adjust and improve our field and nutrient fertility plan moving forward.

One of the biggest challenges that corn fertility planning faces is with nitrogen (N) (see image at top). The right rate of N is dependent on so many other factors that it remains one of the hardest to predict in any given year. Too much N leads to higher chance of losses to the environment, and too little N prevents a crop from maximizing yield, economic return, or both. One of the most difficult aspects of N rate planning is accounting for the plant available nitrogen that is mineralized in the soil and is accessible to the crop through the growing season.

Water-extractable mineral nitrogen (WEMN) testing has shown to be a strong indicator of the soil’s ability to provide plant available nitrogen to a corn crop in Ontario, according to a recently published study.

Research completed by Stoeckli et al., and funded by the Grain Farmers of Ontario, indicates that Ontario corn producers may soon have a more reliable tool for N fertilizer rate recommendations ahead of planting.

Currently, corn growers have a few tools at their disposal for N fertilizer recommendations, including the Pre-Plant Nitrate Test (PPNT), Pre-Sidedress Nitrate Test (PSNT), and the Ontario Corn N Calculator, among others. The PPNT and PSNT involve nitrate testing of the soil, and can improve accuracy of N recommendations, but can be limited by variables like early season rainfall or timing and method of testing in-field. PPNT and PSNT recommendations are also calibrated to a fertilization system that has not yet received broadcast N fertilizer and doesn’t have organic sources of N such as manure or legumes in the previous year. When used in situations that don’t meet these criteria, it’s very difficult to identify where the soil test nitrate value is coming from, making it difficult to adjust N fertilization rates. 

The study evaluated different methods of measuring nitrogen and carbon portions of the soil in lab-based testing. Soil cores were taken to a 30cm (12”) depth consistent with standard N sampling procedures. Researchers compared the soil test results from the lab to yield and Maximum Economic Rate of Nitrogen (MERN) to identify correlation. The study was conducted on 13 different sites over 2 years within Ontario corn fields with a history of synthetic N fertilizer use, with most sites being part of OMAFRA’s corn response trials.

Water-extractable mineral nitrogen is the readily available form of N in the soil solution. WEMN testing is performed in the laboratory using room-temperature water, as the name suggests, instead of a Potassium Chloride (KCl) solution that is used in PPNT and PSNT tests. Among the lab-based testing done on these sites, WEMN showed the strongest correlation with yield and MERN, outperforming tests looking at the organic-N portion of the soil. 

Soil texture has a great impact in the availability of nitrogen in the soil. Heavier clay soils were found to have lower N availability to the corn crop in season, when compared to coarser textured soils. Soil texture leads to differences in N loss pathways as well, where fine-textured clay soils are less prone to leach nitrogen, but more likely to have denitrification occur in wet conditions.

However, WEMN tests showed consistent results across soil types, leading researchers to conclude that this testing should be suitable for use throughout the highly variable soils across Ontario. Since the research focused on fields receiving synthetic N fertilizer, fields receiving a high amount of their plant nutrition from organic sources like manure may not follow the same trend with WEMN testing. Nevertheless, for farmers without livestock, this testing could be a game-changer in the future.

Part of the reason for uncertainty with pre-plant N recommendations is because of nitrogen’s relationship with water. Nitrogen moves into the plants as nitrate or ammonium in a solution with water in the soil. In very dry conditions, if the plant can’t access water, it won’t be able to utilize the N, no matter how much is in the soil.

The next step for this research should involve a multi-year study looking at the relationship between WEMN and N fertilizer recommendations, with more field sites to achieve proper calibration throughout Ontario soils. Still, this gives hope that Ontario agriculture may soon have a more reliable way of generating corn N recommendations on an individual field level.

With an ever-increasing focus on reducing nutrient losses from the field, as well as reducing greenhouse gas emissions from our fertilizers, fine-tuning N recommendations for corn using WEMN tests could optimize N rates for the growing crop and reduce the risk of adverse environmental effects. This includes limiting the amount of residual Nitrate-N left in the soil after corn harvest, which would improve Nitrogen Use Efficiency.

At the same time, an improved N fertilizer calculation could increase yield and profitability, especially in times of volatile input costs and grain markets. Better determination of N fertilization rates helps improve the sustainability of Ontario’s corn production. As the research continues, Ontario’s farmers can hope to put together one more piece of the nitrogen fertility puzzle in the coming years.

– Click here to read the August 3 field crop report at the Field Crop News website.