By: Jessica Wayment 

Nitrate leaching is a major concern in coarse-textured agricultural soils because it can cause economic losses for farmers and contaminate groundwater. While some nitrate leaching may be inevitable when growing corn on sandy soils, there are several management strategies that can be implemented to limit nitrate loss. Here are some key takeaways from a recent 5-year study looking at 3 major factors impacting nitrate leaching: drainage, nitrogen availability, and cropping system.

Drainage

Nitrate leaching occurs when rainfall, irrigation or the two combined exceed the water holding capacity of the soil and the excess water drains below the root zone, carrying nitrate with it. Climate models for Minnesota project a greater likelihood of extreme precipitation events during the spring and summer months. This means that it will become increasingly more challenging to ensure that precipitation and irrigation events do not overlap and exacerbate nitrate leaching. While we cannot do much about the rain, we can manage irrigation by applying only what the crop needs when the weather forecast demands it to minimize drainage. In our study on an irrigated sandy soil at the Rosholt Research Farm in Westport, Minn., from 2017 to 2021, we observed that 50% of the nitrate loss happened in May and June, 30% in July and August, and 20% in September and October. Nitrate loss was higher during the spring months because precipitation often exceeded crop needs. During the summer months, water and nitrogen use by an actively growing crop minimized the potential for nitrate loss.

Nitrogen availability

Another factor that affects nitrate leaching is the amount of nitrogen present in the soil. When nitrogen is applied at the rate suggested by the University of Minnesota’s corn fertilizer guidelines leaching potential decreases compared to when nitrogen is applied above (or below) the optimal rate. While you would assume that applying nitrogen below the optimum rate would reduce nitrate leaching, our study results showed that this reduced crop growth, water use and grain yield, which resulted in more water drainage and nitrate leaching. Applying more nitrogen than what the crop needs to optimize growth and yield results in reduced economic returns and more nitrate in the soil, which is available for leaching. Therefore, applying the optimal nitrogen rate is the best strategy to maximize profit and minimize nitrate leaching. An additional way to further reduce nitrate leaching is to split-apply nitrogen during the crop growth stages when the crop’s nitrogen uptake is highest. Previous research clearly show a consistent large yield benefit when split-applying nitrogen. For this reason, in this study we applied 25% of the total nitrogen at V2, 25% at V6, and the remaining 50% at V10.

Cropping System

Soils naturally convert organic nitrogen in the organic matter into inorganic nitrogen through mineralization. In irrigated sandy soils, this pool of nitrogen is much lower than other soil types and is not sufficient for corn production, so additional nitrogen inputs are needed to meet the crop’s nutritional demands. However, we learned from this study that nitrogen fertilizer is not the only factor influencing nitrate leaching. Annual cropping systems grown in sandy soils are simply “leaky.” The amount of nitrate leaching during the soybean year of a corn-soybean rotation when no nitrogen fertilizer is applied is similar to nitrate leaching observed during the corn year. Similarly, nitrate leaching was roughly the same in our continuous corn system compared to the corn-soybean rotation (even though, over two growing seasons, the amount of nitrogen applied at the economic optimum nitrogen rate for continuous corn is more than double the rate for corn following soybean).

Cover crops

Using cover crops may be a way to fix a leaky annual cropping system and improve soil health, but unfortunately cover crops are not a silver-bullet solution to our water quality problems. Two cover crops that have shown potential to reduce nitrate leaching are winter rye and kura clover. Rye is a winter-hardy annual grass and is one of the most common cover crops grown in Minnesota, and it grows well in sandy soils. It is typically planted in the fall and terminated in the spring before planting the cash crop. Kura clover is a perennial winter-hardy legume that is intercropped as a living mulch with strip-till after one to two years of fallow for establishment.

A well-established stand of rye can act as a nitrogen “scavenger” to reduce nitrate leaching while no cash crop is actively growing. Rye is considered low risk compared to other cover cropping systems because it has lower seed and management costs, and there is typically little impact on grain yield. There are two keys for proper rye management:

1. Get a good stand establishment in the fall: Planting rye as early as possible and ensuring good seed-to-soil contact are the best strategies to help get your rye cover crop off to a good start.
    
2. Don’t terminate the rye too late in the spring: The timing of rye termination in the spring is crucial because you want the nitrogen that the rye scavenged to be available in time for uptake by the cash crop. You need to give the terminated rye residue enough time to decompose and release the nitrogen back into the soil before the cash crop needs it.

As a perennial living mulch, kura clover requires at least as much, if not more, precise management than rye to reap all of the benefits it can offer. Nitrate leaching reduction from kura clover is closely correlated with reduced drainage. This is especially beneficial when extreme precipitation events occur. The more biomass in the stand of kura clover, the greater the drainage reduction benefit. That said, establishing a strong stand of kura clover without it competing with the cash crop for nutrients and water is tricky. Suppressing the kura clover stand too much can lower the cover crop’s capacity to reduce nitrate leaching, but if you let the stand get out of control it can significantly reduce grain yield. Thus, while cover crops can provide a way forward to produce annual crops with lower nitrate leaching in irrigated sands, proper management of these cropping systems takes time and skill.