There is increasing interest in applying sulfur ahead of crops to increase yield. It is commonly known that the conversion of nutrients from one form to another in the soil can create acidity. Most common sources of sulfur can result in greater acidification of the soil profile which can have a negative impact on soybean over time. For example, it is reported that 5.4 lbs of calcium carbonate is needed to neutralize the acidity produced through the nitrification of 1 lb on N contained in ammonium sulfate. In addition, 3 lbs of calcium carbonate are needed to neutralize the acidity produced through the oxidation of 1 lb of S as elemental sulfur. Other nitrogen sources such as anhydrous ammonium, urea, and P fertilizer sources such as MAP and DAP can also produce acidity. I am increasingly finding surface soil pH less than 6.0 which is considered optimal for corn and soybean production. With more sulfur being applied, are soybean growers creating issues where surface soil pH is decreasing more rapidly resulting in the potential for lost yield?
Limestone is used to correct soil acidity, but limestone is not always easily available or cost effective to apply for some soybean growers. Pelletized lime (pell-lime) is available at a higher cost, and it can be mixed with granular fertilizer. Other soil fertility researchers and I have discussed whether correcting the acidity in a smaller area of the soil, such as a band of fertilizer, may be sufficient for most crops to maximize nutrient uptake. With RTK guidance being more common it is relatively easy to band lower rates of nutrients and then plant over top the bands so newly emerging roots have access to the nutrients applied and not all the soil needs be fertilized. While a fertilizer band may be mixed with more aggressive tillage, repeated application in the same area could give a more optimal zone for nutrient uptake for crops that over time may increase yield with repeated applications. I have also received some comments from crop consultants on the benefits of banding sulfur. Combining the pell-lime with a band application of fertilizer could slow soil acidification reducing the needs for high rates of crushed limestone creating a zone where nutrient availability is increased. The research needs to be assessed over several years as the change in soil pH is not rapid and may take some time before benefits can be achieved. Since it is most common for sulfur as well as other forms of fertilizer to be applied ahead of corn in a two-year corn-soybean rotation, treatments should be focused on the corn side of the rotation while measuring the follow up effects on soybean to get a full picture of rotational benefits.
Current limestone guidelines are based on the Sikora buffer. Historical lime guidelines were based on the SMP buffer index and when changed it was assumed the Sikora and SMP buffers would return the same results. Changes were made to the current guidelines based on data from neighboring states so local data would be beneficial to re-evaluate the current lime guidelines. Little work on lime application has been conducted in the past 15 years in Minnesota even though questions arise as to the economic benefits, specifically to soybean which is thought to be more sensitive to low soil pH. Minnesota is divided into two zones that differ based on subsoil pH. Area 1 constitutes the eastern part of Minnesota where subsoil pH is generally acid due to carbonate layers that are deeper in the soil profile and Area 2 where subsoil pH is higher due to shallower carbonate layers. Even though subsoil pH values are higher in Area 2 the surface soil pH can be very low. However, past research has not demonstrated large economic benefits to lime for Area 2. The result of this project should provide evidence whether the current lime calibrations are correct and give some direct evidence on the economic benefits to lime application. I am also looking to generate data on the potential to acidify soils with a high soil pH. Some suggest acidifying soils as a method to reduce IDC severity. The buffering capacity of most soils makes acidification generally not feasible and likely cost prohibitive to lower soil pH. Data could be generated in a lab setting to provide Minnesota soybean growers whether soil acidification is practical.
The fourth objective of this project is to establish some product testing trials, specifically testing in-furrow or near-seed liquid fertilizer application to soybeans. While not suggested I do receive questions from soybean growers about liquid fertilizer and whether new placement methods or low-salt sources could be used to increase soybean yield. Growers trying to reduce costs that do not want to broadcast fertilizer and have the option for in-furrow placement would benefit from additional information regarding different placement options. I am specifically interested in testing the Furrow-jet system offered by precision planting which places fertilizer off to the side of the seed which may reduce the potential for seedling damage and reduced emergence. It is much easier for me to test these types of systems instead of a soybean grower who is at greater risk for loss should the fertilizer and placement significantly reduce emergence. There is data on the negative impacts of in-furrow fertilizer application to soybeans collected in Minnesota about 20 years ago, the Furrow-Jet system was not available at that point in time. The goal of this project would be to evaluate emergence and yield effects on soybeans grown in medium to slightly high soil test P and K concentrations.
The fifth objective is to look more into nitrogen cycling in soils and how soybean impacts nitrogen availability to future crops. A study was established in 2022 to investigate how potassium and nitrogen may influence each other in soils. There exists in soils a pool of potassium and ammonium (nitrogen) that are considered non-exchangeable. The non-exchangeable pools are sometimes thought to be “fixed” and not available to crops. However, research on non-exchangeable potassium in Iowa has shown that what we consider to be not available may be influencing nutrient availability more than what was once considered. The non-exchangeable pool of potassium and ammonium exists in the soil between the outer layers of clay where pockets in these layers are the exact size to hold potassium or ammonium ions. Being between clay layers, ions between the layers can be held tightly if soil clay layers collapse yet these nutrients may slowly move into the soil solution as potassium or ammonium is depleted over time.
Non-exchangeable ammonium has been mostly overlooked by research and we do not know whether this fraction is a part of the nitrogen cycle that would supply nitrogen to a growing crop. A study was established in 2022 to look at how non-exchangeable ammonium is impacted by fertilizer application and how non-exchangeable ammonium concentrations change in the soil over time. What we have found is that high rates of nitrogen have a very small, if any, impact on non-exchangeable ammonium and the largest factor impacting non-exchangeable ammonium is time. At two locations we have found a large decrease in non-exchangeable ammonium from the first-year corn following soybean to when corn followed corn. We are wondering whether this decrease could be reflective of the increased need for higher rates of nitrogen when corn follows corn. What would be beneficial is knowing whether the non-exchangeable ammonium pool may be replenished in a year when soybean is grown as this could reflect what we would term to be the soybean N credit. This data could give us a much better understanding of how inorganic nitrogen pools affect each other in the soil on an annual basis.