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 each access to the nutrients applied and not all the soil need 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 to 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 final goals of this project are to establish some product testing trials, specifically testing in-furrow or near-seed liquid fertilizer application to soybean. 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 the negative impacts of in-furrow fertilizer application to soybean 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 soybean grown in medium to slightly high soil test P and K concentrations.
This pre-proposal submitted to the Minnesota Soybean Research and Promotion council will continue new or on-going research project that were funded for 2023.