Why Sulfur Matters for Organic Soybean Success
Organic farming systems are often considered low-input due to the higher costs, limited availability, and challenges associated with applying organic fertilizers. Additionally, many farmers are hesitant to invest in inputs due to uncertainty about the return on investment. However, in legume production—particularly with soybeans—supplying essential nutrients to deficient soils can lead to significant economic benefits, including improved yields and higher feed quality for the farm.
Soybean vs. Alfalfa: Fertilization Considerations
Soybeans are often overlooked in nutrient management discussions, especially when compared to alfalfa. Alfalfa typically receives more attention because it offers more immediate returns from fertilization, with easily verifiable results through weighed yields and regular feed testing. This makes it easier to assess the economic impact of inputs in alfalfa production, while the benefits for soybeans may not be as immediately apparent.
Identifying Nutrient Deficiencies: Potassium, Phosphorus, Sulfur, and Boron
Potassium and phosphorus are generally easier to identify as deficient through regular soil testing. Extensive research supports the correlation between their application and increased bushels per acre relative to input costs. In contrast, soil tests taken at multi-year intervals are less effective at predicting deficiencies in sulfur and boron, as these nutrients are highly mobile in the soil and prone to leaching.
For sulfur and boron, a more comprehensive approach is needed to accurately determine the appropriate application rates.
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Factors Influencing Sulfur and Boron Availability
Approximately 95% of the total sulfur and boron in the soil is contained within the soil organic matter. The availability of these nutrients is therefore highly dependent on several key factors, including:
a.) Total organic matter
b.) Soil type
c.) Rate of decomposition of organic matter
d.) Rate of decomposition of crop residue
e.) Volume of slow-release sources used previously in the rotation
Soil Test Interpretation and Sulfur Release:
Previous soil test results can serve as valuable indicators of the expected nutrient release from a given soil on an annual basis, as long as they are interpreted with the aforementioned factors in mind. With approximately 3-5 lbs of sulfur released per point of organic matter from an average soil, the total organic matter content on a soil test can also help determine whether additional sulfur is necessary.
Soil Variability and Sulfur Response:
In Minnesota, soils with over 4% organic matter typically do not show a yield response to sulfur applications, although this can vary based on soil sample variability and whether the residue has a high carbon-to-nitrogen/sulfur ratio, which may immobilize sulfur similar to nitrogen. This factor explains why soybeans in a rotation with heavy corn residue may show sulfur deficiency, whereas in the same field, corn following a low-residue crop like soybeans or a harvested small grain (with more time for residue breakdown) does not.
Sulfur Management for Soybeans:
To optimize organic soybean success defined by the yield, total sulfur from various sources should ideally exceed 25 lbs per acre in a given year. Sulfur deficiencies can be identified through visual symptoms, such as lighter-colored leaves that may turn yellow in severe cases, as well as through tissue testing. Common sources of supplemental sulfur include Potassium Sulfate, Gypsum, K-Mag, and Elemental Sulfur. The choice of source depends on the soil’s other nutrient deficiencies and the desired release speed for the current growing season. Here is a link to Organic Dry Fertilizers.
In organic rotations, where dry fertilizer applications are less frequent, adding boron to the nutrient blend is often recommended, as it can help maintain adequate levels for the next couple of years.
Addressing Potassium, Calcium, and Aphid Pressure:
Addressing potassium and calcium deficiencies is also crucial, as research has shown that both are linked to increased aphid pressure when lacking. With limited options for aphid control in organic systems, fostering plant health to naturally resist insect pressure is key to boosting yields and maximizing bushels at harvest.
Beyond NPK: Why Sulfur Matters for Organic Soybean Success – was written by James Schroepfer, Lead Agronomist and COO of Crop Fertility Services. His expertise and contributions have been instrumental in shaping the content and ensuring its relevance to current organic agricultural practices. Crop Fertility Services provides Agronomy Services as well as Organic Dry Fertilizers to address soil deficiencies to improve plant health and overall yields.
