Many people think supplemental N isn’t needed on soybeans because the plant fixes its own N from the atmosphere. While soybeans do naturally fix about 50% of the N they need, supplemental N is necessary to close the gap to achieve maximum yields. The higher the yield potential, the greater the gap.
The soybean plant needs S just as much as N, but stricter EPA emissions regulations have minimized atmospheric S depositions. In fact, most soils across the U.S. are currently receiving no more than about 5 lbs. S per acre from the sky. The lack of S available to the plant can become problematic if not addressed, as S is essential in the synthesis of S-amino acids and proteins, chlorophyll formation, and nodulation.
The application of N and S at the time of planting gives soybeans the jump start they need early in the spring. In high yield environments (>70 bu/acre), an additional N and S application may be necessary later in the season to make sure these nutrients are not limiting during pod filling.
It’s critical that both N and S are available to the plant throughout the season in order to ensure the plant achieves maximum yield and produces high-quality soybeans. Both nutrients are vital to photosynthesis, thus impacting yield. They are also building blocks of amino acids, which impacts soybean quality.
Provide what your seeds need
Today’s high yield soybean varieties require more N and S than ever before to maximize yield potential and ensure the plant is healthy throughout the season. It’s important to work with your agronomic advisor on developing a plan that will balance your crop’s nutrition with varieties and management practices to maximize yield potential. These varieties not only have higher overall S needs, but also half of their total S needs are required during the reproductive stages (similar to the case of N).
Mobile Nutrients Require More Planning
Recent wet years have increased the likelihood of leaching losses. Because S and N are both mobile within the soil, it’s critical to manage these nutrients in each field, particularly on coarser soils following a rainy season or high yielding harvest.
Organic Matter (OM) is the main reserve of both N and S within the soil, and S-responsive soils have traditionally been associated with low OM levels. Recently, however, yield responses to S have been showing up even in heavier textured soils with higher OM levels as a result of less “free” S from atmospheric depositions and higher yielding harvests. Yield responses to S can also be influenced by the effects of colder weather on mineralization as a result of earlier planting or by N and S immobilization by cover crops.
Know your starting point and work with your advisor to understand what your limiting nutrients are within each field. Ensure that soil pH is within the optimum range to make the most out of all nutrients: those already in the soil and those applied via fertilization.
While fields receiving S fertilization have traditionally been lower organic matter and coarser soils, in recent years S deficiencies have been recorded in higher organic matter and heavier soils, mainly as a result of cleaner air and higher yields.
The higher the yield potential of the chosen variety, the more opportunity to gain ROI from a second AMS application as the crop approaches reproductive stages.
With clean air regulations, total S depositions have declined from 20 lb S/acre to ~0, but a 75 bu/acre crop needs ~15 lb S/acre. This is a critical gap in many soils. Notably, 20-25 lb N/acre is all that is needed to boost early growth and/or to complement soil N and biological N during reproductive stages.
- Soybeans can benefit from S and N applied within 2-3 weeks of planting. S is essential for biological N fixation, and N can boost early growth while nodulation establishes.
- Soybeans may benefit from N and S applied at bloom or shortly after – soil N plus biological N fixation may not be enough to grow 75+ bu/acre, plus 50% the N and S is taken up after flowering.
A 100% sulfate-S source can be applied as a granule on the soil surface and will be immediately available for root uptake with the first rain. An N source that resists volatilization loss, such as AMS, may be broadcast- applied over previous crop residue or over the soybean field.
- Sulf-N® resists N loss and provides immediately-available S.
- Soybean field soybean studies have shown S responses of up to 13 bu/acre.
- There is no need to incorporate into the soil; it will solubilize and become available to roots with the first rain.
- It is proven to be 2X more efficient than fertilizer applications composed of 50% of S in the elemental form.