Recycling sulfur with Brassica cover crops

S def wheat (Montana State)

Sulfur deficiency in wheat (left). Photo credit: Montana State University

Among the macronutrients, sulfur (S) doesn’t get a lot of attention. Before coal-fired power plants were forced to clean up their emissions, atmospheric deposition of S each year far exceeded most crops’ needs in most areas. The downside of cleaner air is increasing occurrence of S deficiencies. This can lower protein quality (see graph at bottom of post), bread wheat quality, and yield of many crops. It’s a downside most of us happily accept, but it does raise the issue of how to maintain adequate S levels on farms and cycle S most efficiently.

Sulfur behavior

Sulfur is a little like nitrogen (N) for several reasons. Sulfate, the dominant mineral form of S in soil, is negatively charged (anionic), so it leaches through surface soil horizons that mostly hold positively charged ions (cations). Sulfur can therefore accumulate deep in the soil profile, out of reach of most crops. Like N, S also takes flight sometimes as a gas. Unlike N, S is not mobile in plant tissue, so symptoms are observed in younger, not older, leaves. Last year, many farmers in Delaware were taken aback by the interveinal chlorosis of corn they saw early in the season, which turned out to be S deficiency.

Sulfur deficiency can be distinguished from nitrogen deficiency by location of the chlorosis. S deficiency appears on younger leaves.

Sulfur deficiency can be distinguished from nitrogen deficiency by location of the chlorosis. S deficiency appears on younger leaves. Photo credit: Ray Weil

Brassica cover crops can recycle sulfur

cover crop roots soil pit

Forage radish roots capture nutrients, including sulfur, from deep in the soil profile.

Brassicas have very high S content. Those bitter flavors when you crunch into a radish or mustard are compounds called glucosinolates, and they are loaded with S. These are also the compounds that have sparked interest in using cover crops like mustard for biofumigation.   Just as forage radish can capture deep N, it can capture deep S and bring it to the soil surface. A good forage radish cover crop can contain 40 lb S per acre in its biomass.

Availability of captured sulfur

The S captured by Brassica cover crops is rapidly available when the tissue decomposes. rotten radishes APIn our research, a substantial increase in surface soil sulfate was measured soon after winterkill in February in Maryland and persisted into May (showing that not all of the S is lost as gas, despite a couple days of stinkiness that may accompany radish decomposition). The increase in surface soil sulfate can be especially important in no-till soils, where S deficiencies are often observed early in the season, before the crop roots have reached deeper pools of S.

Sulfur can’t be “fixed” from the atmosphere

Growing legumes is a great way to replenish soil N, but there is no comparable way to replenish soil S. Mineral S additions as gypsum, elemental S, sul-po-mag, or organic S additions with manure and compost will be necessary to maintain S levels in the clean air future, especially if many high-S cash crops (like Brassicas) are grown. Using cover crops efficiently and effectively may reduce the amount of inputs needed by keeping S closer to the soil surface.

To get deep root growth and nutrient capture, early seeding of Brassica cover crops is necessary. It’s too late to get good root growth in New England, but it’s not too late in the mid-Atlantic. But don’t fret, that deep S pool will probably be there next year… it’s not quite as leachable as N.

Sulfur deficient wheat in sandy soil on Maryland Coastal Plain. Photo credit: Ray  Weil.

Sulfur deficient wheat in sandy soil on Maryland Coastal Plain. Some farmers have reported increased winter wheat yields when forage radish is seeded at ~1 lb per acre with the wheat. Photo credit: Ray Weil

Total sulfur content of soybeans is related to the S-containing amino acids cysteine and methionine. Increasing these two amino acids increases the protein quality. Data: Ray Weil

Total sulfur content of soybeans is related to the S-containing amino acids cysteine and methionine. Increasing these two amino acids increases the protein quality. Data: Ray Weil

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