Is it time to start thinking about adding sulphur?

Saturday, November 3, 2012

For the first time, analyses of plant samples from a number of alfalfa fields have been showing low sulphur levels. Is this an anomaly or the new normal?

by KEITH REID

Those who have followed my writings for the past couple of decades know that my message regarding sulphur has been pretty consistent:  "All of Ontario, except the far northwest, gets enough sulphur from air pollution, so we don't need to add any sulphur fertilizer."

Not everyone agreed with me, but the evidence I could find all supported this position.  Even canola, the crop with the greatest hunger for sulphur, only showed sporadic responses if more was added.  

This year, however, may mark a fundamental shift in this picture. For the first time, analyses of plant samples from a number of alfalfa fields have been showing low sulphur levels in the first cut. One field with a sulphur trial run by Bonnie Ball of the Ontario Ministry of Agriculture, Food and Rural Affairs and John Lauzon of the University of Guelph almost doubled the yield of first cut when sulphur was added.

The question is whether this was an anomaly, or if it is the new normal?  And if we are starting to need more sulphur, where and when should we put it for the greatest benefit?

Sulphur deposition trends. There is no doubt that sulphur deposition from the atmosphere has been declining over the past decades, as industries reduce their emissions. The Clean Air Act in the United States is partly responsible for this, but a bigger factor in recent years has been the economic slowdown. Shuttered factories in the "rust belt" along the Mississippi and the lower Great Lakes don't emit much sulphur.  

On top of the reductions in sulphur entering the atmosphere, the weather this year has been much drier than normal. Some of the sulphur in the atmosphere will settle out as dry deposition, but most of it is deposited when it is washed out of the air in precipitation.  Less precipitation means less sulphur, so being short of sulphur in a dry year may not mean you will need to add sulphur every year.

Soil supply. Sulphur cycles through the soil in much the same way as nitrogen, with a large reserve held in the organic matter. Soils with high organic matter content will usually have enough sulphur released through mineralization to meet the needs of crops, even if the atmospheric deposition is low. In Minnesota, where sulphur deficiencies are common, the organic matter level of the soil was a better indicator of sulphur status than a soil sulphur test, since the amount that was in solution varied so much. We would expect this to be true for Ontario soils as well.

The soils most likely to show deficiencies of sulphur are sandy and low in organic matter. These soils have less sulphur to start with and can lose some of that through leaching below the root zone. Clay soils should have lots of sulphur, along with most medium-textured soils, except on severely eroded knolls.

Meeting crop demand. There is no doubt that high crop yields have increased the removal of sulphur from the soil in recent years, although this is less of an issue for sulphur than for phosphorus or potassium. Crop removal of potassium can range from seven to 15 times as much as sulphur removal, so it will take much longer to deplete the soil sulphur pool.

There is a range of removal depending on the crop, with highest demand from canola and the other brassica crops, followed closely by alfalfa. Corn and wheat (particularly the hard reds) have moderate requirements, while soybeans will not benefit from added sulphur.  

Deficiency symptoms look similar to nitrogen deficiency, with stunted and yellowing plants, although the symptoms are more likely to affect the entire plant rather than just the older leaves. Corn may also exhibit some striping on the leaves and canola will show cupping and purpling of the leaves, along with pale-coloured flowers.

If you do want to test whether adding sulphur benefits your crops, use fertilizer with sulphur in the sulfate form. Elemental sulphur may appear cheaper, but it is not available to the crop until the sulphur has been converted to sulfate by soil micro-organisms and this process takes weeks to months, depending on soil temperature and moisture.  To work effectively, it should be applied either in the spring or fall before the crop that requires it.

There are several sulfate fertilizers available, including ammonium sulfate, ammonium thiosulfate, potassium sulfate and calcium sulfate (gypsum). Prices vary significantly, so you will need to check your local market to find the most economical option that fits your nutrient program. Fifteen to 20 kilograms of sulfate-sulphur per hectare should be adequate for any crop you are growing. If you are banding fertilizer, watch the safe rates since sulphur in a band carries about the same risk as nitrogen.

Should we start routinely using sulphur? While it is clear that there will be more instances of fields, or parts of fields, that would benefit from added sulphur, it appears that 2012 is a blip on a long-term trend.  There will be fields that showed sulphur deficiency this year that won't again for the next decade or more.

On the other hand, the supply from the atmosphere is continuing its long downwards trend, so you may want to consider trying sulphur when you are growing high-demand crops on coarse-textured soils.  BF

Keith Reid is manager (Eastern Canada), Soil Nutrient and GHG Management, Agriculture and Agri-Food Canada, Guelph.