by Amy Sangster
The old saying that every cloud has a silver lining actually may hold some relevance to air pollution and acid rain. Acid rain has been recognized as an environmental problem for decades. It has devastated aquatic ecosystems in streams, lakes, and marshes, and has also impacted plants and animals that are sensitive to its effects. However, there is a silver lining: acid rain on agricultural fields provides additional sulphur, an important essential nutrient for plant growth.
Just how much sulphur has been deposited? The Bedford Institute of Oceanography in 1986 quantified the amount of sulphate deposition by precipitation between 1979 and 1983 at four sites along Nova Scotia’s South Shore. The mean monthly depositions via precipitation during that time ranged from 1.5 to 3.9 kilograms per hectare of sulphate per month. Annually that equates to 18 to 46.8 kg/ha of sulphur.
Recognizing that these concentrated deposits were detrimental to the natural ecosystem, efforts to reduce emissions from combustion of fossil fuel and other industrial processes were implemented in the 1990s via Canada’s Environmental Protection Act and the United States’ Clean Air Act. Since then, significant reductions, typically reported around 50 percent, have limited atmospheric deposition of sulphate. As a result, questions arose around the need for supplemental application of sulphur to replace this silver lining.
Sulphur for plant growth is taken mainly from two reservoirs: the soil and the atmosphere. Sulphur in the atmosphere can also originate from naturally occurring sources such as volatile sulphur compounds from tidal marshes and decaying organic matter. In the soil, sulphur exists in either an organic or a mineral form. Approximately 90 percent of sulphur exists in the organic form. Organic sulphur is mineralized by microorganisms, producing sulphate, which is the form of sulphur that can be taken up by the plant.
The Canola Council of Canada provided some context for the amount of sulphur that may be supplied by the soil’s organic matter. The council produced a fact sheet estimating that each percentage point of organic matter in a soil equates to roughly 2.2-3.4 kg/ha of available sulphur from the organic matter. In other words, a soil with four-percent organic matter arguably could provide 9-13 kg/ha of available sulphur. That estimate coupled with a conservative estimate from atmospheric sulphur indicates that approximately 20 kg/ha of sulphate could be provided.
Sulphur removal rates depend on crop and yield. Removal rates reported in Ontario are high for crops such as cabbage and onion. Removal rates for a 78 tonne/ha cabbage crop were estimated to be 72 kg/ha and for a 44 tonne/ha onion crop were 32-54 kg/ha. An 18 tonne/ha DM (dry matter) crop of corn silage came in at 14-18 kg/ha sulphur removed, a 3.36 tonne/ha soybean yield would remove 5.6 kg/ha, and a legume hay crop of 11 tonne/ha would remove 21-30 kg/ha.
The risk for sulphur deficiency varies with soil type, crops grown, manure inputs, compost inputs, and organic matter levels. Deficiencies are more likely to occur in sandy soils, which are prone to leaching and tend to have lower organic matter levels. Soils with adequate soil organic matter alone may provide enough sulphur to meet the requirements of crops with a lower demand for sulphur.
In 2010, an assumed critical level was set for soil sulphur levels of 40 kg/ha in Nova Scotia (Mehlich 3). However, research on the correlation and calibration of soil tests for sulphur across different soil types is limited. The general conclusion is that no one procedure has proved consistently superior in predicting responses to sulphur fertilization.
A survey was also conducted in 2010 of 17 farms between Windsor and Annapolis Royal. Most of these samples were from soils with forages, and had a history of manure application, which is also a source of sulphur. Despite this, the survey results showed that sulphur levels were below 40 kg/ha sulphur. In my own experience, I rarely see a soil test report that has a sulphur level higher than 40 kg/ha. We may need to carefully revisit our set critical limit.
Additional sulphur to meet crop demands can include manures and composts, sul-po-mag (sulphate of potash magnesia), ammonium sulphate, potassium sulphate and magnesium sulphate, gypsum, and elemental sulphur. Keep in mind that elemental sulphur cannot be directly used by plants and must be converted to sulphate by microorganisms. It is difficult to accurately predict the rate of conversion of elemental sulphur to plant-available sulphate as it is sensitive to a variety of soil and environmental conditions. But it would be safe to assume that it will not be available during the year of application.
A better understanding of the fate of soil sulphur coupled with more efficient use of sulphur inputs will benefit farmers growing high-sulphur-demand crops in this region. Until then, using some simple math and some common sense should help you determine if your crops need more sulphur than what is being provided to you free of charge.
(Amy Sangster is a soil specialist with Perennia Food and Agriculture Inc. based in Bible Hill, N.S.)