This is the fourth in a series of blog posts aimed at helping you better understand soil nutrient & testing data for improved horse pasture management, healthier horses, and a better environment. In this series, we will explore traditional soil data like N, P, and K levels and soil pH, along with less well understood data including micro-nutrient levels and biological measures of soil health.
Previous posts
In the first three blog posts, we discussed the “big three” soil macro-nutrients, nitrogen, phosphorous and potassium (NPK). In this fourth post in our soil health series, we discuss sulfur, the soil nutrient that is increasingly recognized as being as important as N, P and K.
Why is sulfur important?
Sulfur is needed by plants, including pasture grasses, to form enzymes and proteins. Deficiencies in plant available sulfur can cause serious problems and loss of vitality in plants, which need about 10 to 30 pounds of sulfur per acre. In the context of horse pastures, if legumes are a desired part of your pasture forage mix, they will show signs of sulfur deficiency (turning light green or yellowish in color) more quickly than will grasses. This yellowing, a result of decreased chlorophyll production, is similar to that observed from nitrogen deficiency. The key difference is that with low sulfur, problems tend to show up on younger leaves first, whereas in nitrogen-deficient plants, the older leaves are first to show signs of yellowing.
The importance of sulfur in soil health is becoming increasingly emphasized. This is due, in part, to several important changes over time that have led to a general decrease in soil sulfur levels. These include reductions in coal-fired power plants using high-sulfur coal (due to concerns over air quality), where the burned sulfur eventually made its way to nearby soil. Areas where industrial activity is still prevalent today tend to have higher soil sulfur levels. Additionally, historical use of certain fertilizers that contained sulfur have changed over time towards the use of high-analysis fertilizers that do not contain sulfur.
Sulfur deficiencies are more likely in sandy soils, soils with low organic matter (less than 2%), and soils that have experienced heavy rainfall. In the US, sulfur deficiencies are most common in the Northwest, California, the Great Plains, and the Southeast.
What are the important forms of sulfur in soil?
Most of the sulfur in soil (~95%) is found attached to organic matter. Similar to other nutrients in soil, organic sulfur must first be converted into a different chemical form (in this case sulfate [SO42-]) through a process called mineralization before it can be taken up by plants. Mineralization is carried out by soil microbes (yet another reason why soil microbial health is critically important). On average, about 3 to 5 lbs of sulfur are made available to plants per year for each percent of organic matter found in the top 6 inches of soil.
How to determine if my pasture grasses are suffering from low sulfur?
Soil tests alone, while important, may not be sufficient to determine if your pasture grasses are deficient in sulfur. This is partly because soil tests are not good at measuring the different types of sulfur found in soil. Instead, soil test results combined with forage analysis is the best way to estimate sulfur sufficiency (most plants should contain between 0.15 and 0.35% S).
If it is determined from soil and forage testing that your pastures would benefit from a sulfur application, typical application rates range from 10 to 25 lbs of sulfur per acre. Common sulfur-containing fertilizers are summarized in the table below.
Fertilizer name % S
Ammonium Sulfate 24
Calcium sulfate (gypsum) 15-18
Magnesium sulfate 14
Potassium sulfate 18
Superphosphate (normal) 12
Potassium magnesium sulfate 22
At Sustainable Stables, we believe that the key to healthy pastures starts with healthy soils. Our soil and forage testing services go beyond simple macro-nutrient (NPK) and pH analysis to include micro-nutrient levels along with biological measures of soil health. Such soil data is key to developing a sustainable management plan to create healthy, resilient pastures. Our testing services can be done independently, or as part of our equestrian property planning and design services. Contact us today to learn more.