What Is the Organic Equivalent of Potassium-Magnesium Sulfate Fertilizer?

Definition

• Although it sounds like a chemical Frankenstein, Sul-Po-Mag is simply the name by which the mined rock salt potassium-magnesium sulfate is known. Scientifically-speaking, the material, also called sulfate of potash magnesia, is an organic double salt. As its commercial name suggests, Sul-Po-Mag, when processed from rock to powdered form, adds sulfur, potassium and magnesium to your soil.

Rates of Application

• Rodale's "Ultimate Encyclopedia of Organic Gardening" suggests that you test your garden soil for nutritional deficiencies. If your soil is severely low in one or more of the three key ingredients of Sul-Po-Mag, work at least 10 pounds of the organic product into your soil for every 1,000 square feet. The depth at which the powdered product should be worked in should be tailored to what you will grow -- a few inches down for shallow-rooted vegetables, to as deep as 24 inches for young trees. Moderately deficient soils benefit from 7 pounds per 1,000 square feet, and those needing an annual boost can get by with about 5 pounds per 1,000 square feet.

Converting from Inorganic Fertilizers

• At times, a professional soil testing service may make fertilizer recommendations based on the assumption that you will use an inorganic fertilizer, especially for nitrogen, phosphorus and potassium, or NPK. These products are usually sold listing numerical sequences, such as 5-10-10, representing the NPK ratio. If you wish to use Sul-Po-Mag as your potassium source instead of the inorganic recommendation, divide the last number of the lab's recommended NPK formula -- for instance, the number 10 from the 5-10-10 example -- by 22. The resulting figure from that example is .45, which should be multiplied by the number of pounds of NPK recommended by the lab. For example, if the service recommended 20 pounds of 5-10-10 for a given area, your total amount of Sul-Po-Mag would be 9 pounds.

Exceptions

• There are times when using Sul-Po-Mag is inadvisable. If your have an excess amount of either sulfur, potassium or magnesium, then you risk harming your plants by adding additional amounts of any of these nutrients. In such cases, it is better to use greensand for potassium, Epsom salts for magnesium or ground elemental sulfur for your sulfur source. Another instance in which you should avoid Sul-Po-Mag is if you add limestone to your soil to raise the pH level. By doing so, you will also be adding the magnesium contained in limestone. In these cases, combining Sul-Po-Mag with limestone can lead to amounts of magnesium that may be toxic to some plants.