Part Three: Greensand
Unlike most components of organic fertilizer blends, Potassium is a notoriously difficult element to derive from contemporary, sustainable sources such as plant and animal waste or carcasses. To keep agricultural soils productive, this elusive mineral is most often collected from mines which occur all over the world. The product sold as Greensand is comprised of a mineral known as Glauconite Most Greensand products are mined from ancient beds of Glauconite which were deposited on the ocean beds of the long past days of the Eocene, Cambrian, and Cretaceous periods.
In spite of its non-renewable nature, its wide range of soil-improving properties make greensand a delightful and necessary soil-building substance. In addition to supplying Potassium, Greensand is a source of many micronutrients (which we learned about when discussion Kelp Meal), it can help break up clayey soils and improve your soil’s water retention capacity. Greensand is a slow-release fertilizer, so its not the best quick-fix solution if you are experiencing a Potassium deficiency in your crops, (use Sulfate of Potash instead), but it is certainly the best Potassium source for your fertility regime that is aimed at improving the long-term health of your soil and crops.
We recommend using Greensand as part of a balanced fertilizer blend and applying Greensand as a stand-alone amendment for soils that are particularly low in Potassium or are incredibly clayey. Rough guidelines are as follows:
Use a balanced fertilizer including Greensand when planting new vegetable crops each season. Application rates for fertilizers vary, follow the recommendations on the product label.
If your soil test results indicate very low levels of Potassium in your soil, consider an application of pure Greensand in the fall or early spring (use product application rates).
Bonus tip: Potassium plays a major role in the efficiency of water use in plants. Both uptake of water through plant roots and its loss through the stomata are affected by potassium. Adding potassium to your garden can improve drought resistance in crops.