Greensand fertilizer is a natural, mineral-based soil conditioner known for its gentle action on plants and long-term soil benefits. This amendment is a finely crushed rock material, named for its distinctive olive-green color and granular, sand-like texture. Unlike quick-acting synthetic products, greensand is primarily used to improve the fundamental health and structure of garden soil over time. It has a history of use in agriculture spanning over a century.
Geological Origin and Primary Mineral Content
Greensand is a sedimentary rock material formed millions of years ago on ancient ocean floors, developing in anoxic marine environments. These deep-sea deposits were compacted over geologic time into the soft rock that is mined today. The component that gives greensand its characteristic color and agricultural value is the mineral glauconite, an iron potassium silicate.
Glauconite delivers greensand’s primary nutritional benefit to the soil. The two most significant nutrients it contains are potassium (potash) and iron, which contributes to the green hue. Greensand is also a source of over 30 trace minerals, including magnesium, calcium, and manganese. The concentration of these minerals varies depending on the specific geological deposit from which the greensand is sourced.
Function as a Soil Amendment
Greensand functions as a soil amendment primarily due to the chemical structure of the glauconite mineral. The nutrients are not immediately water-soluble but are locked within the crystal lattice, leading to a gentle, slow-release effect that minimizes the risk of plant “burn.” This characteristic makes it a sustainable choice for building soil fertility over several seasons.
Greensand is particularly noted for its ability to improve the soil’s physical structure, acting as a conditioner for both heavy clay and light sandy soils. When added to dense clay, its physical properties help loosen the particles, improving drainage and aeration for healthier root growth. In contrast, when incorporated into sandy soils, it increases the soil’s capacity to hold moisture and nutrients.
This increased holding capacity is linked to greensand’s relatively high Cation Exchange Capacity (CEC). CEC measures the soil’s ability to retain positively charged nutrient ions, preventing them from leaching away. By boosting the CEC, greensand helps the soil become a more efficient reservoir for nutrients, making them available to plants as needed. It can also absorb a significant amount of water, sometimes up to one-third of its own weight, which is beneficial for moisture retention.
Practical Application in Gardening
Greensand can be applied to the garden using specific rates depending on the area being treated. For vegetable gardens and flower beds, a common application rate is approximately 25 to 30 pounds per 1,000 square feet. For severely depleted soils, a heavier initial application of 50 to 100 pounds per 1,000 square feet may be used.
The most effective method for application is to broadcast the greensand evenly across the soil surface and then lightly incorporate it into the top three to six inches. This mixing helps speed up the natural weathering process. For existing plants or trees, greensand can be applied as a top dressing around the base of the plant.
Application is best performed in the early spring before planting or in the fall, allowing the winter weather and microbial activity time to begin breaking down the mineral structure. Greensand is a good choice for acid-loving plants, such as blueberries and rhododendrons. For container plants, two to three tablespoons per gallon of potting mix can be blended into the soil before planting.