A marine deposit called glauconite which is iron potassium silicate and an excellent source of trace minerals. Texas greensand contains 19-20% iron.
Here's some information by John Ferguson, Nature's Way Resources:
Traditionally, we were taught that plants can grow with only 16 elements. However recent research has shown that plants grown with a much wider assortment of nutrients have less disease and insect problems, use less water, taste better, have larger and more fragrant flowers, etc. The human body has 90 elements in it. If these elements are not in the soil then plants cannot absorb them, and we do not get them when we eat the plants and fruits, nuts, etc. Many health problems are the end result of this type of scenario.
There is an excellent lecture available on CD on the importance of trace elements. It is called "Dead Doctors Don't Lie", By Joel Wallach, DVM, N.D. It explains why we have so many health problem associated with the lack of nutrients in our food supply. Almost all of Dr. Wallach's statements have been confirmed by other researchers since this lecture was recorded many years ago. This is a fun lecture to listen to, as Dr, Wallach has quite a sense of humor as he explains the importance of trace elements for both animal and human health.
So what does this have to do with greensand? As we all know seawater has almost all the elements known to mankind in it. Greensand is a naturally occurring mineral mined from ocean deposits from a sedimentary rock known as "Glauconite," hence it contains these nutrients. It is often an olive-green colored sandstone like rock found in layers in many sedimentary rock formations.
Origin of Greensand
Greensand forms in anoxic (without oxygen) marine environments that are rich in organic detritus and low in sedimentary inputs. Thus when greensand is exposed to oxygen, the complex minerals break down and the nutrients are released into the soil.
Greensand in our area is a dark greenish gray color when dry and turns almost black when wet. Greensand is a very heavy mineral with a density of approximately 90 pounds per cubic foot (over 1 ton per cubic yard). The minerals are normally released slowly over time but occur much faster in organic rich soils full of beneficial microbes (microbes produce organic acids as they break down organic matter which facilitates the release of the minerals for plant absorption).
The pH of greensand varies from slightly acidic to slightly alkaline depending on the source and has little effect on soils. For those that want a more technical explanation and references see the paper on Greensand on the Nature's Way Resources website at www.natureswayresources.com.
Uses of Greensand
Greensand has been used for over 100 years as a natural source of slow release fertilizer and soil conditioner. The slow release of potash and phosphate does not burn plants and the minerals improve the moisture holding properties of soil.
Greensand often has the consistency of sand but is able to absorb 10 times more moisture, making it a good amendment for use in agriculture and horticulture for many soils types. Greensand does not burn plants and helps the beneficial microbes to grow in the soil. It also has been found to be a good conditioner to help loosen heavy and tight soils and help bind loose soils.
Recommended application is 2-4 pounds of greensand per 100 square feet or 1 ton per acre. For potting soils 5-20 pounds per cubic yard can be beneficial. It is a good idea to repeat this every few years to replace the nutrients that have been used up or leached from the soil.
A field test by Rutgers University in a sandy loam soil with greensand applied in the row at the time of planting, found that the application of greensand increased the yield of potatoes by 16%.
The benefits of greensand, largely unexplained by scientific research are far more than a laboratory analysis would indicate. However numerous greenhouse and field studies have shown significant improvement in the growth of plants. Other studies have shown that the use of greensand improves the taste, color, nutritional value, the health of plants and the health of soils.