Greensand is an organic fertilizer that has been around for many years but it has never become popular except in certified organic farms. Are gardeners missing out on an important soil amendment and source of potassium?
Key Takeaways
- Greensand is not a good source of potassium.
- It does have some soil conditioning benefits but compost is a better option.
What is Greensand?
Greensand fertilizer is a natural soil amendment derived from ancient marine deposits, primarily composed of glauconite, a green mineral that contains nutrients plants can use. It has been used for centuries in agriculture for its ability to provide potassium and some micronutrients to soil. The mineral’s green color comes from iron, potassium, magnesium, and trace elements.
Since the mineral is mined and minimally processed it does qualify as an organic amendment that slowly releases nutrients. Gardeners typically mix it into the soil before planting or apply it as a top dressing around established plants. It can be used in vegetable gardens, flower beds, and even lawns.
It is mined from various locations and the specific composition will depend on its origin.
NPK of Greensand
A common NPK found in commercial products is 0-0-3, but the potassium level can be as low as 0-0-0.2. Claims that this is a good fertilizer are wrong since it provides no nitrogen or phosphorus.
It does provide other nutrients. One product claims that it has “68 Minerals and Trace Elements”. It may contain that many minerals, but most of those are not used by plants that only use about 24.
Some types of greensand do have higher levels of calcium, magnesium, and iron, but these are not deficient in most soils.
Soil Conditioner
Glauconite often exists as sand-sized pellets, as shown in the photo below, and does not behave like sand when added as a soil conditioner. Unlike sand, it holds a fair amount of water and can increase the water retention capabilities of sandy soil. It also has a relatively high cation exchange capacity (CEC) giving it the ability to hold plant nutrients.
Comparison of CEC values:
- Greensand: 0-30 meq/100g
- Clay: 20-60 meq/100g
- Organic Matter: 100-200 meq/100g
When added to clay soil greensand adds porosity and loosens them.
The Problem With Greensand Fertilizer
This fertilizer is a form of rock, similar to rock dust, and has been in the ground for millions of years. It is quite stable, so the potassium and other nutrients in it will be released very slowly.
The critical question is, how slow is slow? If it is too slow it is useless for gardens.
Clemson University reports that nutrients are released “very slowly”, meaning years. Rutgers University did an extensive review of the literature in 2002 and found mixed results with some projects showing increased yield and others with no effect. The general consensus was that potassium is released too slowly in most soil to be effective.
Greensand also contains heavy metals including arsenic, cadmium, chromium, and lead.
A pot experiment using acidic (pH 4.5), tropical soil found that greensand did increase plant growth. Potassium is released faster in very acidic conditions.
Another study concluded that greensand released potassium very slowly over 50 years and that other sources of potassium would be required to meet crop needs. They recommend it only be used in naturally acidic soils and soils that contain manganese oxides because both conditions increase the release rates of potassium.
In an article called, Managing Potassium for Organic Crop Production, Robert L. Mikkelsen concluded that “the K release rate is too slow to provide any significant nutritional benefit to plants at realistic application rates”. Any increases in yield are probably due to improved soil characteristics.
Should Gardeners Use Greensand?
Greensand products marketed to gardeners promote the idea that they add potassium to soil, but unless the soil is very acidic, below 5.0, it is not going to add enough potassium to make a difference in plant growth. Don’t use it as a potassium source.
It does have some value as a soil amendment when added to clay or sandy soil. It improves moisture retention and increases the soil’s ability to hold nutrients. It also loosens clay soil.
The problem with this product is that it is usually sold in small bags at a very high price, making it unsuitable for use as a soil amendment. It is less expensive and much better to use compost.
I can’t think of any good reason for gardeners to buy this product unless they can get it in bulk at a low price. Even then, compost would be a better option.
Not to mention the environmental impact of trucking ground rocks around.