Rock minerals are a bit of a mystery to most people, even good gardeners with whom I associate. Many don’t understand the reason that rock minerals are so important to your soil health and, indirectly, to your health. To better understand why rock minerals are so important to the whole ecology and biology of your garden, we first have to understand what rock minerals bring to the table.
We are all familiar with the big three in the nutrient category, N-P-K. Most of us also recognize many of the lesser-known nutrients that our soil requires, such as manganese, iron, copper, etc. In all, depending on which expert you consult, there are between 17 and 20 nutrients that must be present in the plant-available form to grow healthy, nutrient-dense foods.
The problem is availability. In West Texas, our soil doesn’t have a readily available supply of the base material from which these nutrients are supplied. Typically, these nutrients are found in hard rock, such as granite and limestone. The nutrients are locked up in these hard to reach sources and are not in a plant-available form. There needs to be another mechanism involved to make these nutrients available. Enter our soil biology in the form of bacteria and fungi.
The Soil Biome
There are hosts of bacteria in healthy soil. Many of these do not directly interact with plants but serve a crucial role none the less. These bacteria and some fungi serve the role of scavengers, attacking the rock structures in the soil and released in the trapped nutrients and returning them to the soil in plant-available form. These microscopic miners work the rock to bring those trapped nutrients where other bacteria and fungi can transport them to the plant roots. In the process, new soil is created as the fungi and bacteria break down the rock even further.
Now you can probably see a bit of the problem. There isn’t a lot of granite in West Texas. In fact, in almost every place you chose to study, there are some of the vital nutrients missing in the soil equation simply because there is not a source in the soil for those nutrients.
For gardeners and landscapers, the solution is simple. Add the proper materials to the soil and allow the bacteria and fungi to do their job. Hence, rock minerals. Adding the proper rock minerals to the soil in your garden and landscape provides the raw materials from which the soil biome can extract the nutrients needed by your plants and provide them in the proper proportions and at the right time.
Rock Minerals come in many forms. Some are specific to certain mineral deficiencies, while others provide broad coverage for several vital elements needed in the soil. What you use is not as important as making sure you use them appropriately. Here are a few of our favorites
Texas Green Sand (Glauconite)
Texas Greensand is mostly an iron potassium silicate. It gets its name from its green color. Texas Greensand is a marine deposit laid down eons ago and, because it is based on marine organisms, it is rich in all the elements contained in seawater. It is a natural source of phosphorus, potash, and other trace minerals. Texas Greensand is about 19 percent iron and is one of the best treatments for chlorosis that we have found. If you are applying Texas Greensand to your landscape, use it at a rate of 10-50 lbs per 1000 square feet. We use Texas Greensand when we mix our potting soil. Typically we use at a ratio of 10 lbs of Texas Greensand per cubic yard of material.
Soft Rock Phosphate
Soft Rock Phosphate is also known as colloidal phosphate and comes from a clay material that is the by-product of mining operations for hard rock phosphate. It contains approximately 20% P2O5 as well as about 25% lime and some other trace minerals. Soft rock phosphate is a very fine material with the texture and consistency of talcum powder. Unlike many other rock minerals, soft rock phosphate cannot be spread on the surface of the soil and left to make its way to where it is needed. Soft rock phosphate must be incorporated into the soil by tilling. We typically don’t apply soft rock phosphate. Our method is to blend soft rock phosphate into the soil of beds when we build them or when we are re-working the soil. Soft rock phosphate is insoluble in water, so it doesn’t leach away and is long-lasting in the soil. Most plants take up the phosphate they need in the first few weeks of its life. Without phosphate in plant-available form, young plants never catch up and remain stressed their entire life. Unfortunately, soft rock phosphate is becoming harder and harder to find and may soon disappear from the market.
Granite Sand or Decomposed Granite
Granite sand or decomposed granite is the natural result of the decomposition by weathering of hard granite rock. It is typically finer in texture than pebbles or other landscaping material. Decomposed granite comes in several sizes and types. For soil amendments, we prefer the smallest sizes and those that come from the natural decomposition of the stone. Many providers of landscape materials offer a decomposed granite that is mixed with stabilizers that act as a binder between the particles. This is great if you are building garden paths but not so good if you are trying to build soil.
Granite sand can have many different compositions based on the rock from which it is produced. The biggest advantage to adding decomposed granite to the soil, other than the nutrient value, is its natural ability to add tilth to the soil. The particles of the material tend to attract bacteria and fungi and become great cores for soil aggregates. We usually add decomposed granite to our fresh bed building soil at a rate of ¼ cubic yard of decomposed granite to every cubic yard of our starter material.
Not many people are familiar with lava sand. Simply put, it is crushed lava rock, scoria, and is one of the most beneficial of all the rock mineral amendments. Lava sand doesn’t bring as much nutrient content to the soil as other rock mineral products. It contains some, but its biggest contribution is to act as a water retention material in the soil. The porous and rough texture of the lava rock particles acts as natural sponges to trap and hold moisture while providing habitat for the micro-organisms that inhabit healthy soil.
We suggest adding a component of lava sand to your potting mixes, raised bed soils, and your landscape soil. Lava sand can be broadcast onto your landscape soil at a rate of 80 – 100 lbs per 1000 square feet. In perennial beds, you can safely increase this rate by almost double. Since sandy soils tend to allow water to drain away quicker than normal, the addition of lava sand to the naturally sandy soil can help water retention. Lava sand can be used in vegetable garden beds at the same rates as landscape use.
Zeolite is a natural volcanic material that contains a wide array of basic minerals. Technically, zeolite is classified as a hydrated aluminosilicate. What is more important to gardeners is the wide range of benefits zeolite can bring to your soil. It is a natural scavenger. It can be used to detoxify contaminated soil, to trap and hold odors, and to trap nutrients which are then released back as needed and at rates beneficial to plants.
Zeolite should be mixed into new beds as the soil is prepared. Rates are usually 10 – 50 lbs ber 1000 square feet Adding more won’t hurt but may be a waste of money. Mixing zeolite with fertilizers, even organic fertilizers, is a great idea. Zeolite has an affinity for raw nitrogen and will trap the excess nitrogen applied to the soil. Nitrogen is notoriously mobile in the soil. Zeolite can hold onto that nitrogen and will release it back to the plants as needed. It makes the use of organic fertilizers much more efficient.
Building soil is like putting together legos with your kids. There are a thousand different pieces of all shapes, sizes, and colors. It can get confusing and frustrating at times, but with a little imagination and focus, the result can be amazing. Don’t despair if you feel overwhelmed. As with most of the other parts of the organic system, it isn’t as much about getting it perfect as it is about getting it done.
Links and Resources
For more information about organic gardening, lifestyles, and living, visit our website at West Texas Organic Gardening.
If you found the information here helpful, you might also find these articles on our website of interest.
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