Soil Minerals and Soil Testing for Organic Gardeners
The Ideal Soil
This chart has been put together over a period of years from many sources, including a lot of personal experience. It is meant to represent an ideal balance of minerals and trace elements for growing nutritionally perfect food for people and animals, not necessarily the ideal soil for pine trees or rhododendron flowers. It is fine for lawns, though.
[Please note that the chart below is an early (2007) version of The Ideal Soil Chart. The latest and more complete and accurate version is only available in The Ideal Soil Handbook.]
The Ideal Soil
(Agricola’s best guess, version 1.5, April 2007)
Based on a standard soil test
Bases (cations) as % of exchange capacity
Other major nutrients (anions)
Minor elements (of Major importance)
Trace elements (also of Major importance)
Read the notes and explanation below that go with this chart. Please.
Caution, Warning and Introduction to Agricola’s best guess:
The Ideal Soil [version 1.5] April, 2007
Do not just go out and buy minerals and throw them on your garden, field, or pasture. Please.
This chart reflects our current level of knowledge. It has been put together over a period of years from many sources, including a lot of personal experience. It is meant to represent an ideal balance of minerals and trace elements for growing nutritionally perfect food for people and animals, not necessarily the ideal soil for pine trees or rhododendron flowers. It is fine for lawns, though.
In high doses, many mineral elements can be toxic to people, animals, plants and soil organisms. This is true regardless of whether they are in a naturally occurring or a purified, concentrated form. Keep them out of ponds and streams. Any of them, if used in excess, can screw everything up, so let’s take it easy. It is much easier to put them in than to get them back out of the soil.
High levels of some minerals in the soil may inhibit sprouting of seeds. Boron is definitely known to do this. High levels of free minerals (not biologically assimilated) can also “plug up” the vascular systems of young plants, stunting their growth. They may sprout fine but stall out after the first set of true leaves. This seems to be particularly true after adding high amounts of calcium. For these reasons it is best to wait until the minerals are chemically and biologically a part of the soil before starting seeds in it. Transplants usually do fine if you wait a few weeks after adding minerals before replanting them, and we have seen few problems with established plantings, trees, or pastures. Adding minerals in the fall or in the very early spring works best.
If minerals are added directly to potting mixes they need time, like a few weeks, to “settle in” before the potting mix is used. Adding a biological activator such as beneficial bacteria or fungi to the soil will greatly speed up the process.
The mineral concentrations shown on this chart are safe enough once they are assimilated into the living soil. If the chart’s guidelines are followed you won’t end up with too much of anything–many soils naturally contain higher levels of available minerals than the chart calls for.
First of all, primarily, and before anything else, please get a professional soil test. Soil testing is not expensive and most testing labs pride themselves on getting your results back to you quickly, usually within a few days of receiving the soil sample. You must have a soil test before you add any minerals at all. This is not to make us happy, but to ensure your happiness in the long run. You will want to know what you started with. And if you insist on adding minerals without a soil test, don’t say we didn’t warn you!
A good soil test will give readings for most of the minerals on this chart and will also tell you the CEC (cation exchange capacity), the TEC (total exchange capacity) and the base saturation percent of Calcium, Magnesium, Potassium, Sodium and Hydrogen in your soil sample. The soil test should also tell you the amount of Boron, Iron, Manganese, Copper, and Zinc. These are minerals whose function we understand well and it is essential that they all be in your soil in sufficient quantities. You do not need to know the amounts of the minor trace elements to start with (those at the very bottom of Agricola’s chart), and ordinary soil tests don’t measure them anyway.
If you are a very cautious or doubtful person, or the expense seems too great, you may choose not to balance the minerals on the whole farm or the whole garden or pasture at once. Start with maybe one-half of the area and see how things go, or divide it into two or more parts and treat them slightly differently, for instance putting the whole amount called for on one part and only half that amount on the other. And it is always a good idea to leave a small representative area untouched as a control. After a year or so you will enjoy pointing out that area and saying “See, that’s what I started with!”
The mineral balancing process will probably take around three years. As the minerals settle into the soil ecology, some will become available to the plants and soil microorganisms and others may get tied up for a while. Adding a little bit of a badly needed mineral nutrient to the soil may greatly increase microorganism and fungal activity, and may catalyze the release of other previously bound-up minerals. Adding a little copper may make more Zinc available (or less Zinc available). You won’t know until the next soil test.
If you are truly serious about gardening or farming and having the healthiest soil and plants possible you will want to get a soil test once a year. For commercial growers, twice a year, in the spring and in the fall is even better. The spring test will show you what you should apply for this years crop, and the fall test will tell you what to add to settle in over the winter. Calcium and Magnesium, for example, become much more bio-available if they are spread on top of the soil in the fall and allowed to leach into the soil with the winter’s rain or snow.
Ignore the pH. It will self- correct as the minerals are balanced.
Mother Nature and the soil are very forgiving and you do not have to be exact in these proportions. It would be unlikely to find two soil samples taken one foot apart that were identical. The soil test will give you the general idea, and as long as you go slow and take it easy everything will be fine.
In the beginning, pay a lot of attention to Calcium and Magnesium. They are fully as important in the soil as they are in the human body. In a sandy soil with a low exchange capacity you will want about 60% Ca and 20% Mg. In a heavy clay soil with a high exchange capacity, about 70% Ca to 10% Mg. This is because the higher the ratio of Calcium to Magnesium, the looser the soil gets, and as the Magnesium portion gets higher, the soil gets tighter. You want to tighten a sandy soil and loosen a clay soil. Once these two are balanced and in the right saturation they will bring many other things into line and a new soil test will guide you to your next step.
Calcium sources: Sweet lime (calcium carbonate) and gypsum (calcium sulfate) are the preferred sources of calcium. Gypsum will not make your soil more acid, it supplies readily available calcium, and is also a good source of sulfur, an element that is seriously lacking in many soils. Sweet lime supplies carbon as well as calcium. Carbon helps make a soil less sticky. If you already have plenty of carbon in your soil as organic matter, but are low on sulfur, gypsum is a better bet. The various rock phosphates are also significant calcium sources.
As a rule, don’t use Dolomite lime, regardless of what you may have read in various gardening books, unless you are sure that you need Magnesium. Dolomite is a high Magnesium limestone. Using dolomite will tighten the soil, reducing air in the soil and inducing anaerobic alcohol fermentation or even formaldehyde preservation of organic matter rather than aerobic decomposition. If the soil test calls for more magnesium, magnesium sulfate (epsom salts) or K-Mag (also known as Sul-Po-Mag or Langbeinite), are generally safer and quicker acting sources of magnesium than dolomite. Dolomite is an inexpensive source of Magnesium and Calcium, but it is slow acting. Magnesium oxide is the purest and quickest acting Magnesium additive, but is not presently allowed under USDA NOP organic rules, for some reason. If one is not concerned with being “certified” organic under USDA rules, Magnesium oxide is the best bet. MgO (Magnesium Oxide)is around 50% Mg, a much higher percentage than dolomite lime (13% Mg) or Epsom salts (10% Mg) and it is also a much cheaper source of Mg. If you are not concerned about being “certified by the government”, I would recommend using MgO.
Agricola’s chart says that Phosphorus and Potassium should be equal, but that’s not as simple as it looks. On a bag of fertilizer, such as 10-10-10, the numbers stand for Nitrogen-Phosphorus-K(Potassium), in that order, but they don't indicate pure elements. The N number tells you how much of some type of Nitrogen compound, while the P number actually stands for phosphate, P2O5, and the K number stands for potash, K2O (K is from the German word Kalium, meaning Potassium). Phosphate is 44% Phosphorus, while potash is 83% Potassium. So, one needs about twice as much phosphate as potash for the P and K to be equal. A 10-20-10 or a 2-4-2 fertilizer would have that correct ratio.
If you are going to learn to interpret your own soil test, it is also important to know what form of P and K the soil test results from the testing lab indicate. Most labs give the P number as phosphate, P2O5, so you can take that times .43 and find the actual amount of Phosphorus in your soil. Some give the K number as K2O, some as actual K. Ask the nice person who does your testing which they are using.
Although this chart emphasizes minerals, you would not have much luck trying to grow food in a soil that wasn’t bio-active even if it contained the perfect mineral balance. The goal is to get these minerals into the soil in a biological or at least bio-available form. We add them gradually and let the soil life assimilate them.
In some cases minerals may be added to the compost pile to start the bio-availability process, but we would recommend doing this only if you keep perfect records of exactly how much of what is in which pile. For example, one could mix a 50# bag of rock phosphate into a good sized compost pile , but it would be nice to know just how much Phosphorus, Calcium etc was in the bag to start with, and that it was all in that pile and could be spread over X amount of area.
Know all the ingredients of anything you add to the soil if at all possible. How much Calcium does that phosphate rock have in it? Montana rock phosphate contains around 30% total phosphate (13% actual P) but it also has around 30% Calcium. The Calcium is chemically attached to the Phosphorus in the form of Calcium phosphate. As the Phosphorus is made available by the soil microorganisms, so is the Calcium.
Glacial rock dust, granite dust etc. are great sources of fresh minerals, but they won’t help that much if your soil already contains plenty of the same minerals. Most of them have low enough numbers of the major nutrients that they won’t hurt anything, though, and because they are freshly ground up and sharp grains of rock, they will generally increase the energy level in the soil.
The top seven inches of an acre of soil is assumed by convention to weigh two million pounds.
This is referred to as the plow layer, and is where most of the growth happens and where most of the available nutrients are.
So, one part per million (1 ppm) of the plow layer equals two pounds per acre.
An acre is 43,560 square feet, or close to 45,000 sq. ft.
A pound is 453 grams, or about 450 grams.
So one part per million = approx. 2 grams per 100 square feet.