Sunday, August 3, 2014

Soil Minerals

While the living systems that comprise soil form the foundation for the ecosystems they sustain, living things also require a number of non-living minerals to support their normal functions. For example, bones are made primarily from calcium and phosphorus, vitamin B-12 contains cobalt, and the essential amino acids methionine and cysteine both contain sulfur. While plants can grow with only a handful of different nutrients, animals have more complex requirements which you can find in the contents of a typical multivitamin. If the soil is deficient in any of these factors, or if plant absorption is impaired by the conditions in the soil, it can lead to negative effects which propagate all the way up the food chain. Soils which are very mineral deficient, such as those in the Amazon region of South America, can only support a very low density of animals- humans included.

The mechanisms by which minerals attach to and are released from soil are fairly complex and are also pH dependent. Mineral nutrients can be broadly divided into two classes, positively charged cations and negatively charged anions, and these nutrients along with pH must be properly balanced in order to support optimal plant and animal health.


The positively charged cationic nutrients include calcium, magnesium, potassium and sodium. In soil they are often found complexed in the mineral structure of silt, but in more weathered soils they can be found attached to negatively charged sites on clay particles, from which they can easily detach. Calcium and magnesium also form complexes with organic matter in the soil, and help stabilize the organic matter against being converted into carbon dioxide. Sodium and potassium are generally found as soluble salts in living things and readily leach out of organic matter, and are primarily retained in soil by attachment to clay particles instead.

The total ability of organic matter and clay particles in a given soil to hold onto cation nutrients is known as its cation exchange capacity or "CEC", expressed in hydrogen equivalents. Cation exchange sites that do not contain minerals are occupied by hydrogen (H+) ions, which plant roots release in order to free nutrients from the soil and make them absorbable. Target levels for cation nutrients are expressed as a percentage of the the total CEC, with the ideal for most plants being 50% calcium, 30% magnesium, 10% potassium and 1.5% sodium. The "extra" CEC is left over as a pH buffer and to catch other cation nutrients which may percolate through the soil out of organic matter or other sources. In order to hold the minimum amounts of nutrients required for optimal plant growth, a soil must have a CEC of at least 7 as reported on a soil test. A soil with a CEC less than 7 will be chronically infertile, however most soils have several times this amount and very fertile soils may have a CEC as high as 35 or so.


The negatively charged anions include phosphorus, sulfur and chlorine. As with cations, they can frequently be found complexed in the rock minerals of silt particles, however anions bind to soil a bit differently than cations do. Like cations, anions become bound to organic matter and are released as it breaks down, but when binding to clay particles anions tend to form strongly insoluble complexes with iron and aluminum (which themselves are cations). As a result anions are less prone to leaching from soil, but are also less available for plant uptake than cations.

While there is such a thing as anion exchange capacity, it is a seldom used or measured figure since most soils are rich in iron and/or aluminum, and since the plant requirements for anions are narrower than for cations. In general only about 100 pounds of soluble phosphorus per acre and 100 parts per million of sulfur are required for optimal plant growth, however most fertile soils contain as much as 1000 pounds per acre of phosphorus in mineral forms or bound to iron and aluminum. For this reason root symbionts are more important for anion nutrition than are the soluble levels in soil, which typically quickly become bound into insoluble forms that can only be extracted with the help of said symbionts.

In general, about 5000lbs per acre of rock phosphate and 500lbs per acre of "agricultural" elemental sulfur (which are both highly insoluble in water) are enough to support plant growth for 10 years without additional fertilizer inputs, and without leaching or other negative effects.


Micronutrients are mineral nutrients which are required in amounts less than 300 ppm total soil levels, and may be either cations or anions. Micronutrients include a variety of minerals such as iodine, selenium, molybdenum, cobalt, manganese, zinc, copper and boron. Even though these nutrients are equally as important as macronutrients in terms of animal and human health, standard soil tests typically omit values for iodine, selenium, molybdenum and cobalt, and testing companies charge as much as a normal test for each additional mineral. Fertilizers for some of these nutrients can also be quite difficult and expensive to obtain, much to the detriment of modern 'scientific' agriculture. Ideal soil values for micronutrients are 20 ppm each for iodine, zinc and copper, 0.8-1.4 ppm selenium, 5-6 ppm molybdenum, 15 ppm cobalt, 50 ppm manganese, 200 ppm iron and 4 ppm boron.

Care must be taken when applying micronutrients, because they can easily reach toxic concentrations if too much is applied at once, or if applied unevenly. Typically only part of the required amounts are applied in a given year, for example you should only apply boron 1 ppm at a time (recommended is 1lb/acre/year until the ideal level is reached for boron). For selenium even smaller values are required, whereas for most other micronutrients larger values are more appropriate.

pH and Nutrient Uptake

The availability of different nutrients depends on the pH of the soil. Some nutrients, such as phosphorus, sulfur, iron and manganese become more available at a lower (acidic) pH. Others, such as calcium, magnesium, potassium and selenium become more available at a higher (basic) pH. However, almost all nutrients have a high availability at a slightly acid pH between 6.0 and 6.5, which is the preferred pH range for agricultural soil. There are exceptions to this, such as blueberries which prefer a pH below 5.0, and lavender which prefers a pH above 7.0.

In many cases, balancing the minerals in the soil will also correct the pH without further intervention. However, there are also many cases where the soil type presents extreme tendencies of pH. For alkaline soils there are two main types, sodic and calcareous. Sodic soils contain high levels of sodium carbonate and bicarbonate, and can only be treated with hydrochloric acid to produce neutral sodium chloride, or with calcium chloride if the calcium levels are also low. Calcareous soils are very high in calcium and/or magnesium, and it is best to plant things which prefer calcareous soils than to attempt to modify them, since removing excess calcium or magnesium would be a monumentous task.

Acid soils also come in two main types, kaolinitic clays and histosols. Kaolinitic soils are highly weathered and typically have a low CEC and few nutrients, while also having high levels of reactive iron and aluminum. Kaolinitic soils tend to be very acidic, but can be remediated with biochar and mineralization. Histosols are soils which are frequently waterlogged, and as a result organic matter never fully breaks down in these soils. The minerals bound to the organic matter in histosols are never released, and the fermentation process releases large amounts of organic acids which acidify the soil considerably. Histosols are difficult to work with because they require a combination of drainage and areation to stimulate the breakdown of organic matter, opposite to the usual goal of supporting the soil ecosystem.