Soil Chemistry

If you grow in soil, understanding the principles of soil chemistry is like flipping on a light in a darkened room. You owe it to yourself to learn about soil.

Soils is composed of three structural elements: sand, silt and clay. These are defined as follows:

– Sand: rock (mineral) particle between 2.0 mm and 0.05 mm in diameter
– Silt: rock (mineral) particle between 0.05 mm and 0.002 mm in diameter
– Clay: rock (mineral) particle less than 0.002 mm in diameter

The soil that is made up of these particles is named according to the predominating percentages of each. The extreme points on the USDA soil texture triangle are bad. We’re shooting for the loam area.

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Sand is big as far as plants and nutrients are concerned. It is big, smooth and not very “sticky” to ionic nutrients. Nutrients are only useable to plants when they are in their inorganic form. What does that mean? Well, to be organic-—chemically organic-—a molecule must have a carbon ring attached to it. It’s that simple. Organic compounds are carbon-containing, inorganic compounds do not have carbon. End of discussion. We are setting politics aside—-this is what a chemist will tell you.

Plants do not use organic compounds; they use inorganic compounds in ionic form. An ion is a molecule or atom that does not have an equal number of protons (+) and electrons (-). Because their electrons and protons are not balanced, ions carry an electrical charge. All of the nutrients a plant needs must be in this ionic form or they cannot be used because they cannot be exchanged via active transport. Moreover, each ionic mineral has either a positive or negative charge. (If it has a positive charge it is called a cation; if it has a negative charge it is called an anion.) The charges on both the ions and the soil particles keep them stuck together like magnets and prevent the nutrition from washing away. The mineral ions adhere to, or are adsorbed onto, the soil particles. When lots of ionic minerals are adsorbed by the soil, it makes for lots of useable plant nutrition.

The Two Components that Make Soil Chemically Active

Sand, because of its large size, has a low surface-to-volume ratio and there-fore a low electrical charge. Silt is smaller and has a better surface-to-volume ratio and therefore it has a higher charge. Clay is the smallest, with particles so small that you need a microscope to see them. It has a high surface-to-volume ratio, and therefore a high charge. Think of it this way: volume “dilutes” the charge; the smaller the particle, the more “concentrated” the charge.

This electrical charge allows mineral nutrient ions to stick to the constituent soil particles. Look at the soil texture triangle. You can see that, based on the percentage of sand, silt and clay, a given volume of soil may have a greater or lesser charge than another. The amount of total electrical charge in a given volume of soil is called the soil’s Cation Exchange Capacity, or CEC for short. The more sand in a given sample, the lower its CEC will be; conversely, the more clay in the sample, the higher its CEC will be. Also, larger amounts of organic matter, or humus, in the sample will result in a greater CEC.

Humus and clay are the two sources of chemical activity in soil. This is be-cause they are the smallest particles in soil. Now before you run off to the compost pile and mix in a bunch of leaf mulch, you should understand a few things about organic matter. The kind of organic matter you want in your soil is not last year’s garden waste. You want the tiny particles that are the result of years of bacterial, fungal and physical breakdown of such waste—the particles so tiny that they neither float nor sink in solution. These are called colloids. Both clay and humus particles are often small enough to remain in colloidal suspension—they have the highest chemical activity and give your soil its CEC.

Cation Exchange Capacity

Cation Exchange Capacity is a measure of how many H ions can be held in 100 grams of soil. The clay and humus particles in the soil are generally negatively charged and they therefore adsorb cations. When the plant needs to absorb a cation, it secretes an H+ cation (recall the process of active transport), which the soil particle attracts (because it is negatively charged) and in exchange releases a cation of useful nutrient. The clay (or humus) particle readily takes on the H be-cause it is a lower molecular weight than the cation it is giving up (NH4+, for example). The number of times 100 grams of soil can make such exchanges for H is referred to as its Cation Exchange Capacity. Specifically, for every milligram of H that each 100 grams of soil can trade out, the soil’s CEC is increases by 1.

CEC 1 = 1 mg of H+ adsorbing potential of 100 grams of soil.

Since there are other cations to account for (in addition to H+), another way to look at CEC is through milligram equivalence, or “MEQ.” Let’s take Ca++ as an example. Since calcium has a double charge to it, it can bind to twice as many soil particle-sites as can H+, which has just the one cation. So although the soil’s CEC is the same, it can only hold half as many Ca++ cations as it can H+. Also, because Ca++ is 40 times heavier than H+ (but the soil can only hold half as many), the total weight of Ca++ held by 100 g of soil with a CEC of 1 is 20 mg to H+’s 1 mg; hence, the MEQ is 20 for Calcium and 1 for Hydrogen.

What about anions? After all, plants also need anions (such as nitrate, phosphorus, sulfur, boron, chlorine and molybdenum). Anions are a bit of a problem. They are not easily adsorbed onto soil particles and therefore tend to wash away. Only rarely does a soil have an AEC, or anion exchange capacity. Plants must get these anions by chance encounter as they tumble and bounce through the soil and eventually contact a root hair.

If this sounds unlikely to occur, consider the amount of root surface area in a plant. It sounds crazy, but this has been measured. A botanist by the name of Dittmer figured the surface area of a typical winter rye plant (not a big plant) back in 1937.1 He counted the roots by category and added surface area. Here’s what he found: “The 13,815,672 roots had a surface area of 2,554.09 square feet… Living root hairs on this plant numbered 14,335,568,288 and had a total surface area of 4,321.31 square feet… The root hair surface combined with that of the roots gave a total of 6,875.4 square feet!”2

Soil pH

Your soil is conspiring against you in an attempt to become acidic, and the longer it is in use, the more dramatic this tendency becomes. The more organic matter your soil has, the lower its pH will be. This is because as organic matter decomposes, it releases carbon dioxide, which then reacts with water to create carbonic acid (H2CO3) in the soil. Similarly, as minerals decompose, high concentrations of aluminum are released in forms that create acidic conditions. Add to this the use of fertilizers (which are generally acidic because of their reliance on ammonium) plus the plant itself (which tends to acidify soil during nutrient uptake by taking up more cations than anions), and you have a situation that is precarious at best and that requires your continual vigilance. Ideally, when growing cannabis, you want to keep your soil’s pH in the 5.7 to 6.5 range. Do this by periodically adding dolomitic lime.

When growing outside, things are even more challenging because you must account for rain, which is always slightly acidic (owing to CO2 in the atmosphere that gets dissolved in the rain). This is true even when we don’t account for the phenomenon known as “acid rain,” which is cause by industrial sulfur and nitrogen emissions. Rain water pH typically tests at about 5.7. And because anions do not adsorb onto soil particles as strongly as cations, they get washed away by rain and watering. This washing away of anions predisposes your soil to acidification even further.

Tight or Loose: The Calcium-to-Magnesium Ratio

The balance of calcium and magnesium determines how “tight” the soil is. These two cations make up the bulk of bound nutrient in the colloidal component of your soil (clay and humus). Calcium has a large ionic radius, meaning it holds colloids at a distance and flocculates (opens) soil, whereas Magnesium has a small ionic radius and holds its colloids close and therefore coagulates (closes or tightens) soil. The measure of a soil’s tightness can be determined by a soils test where Ca and Mg are calculated on a MEQ/100 g basis then the level of Ca is divided by the level of Mg to give a numerical index such that:

  • <2 = tight, sticky (or hard, if dry) soil
  • 5-7 = just right soil
  • >10 = too loose soil.

It is the ratio that matters here, not the total level of each component cation. If you have ever walked through a muddy field and had soil build up on the bottoms of your shoes, you know what a high Mg-to-Ca soil is.

Sand, silt and clay make up soil structure, while the organic component humus along with the clay are responsible for the soil CEC. Soil pH, plus a good initial mix, is the key to growing healthy plants in soil. Now that you know what’s going on in the root zone, you can more easily identify the problems when things go wrong.

  1. Dittmer, H. J. (1937). A quantitative study of the roots and root hairs of a winter rye plant (Secale cereale). American Journal of Botany, 417-420.
  2. Ibid.