Work in Progress: Basics and Fundamentals

^{Remember to mouse over underlined words for the definition tooltip}

When you see a tree towering up above a house, or a dandelion in your front yard where do you think its mass comes from? You may be tempted to answer the soil, but in fact, most of its mass comes from the air. A tree is majorly carbon after all, and the sapling obtained this carbon from carbon dioxide during photosynthesis. (If this is unfamiliar I recommend watching this short 5 minute dialogue from Richard Feynman). This is true of basically every plant, give it the correct conditions, and it will continue to put on mass until most of its cells are undergoing senescence. But what are the correct conditions? In our gardens or farms this could cover anything from soil condition/composition, fertility, aeration, available root space, light / sun intensity, weather, microbiota interactions, or even fungi associations.

It’s been often said that the quality of cannabis will be determined by the weakest link in its production. If you’re unfamiliar with this saying, whether it’s the genetics being grown, or one of those conditions mentioned earlier, whatever comes up shortest is going to determine the end quality. Considering the number of variables involved this might seem a little daunting, but with some time, practice, and most importantly, knowledge of the fundamental principles at play, most of these will become second nature.

Soil

Soil and the Cation Exchange Capacity –

If you’ve picked up a grow bible in the past, you probably learned to feed plants through liquid fertilizer; Usually covering N-P-K (N) Nitrogen – (P) Phosphorus – (K) Potassium as so called ‘primary nutrients’, and then speaking on ‘secondary nutrients’ with particular emphasis on Calcium (Ca), Magnesium (Mg), and Sulfur (S). On the other hand, if you’ve been reading up on No Till gardening, or similar organic methodologies, you’ve undoubtedly come across people telling you N-P-K don’t matter, the soil will feed the plants. Both of these ways of looking at fertility are a little flawed however.
When we say that the soil will feed the plants, it does so by providing the elements discussed above, among quite a few others, so to say you can ignore the measures of fertility a given input has is silly. Likewise, looking at elements as primary and secondary nutrients  ignores the interconnected web of their interactions. Even worse, it doesn’t teach any familiarity with the concrete science at play.

When we talk about the fertility of a soil we are talking about the capacity and availability of ions, or the Cation Exchange Capacity (CEC). If you recall back to high school chemistry you’ll remember that an atom has a nucleus, made up of protons and neutrons, and orbiting around that nucleus are electrons. The protons have a positive charge, the electrons a negative charge, and the neutrons have no charge at all. A ion is simply an atom, or groups of atoms, with an electrical charge. When we talk about ions, we say it’s either a cation or anion. Cations(+) have a positive charge from losing electrons, Anions(-) have a negative charge from the gaining of electrons. In order for a plant to uptake nutrients, the ions have to be both in the correct form and available. Ions, like magnets, obey a positive charge attracting a negative, and likewise charges repelling. Clay content and organic matter have a large amount of negative charge, these negative charges provide attraction for free cations, as well as repelling free anions. Increasing the clay content, or to a slightly lesser extent, our organic matter increases our Cation Exchange Capacity, with a few minor considerations. The more cations a soil will hold, the more amendments and liming we have to use to fill those negative charges and balance the soil. Once the soil is properly balanced, the high CEC acts as a large nutrient reserve, making for what would be considered very fertile soil. A low CEC soil will have less capacity for cation exchange, and will tend to have more mobile anionic nutrients in the run off. Knowing this, let’s take a look back at those elements mentioned earlier.

Nitrogen is brought into availability in the soil through organic matter decomposing (manure, plants, ect), and from fixation from the atmosphere. Our atmosphere is 79% nitrogen gas( N₂ ), however this form of nitrogen is of no use to use or the plants, as the plants uptake nitrogen as either amino acids, nitrate( NO₃– ), ammonium( NH₄⁺ ), or as urea (NH₂)₂CO. Thankfully, in comes the Nitrogen Cycle!
Nitrogen Fixation takes place from nitrogen fixing bacteria in the soil. Some bacteria require legumes in order to fix nitrogen, but other variety, such as some Azospirillum and Azotobacter species, will work wonderfully for use with cannabis. These bacteria produce an enzyme called nitrogenase that ‘fixes’ nitrogen gas (N₂) into ammonia (NH₃). On leaving the nitrogenase complex NH₃ is quickly ionized to ammonium (NH₄⁺).  This is wonderful for us as ammonium is a cation that is plant soluble, so it can be utilized immediately or it can bind with a negative charge to be of use later. However just because the nitrogen is now in a plant friendly form doesn’t make this the end of the cycle!
Next comes nitrification where the NH₃ / NH₄⁺ from before is oxidized by nitrifying bacteria into nitrite (NO₂-), then further oxidized by the nitrifying bacteria into nitrate (NO₃-). Nitrate (NO₃-) is also plant soluble, but is subject to leaching, as the anionic charge repels the negative charges of the soil, instead being carried away with water as runoff. This tendency to end up in the runoff is precisely why (liquid) nitrogen fertilizers are so damaging. After this come denitrification, where denitrifying bacteria utilize nitrate (NO₃-) as an alternative to oxygen during their respiration. This reduction brings nitrate (NO₃-) back into the form of atmospheric nitrogen (N₂), starting the process over.
Lastly, we have to talk about immobilization vs mineralization, and with them, our C:N (Carbon to Nitrogen ratio). Nitrogen is needed by bacteria, and really most organism, to live. If the Carbon:Nitrogen ratio is above 30:1 then the decomposing bacteria can compete with the plant for nitrogen, causing the bacteria to uptake nitrate, at which point it is non-plant soluble, or immobile. With C:N ratio of 25:1 or under, decomposition of organic materials instead yield ammonium, which is plant soluble, a process called mineralization. As you can see looking at nitrogen as simply nitrogen really makes you loose the picture of the role it plays.

Soil Composition –

Soil is composed of a great many things, but its texture is described by the ratio of only a few types of molecules. From our gardening perspective, these are sand, silt, clay, and organic matter. When it comes to feeding the plants, organic matter and clay are of the most importance, as both play a very large role in the Cation Exchange Capacity (CEC), whereas sand and silt are largely inert. ¹ This isn’t to say sand and silt have no place, they greatly affect the compaction of the soil, as well as the amount of free space. The exchange of gasses within a soil is incredibly important, so particle size and the resulting free space can be just as important as balancing your CEC.

Clay particle under a electron microscope

Most people can easily identify sand, or clay, but know very little about what makes them unique. Sand, the largest particle of the group does not have the most surface area. Clay, while being the smallest average particle size boast by far the highest surface area, due largely to its unique structure. Clay particles are series of platelets, this structure allows many reactions to take place. Clay has a tacky, somewhat sticky texture when wet, and rather hard when dry. Clay is said to adsorb water, gas, or other dissolved substance rather than absorb. That might seem like a bit of a complex idea, but anyone that’s tried gardening in clay soil will be familiar with water collecting on the surface, refusing to drain.  Now of course, there are quite a few different variety of clay, and not all of them behave exactly the same. The varieties fall under two distinct category, those that swell when wet and shrink when dry, these are the expandable clays, smectites, and montmorillonites; and those that do not shrink or swell, kaolinitic or vermiculite.

When we describe the texture of a soil, we are doing so when it is moist. In your garden, or out on a farm, this will be done by feel but don’t forget you can get a much more precise measurement by sending it into your local laboratory for particle size analysis, usually with a hydrometer and pipette. When we describe the soils texture we do so using the three texture components, sand, silt, and clay. There are 12 textures of sand officially (according to the USDA³), clay, sandy clay, sandy clay loam, clay loam, silty clay, silty clay loam, sand, loamy sand, sandy loam, loam, silt loam, and silt. The suffix tells you what the major constituent of the soil is, so sandy clay loam would be very loamy texture, have a clayey feel, clay loam, and still have a some large sandy particles.

You may ask yourself how these textures form naturally over time, and indeed, this question covers yet another way soils are grouped. Soils can also be described by the forces that produced them, as shown in table below^{1, 4}.

If instead we looked at soil from a geotechnical engineers perspective, we would say the it’s comprised of sediments and rocks. Rocks are generally classified into three groups – igneous, sedimentary, and metamorphic, based on how they were formed. ⁵ 

When we as gardeners and farmers talk about increasing mineral content, we refer to our amendments as rock dust because that precisely what they are. Even amendments such as gypsum are rocks bound with other molecules, in the case of gypsum it’s a clastic sedimentary rock, basically a rock bound with sulfates (gypsum, CaSO₄[+2H₂0])⁵.  Gypsum is usually formed when sulfates, that are freely dissolved in water bond with other minerals in a rock face. Similarly, if it were to be carbonates dissolved in the water, calcite could form over time (calcite, CaC0₃). This is why when we utilize gypsum, and provide conditions for its weathering, the soil can make use of both its calcium content (Ca), as well as the sulfur (S).

^{1 – Retaining Soil Moisture in the American Southwest, pg 30, Kelly J. Ponte Ph.D.
2 – Soil Mechanics and Foundation 3rd Ed, pg 12, M. Badhu
3 – Soil Survey Division Staff, 1993, Soil survey manual USDA Handbook #18
4 – Soils and Fertilizers, 1926, Lyton T.L.
5 – Soil Mechanics and Foundation 3rd Ed, pg 7, M. Badhu}