Soil Health

A high level introduction to soil, agronomy, and nutrients.

Note: This content is largely copied from the work I did for The Growing Group. At that time, I spent many weeks studying and simplifying the dynamics of soil and how nutirents are delivered into plants and crops. This is a single (albeit long) article which introduces some of the main topics that factor into soil health.

Soil health

All of the food on the planet is an indirect result of a thin layer of topsoil across the planet. Adjustments to this thin layer through fertiliser or other means can have an outstanding effect, although not always in a positive sense. As an example, overuse of fertilisers has lead to nutrient run offs which pollute waterways. However, without fertiliser we would not be able to sustain a planet of 7.6 billion people (and this is indeed a problem we may soon face when we run out of readily available phosphate).

One of the most basic principles that is overlooked by the fertiliser industry is this: what goes into your soil is not what is available to your plant. Applying fertiliser is a bit like baking a cake - you wouldn't just pour flour, eggs, and chocolate into a bowl and expect a perfect cake to result.

The same is true of soil. The particular order, quantity, combination of nutrients, and countless external factors lead to vastly different outcomes. Within the industry it's starting to appear that is as complex as human health, where it's not entirely difficult to survive but the factors required to thrive are still largely under adolescent scientific scrutiny. Nevertheless, there is a lot that we already do know and, although there a huge number of factors, we can achieve incredible results from optimising a few major elements that lead to soil health.

Soil Organic Matter

Soil Organic Matter (SOM) is arguably the most important component of healthy soil. SOM consists of any organic matter that is decomposing in the soil, such as compost, decaying plant material, and animal waste. Once SOM breaks down far enough to resist further decay it is becomes Humus. Humus is able to hold large amounts of water, and this water (called the soil solution) is where almost all nutrients are delivered to the plant.

Soil pH

Soil acidity is another very important variable in healthy soil. It is sometimes called the soil “water” pH. This is because it is a measure of the pH of the soil solution, which is considered the active pH that affects plant growth. Soil pH is the foundation of essentially all soil chemistry and nutrient reaction and should be the first consideration when evaluating a soil test. The total range of the pH scale is from 0 to 14. Values below the mid-point (pH 7.0) are acidic and those above pH 7.0 are alkaline. A soil pH of 7.0 is considered to be neutral. Most plants perform best in a soil that is slightly acid to neutral (pH 6.0 to 7.0). Some plants, like blueberries, require the soil to be more acid (pH 4.5 to 5.5), and others, like alfalfa, will tolerate a slightly alkaline soil (pH 7.0-7.5).

The soil pH scale is logarithmic, meaning that each whole number is a factor of 10 larger or smaller than the ones next to it. For example if a soil has a pH of 6.5 and this pH is lowered to pH 5.5, the acid content of that soil is increased 10-fold. If the pH is lowered further to pH 4.5, the acid content becomes 100 times greater than at pH 6.5. The logarithmic nature of the pH scale means that small changes in a soil pH can have large effects on nutrient availability and plant growth.

Cation Exchange Capacity

In order for a plant to absorb nutrients, the nutrients must be dissolved. When nutrients are dissolved, they are in a form called “ions”. This simply means that they have electrical charges. As an example table salt is sodium chloride (NaCl), when it dissolves it becomes two ions; one of sodium (Na+) and one of chloride (Cl-). The small + and – signs with the Na and the Cl indicate the type of electrical charges associated with these ions. In this example, the sodium has a plus charge and is called a “cation”. The chloride has a negative charge is called an “anion”. Since, in soil chemistry “opposites attract” and “likes repel”, nutrients in the ionic form can be attracted to any opposite charges present in soil.

Some important elements with a positive electrical charge in their plant-available form include potassium (K+), ammonium (NH4+), magnesium ( Mg++), calcium (Ca++), zinc (Zn+), manganese (Mn++), iron (Fe++), and copper (Cu+).

Some other nutrients have a negative electrical charge in their plant-available form. These are called anions and include nitrate (NO3-), phosphate (H2PO4- and HPO4–), sulfate (SO4-), borate (BO3-), and molybdate (MoO4–).

A significant percentage of most soil is clay and a smaller percentage is organic matter. Both of these soil fractions have a large number of negative charges on their surface, thus they attract cation (positive) elements. At the same time, they also repel anion (negative) nutrients since "like" charges repel eachother.

Low CEC soils hold fewer nutrients, and will likely be subject to leaching of mobile “anion” nutrients. The particular CEC of a soil is neither good nor bad, but knowing it is a valuable management tool.