Soil

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

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

The importance of soil health can not be overstated. 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 compounding effect, and not always in a positive sense. 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 face in the future if/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. Imagine you were making a cake - if you just poured all the ingredients into a bowl and expected that to result in a perfect cake you may find it doesn’t taste as good as you hoped. The same is true of soil. The particular order, quantity, combination of nutrients, and countless external factors all lead to vastly different outcomes. Soil health seems 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, just as with human health 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. For this article we include Humus in SOM, and it is commonly referred to as ‘Top Soil’.

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.

Nutrients

Phosphate

Phosphate is essentially a rock that is mined throughout the world, which contains a macro-nutrient used by plants – Phosphorus. Phosphorus plays a role in many of the complex functions of a plant. When adequate supplies of Phosphorus are available within the plant it can promote or enhance the following benefits:

  • Early root formation and growth
  • Greater flowering and seed production
  • Fruit, vegetable, and grain quality
  • Better growth in cold temperatures
  • Water use efficiency
  • Early maturation of fruit and grain

Phosphorous is a highly reactive element. The majority of Phosphorous in most soil is in essentially insoluble forms and unavailable to plants. In most situations there is very little soluble Phosphorus in the soil at any point in time. It has been estimated that at any point in time, the solution/available forms of Phosphorus in many soils may only amount to from 0.01 to 0.06 ppm (0.02-0.12 lb P/acre). This Phosphorus will typically move no more than about one tenth of an inch in the soil. Roots quickly deplete the 0.10 inch cylinder of soil around each root and must continually grow into new areas of the soil to maintain adequate Phosphorus intake. Phosphorus is held within the soil in 3 different “Pools”:

The Solution Pool

This is the primary pool in which phosphorus is available to the plant, and is stored in the soil solution

The Active Pool

This is the pool that feeds the Solution Pool, or conversely may withdraw phosphate from the Solution Pool if the quantities of phosphorus in the Solution Pool are extremely concentrated. In this pool the phosphorus is adsorbed to particles in the soil, ready to feed the Solution Pool. It can also provide phosphorus in small amounts to the plant roots.

The Fixed Pool

The Fixed Pool contains very insoluble phosphate that may remain unavailable to plants for many years. Conversion into the the Active Pool occurs very slowly. Within the Solution Pool plants will take up nearly all Phosphorus as either Primary orthophosphate anion or Secondary orthophosphate anion.

Primary orthophosphate is taken up about 10 times as readily as the secondary orthophosphate form. All Phosphorus sources applied to the soil must be converted to the orthophosphate forms before a plant can utilize them. However, applying these forms of Phosphorus to the soil does not guarantee that they will remain in that form for very long. Because phosphorous is highly reactive, it is readily converted to other, less soluble forms. The particular forms that are created depend on other soil factors such as the soil pH, temperature, moisture, other elements, and others. This is one reason all aspects of the soil must be optimized before plants will perform at their best.