Every soil inherits traits from the parent material from which it formed. For example, soils that form from volcanic soil are rich in iron. Soils that form from limestone are rich in calcium. Soils that form from materials at the bottom of lakes are high in clay. The parent material is changed through biological, chemical and environmental processes, such as weathering and erosion.
"There is much more to soil than first meets the eye.
"Soil is composed of organic matter and mineral particles. The mineral particles are split into three distinct groups, based on their size. There's sand, with particles about the size of a pinhead, then there's silt, about the width of a human hair. And lastly, and the smallest particles, are clay, with some as small as a single bacterium.
"Every soil has a different composition. Some have more clay. Some more sand. It's the unique blend of these components that determines the properties of soil (texture, pH, drainage, water-holding capacity, color, nutrients). This is termed soil texture.
"After these three mineral particles, it is the amount of clay that determines how good a soil is at holding onto nutrients. This is because of two key characteristics.
"Clay particles have a huge surface area, as much as 1,000 times more than sand. This is because the surface of the sand particle is smooth, whereas a clay particle has an incredible has an incredibly uneven surface. If I were to take every sand grain in this teaspoon and lay their surfaces flat, it would just about cover an area the size of this table.
"But if I were to do the same with this teaspoon of clay, it would cover an entire football field. And all that space means more nutrients can be stored. But clay has another important characteristic, one that helps lock up the nutrients, preventing them from leeching out.
"A bit like a book, clay is formed in sheets. There are layers of positively charged cations like silica and aluminum, and these surrounded by negatively charge hydroxyl molecules and oxygen.
"The negatively charged sites on the outside of the clay particle attract positive ions including plant nutrients such as potassium, phosphorus and magnesium, but because they are not part of the clay particle's structure, these cations can be readily exchanged for other positively charged ions. Plants can exchange nutrients from the clay in return for positive hydrogen ions that they release from their roots.
"Now multiply these attachment sites with its huge surface area and a clay particle will look more like this (a big pile of shredded newspaper), an intricate mass of attachment sites with a huge potential for locking up nutrients. The more attachment sites the soil has, the better its potential for holding onto nutrients. This is the cation exchange capacity.
"However, it's not just the chemistry that determines the nutritional value of the soil, there are other factors at work.
"Even the poorest soils can produce impressive yields if there are microbes present. Beneficial soil organisms (bacteria, fungi, protozoa) unlock nutrients from mineral components and transform organic matter into complex compounds (humic substances).
"What this means is that although it's important to recognize soil texture and soil chemistry when planting, fundamentally, it's the soil biology that enables healthy plant growth.
"In our next video, we'll delve further into the complex relationships that microbes have with soil chemistry."
| CLICK to learn about the beneficial presence of microbes (4:05min) - also linked above