Written by Adrienne Wild
Soil is a reservoir of nutrients and moisture, and fertile soil is teeming with life. The measure of any good soil is the presence of earthworms. Referred to as the caretakers of the earth, they help cultivate the soil by burrowing and aerating the ground and breaking down organic matter.
Earthworm activity helps improve aeration and drainage as their casts help to glue soil particles together into crumbs. This enhances the way that solid particles of sand, silt, clay and organic matter are organised, including the pore spaces between them.
Without the activities of earthworms and other soil organisms, dead matter would accumulate and litter the soil surface, and there would be limited food for plants. For plants to grow and flourish, good soil structure is essential.
Well-structured soils are usually crumbly and fragile and have plenty of pore space to allow water and air movement and healthy root development. A poorly structured soil tends to be too wet in winter and too dry in summer. Soil like this is challenging to cultivate, and in summer may become hard and concrete-like. As a result, plant roots have difficulty penetrating down into the ground, so their growth is adversely affected.
Soil structure is not particularly stable, and cultivating and even walking on wet soil will damage the structure of the surface layer by compacting it and squeezing out the air. As a result, only the finest silt particles remain at the surface, which forms a crust, known as ‘capping’. The crust prevents air and water from reaching the roots and also prevents seeds from germinating.
Looking at root development and for evidence of worm activity will give an indication of how well structured your soil is. Shallow, restricted rooting and lack of earthworm activity indicate a problem with the soil structure.
As organic matter is the glue that binds soil particles together, the remedy is to add plenty of homemade compost, bagged soil conditioner or well-rotted manure. No matter the kind of soil structure you currently have, by making some amendments this winter, you can improve your soil structure and have a great gardening season.
Soil texture refers to the look and feel of the soil and depends on the kind of rock from which it initially came from. You will never be able to change the basic soil texture, but there are ways in which you can improve and condition it.
The largest particles present in any soil are sand. The medium-sized particles are silt and the smallest particles clay. Particle size determines the soil’s texture and its fertility.
To assess your own soil’s texture, simply rub it between thumb and fingers under wet conditions.
Different layers of soil have different textures
Sandy soil feels gritty, and its particles can easily be seen. It is also warm and airy, making it excellent for growing bulbs like tulips, roots and asparagus. But as water runs away quickly after rain, nutrients leach out, so it needs plenty of organic matter adding throughout the season.
Silt is like flour or talcum powder when dry and feels like Plasticine or soft modelling clay when wet. It is incredibly fertile, holding onto nutrients and moisture for longer than most soil ingredients. Silty soils are therefore excellent for growing fruit and vegetables.
Clay is made up of minute particles that crowd together into compact masses. Clay soils tend to feel like plastic and are sticky when wet, making it easy to roll into a ball. They are especially hard and impenetrable under dry conditions and crack into wide fissures.
Clays soils can be lightened and made more crumbly by forking in well-rotted manure. Regular liming makes clay soil less sour and helps break it down into crumbly granules. Fertile clay soils are preferred by brassicas such as cabbage, as well as beans, peas, leafy crops and roses.
"SoilComposition" by Richard Wheeler (Zephyris) (CC BY-SA 3.0)
Knowing the soil texture is useful, as it determines how easy the soil is to work and how prone it will be to structural damage through cultivation.
Amendments do not change the texture unless you are adding large quantities of sand, clay or silt to existing soil to create loam. Loamy soil makes good garden soil as it is well structured, with a balanced mix of coarse pores for rapid drainage and a network of smaller pores that hold water and air for plants to access.
For World Soil Day we are creating a week worth of content to educate and inspire gardeners into loving the soil, tardigrades and the untold truths about soil. We want to empower everyone to take ownership over their own plot of land to help the environment, prevent climate change and to help meet UN Global Goals.