Soil – What is it?

Virtually all plants grow in it and its probably the most important element in your garden. But what is it and how should we look after it?

Soil, dirt, muck, humus, fertile, well drained, clay, sand, chalk…..there’s lots of different terms to describe this medium which plants need to grow and flourish, but what do they all mean? So let’s briefly explore what soil is made of and how we as gardeners can nurture it.

Soil is made up of 4 components: Soil Structure (45%), Organic Matter (5%), Water (25%) & Air (25%)…..lets look at each one of these in turn.

Soil Structure – this is the largest part of soil made up of Sand, Silt & Clay. They are different grades of Quartz (silicon dioxide), the second most abundant mineral found at Earth’s surface, that has been ground down over millenia.

Soil structure can be grouped into 12 soil textures depending on the ratio of sand, silt & clay….. including the mythical Loam, which we’ll talk about in a minute. Knowing these helps us understand our soil and what plants will thrive in it.

A lot of people say they have a Clay soil. Actually most gardens have Loam or Sandy Loam. Your soil would have to be 40% clay to be classed as a ‘Clay Soil’.

You can find out what soil structure you have by performing a ‘Mason Jar Soil Test‘.

Loam – consisting around 20% Clay, 40% Sand & 40% Silt, is considered to be ideal soil structure. It has the right balance of moisture retention, nutrient retention and aeration, allowing oxygen to flow freely to plant roots.

Clay soils are rich in nutrients and very fertile. They hold water, are slow to warm up and plant roots find it hard to penetrate. It can be improved by adding organic matter to break up the clay structure.

Sandy Soils are usually low in nutrients, lose water very quickly (free-draining), warm up quickly and roots find it easy to penetrate. It can be improved by adding organic matter to increase water retention.

Chalky Soils – have high proportions of limestone and can be alkaline. They are highly variable and range from gravelly/sandy to clay like in structure. Often poor in nutrients, then can be improved by adding organic matter.

So how do I know what will thrive in my soil – Usually the plant label will tell you what soil structure the plant likes, for example:-

• Acer – Moist, but well drained. This is loam or clay loam.
• Sempervivum – Well drained. Sandy loam or loamy sand
• Salix – Moist, poorly drained. Clay soils.

So select plants to suit your soil structure. The good news is most soils are ‘Loamy’ and most plants thrive in these soils.

Organic Matter – ‘Organic’ means derived from living matter. It comprises everything that lives in the soil, like worms and decomposing plant or animal material. Its packed full of nutrients, improves water retention and is the powerhouse for plant growth and health. Fertile or Humus refers to the amount of organic matter in soil. So if a plant label says ‘Fertile’ the plant requires lots of organic matter and is probably a woodland type plant.

Most soil organic matter originates from plant tissue. Plant residues contain 60-90% moisture. The remaining dry matter consists of carbon (C), oxygen(O), hydrogen (H) and small amounts of sulphur (S), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg). Although present in small amounts, these nutrients are very important for plant growth.

As organic matter decomposes it makes these nutrients available to plants but the levels deplete over time. Hence you need to keep adding organic matter to your soil. This is the single most important thing you can do for plant health.

Add a mulch of garden compost (not to be confused with ‘potting compost’ you buy in bags from garden centres) or well rotted manure (manures are animal derived) to your garden in late autumn and again in spring. No need to dig in. Nature will do the rest and the worms will pull the organic matter into the soil.

Do this year after year and you will generate the perfect soil for most plants to grow.

Water – essential for life. Its held in ‘pores‘, the spaces between soil particles. The soil’s ability to retain water is strongly related to particle size; water molecules hold more tightly to the fine particles of a clay than to coarser particles of a sand, so clay soils retain more water.

Adding organic matter to your soil improves its water retention. This allows a reserve for plants to use and means you don’t have to keep watering your plants.

Air (Soil Air) – Oxygen is needed by many of the microorganisms that release plant nutrients to the soil. An appropriate balance between soil air and soil water must be maintained since air is displaced by water. Air can fill soil pores as water drains or is removed from a soil pores by evaporation or root absorption. 

Soil air is very different to atmospheric air. It has higher levels of carbon dioxide and hence oxygen levels may become depleted. As plants must have oxygen to live, it is important to allow proper aeration in the soil. Again adding organic matter opens up the soil structure allowing more aeration.

So that’s soil. A wonderful resource we should all nurture and look after……..now don’t get me started on Peat (more on that later).

Most of all, enjoy your garden.