Soil is the heart of the organic garden. A living mix of minerals and organisms, soil provides vital nutrients and a healthy environment that nurtures plants as they grow.
What is Soil?
Soil starts out its life as rock, worn down over thousands of years. It’s made up of particles of sand (the largest), silt (smaller), and clay (the smallest). Well‐balanced soil, which contains equal amounts of all three types of particles, is called loam. Often one type of particle is dominant, as in “sandy loam” for example.
Soil isn’t just rock particles, however: it’s a complex mixture of living and non‐living matter. While soil start out as rock, over time organic matter accumulates as plants and animals die. (Technically “organic” means any material made up mostly of carbon.) Insects, earthworms, and microorganisms such as bacteria and fungi live in the soil and perform the important job of breaking down the organic matter, which in turn nourishes the next generation of plants.
Determining Your Soil Type
To figure out what type of soil you have, pick up a handful. If it’s dry and gritty and runs easily through your fingers, it’s sand; if it’s sticky and can be formed into a compact ball it’s clay; if it’s neither sticky nor gritty and has a silky feel, it’s silt. If your soil can form a soft ball and is brown and crumbly like chocolate cake, it’s loam. Loam is ideal garden soil.
In some parts of the UK, soil may also be chalky or peaty. Chalky soil is alkaline and fast‐draining, requiring extra watering and nutrients. Peaty soil is dark, spongy and organically‐rich – wonderfully fertile but usually acidic.
Compost and other amendments can be added to any type of soil to improve its structure and create a better‐balanced mix.
Like people and animals, plants need sunlight, air and water, which are essential to plant growth. Plants get carbon, hydrogen and oxygen from the air or from water. They also need minerals, which occur naturally in rock particles and organic matter in the soil: the minerals, dissolved in water, are taken up by the plants’ roots.
The most important minerals are called macronutrients because plants require high amounts of them. The three most important macronutrients are:
- Nitrogen (N) – encourages leafy growth
- Phosphorus (P) – needed for root growth
- Potassium (K) – for healthy fruits and flowers
These are the nutrients you see on bags of fertiliser, listed as 5‐10‐5 (N,P,K) for example. Other macronutrients are calcium, magnesium and sulfur, but these do not usually need to be added, or they are included with other additions such as lime.
Plants also need micronutrients – or trace minerals – including iron, boron, copper, manganese, molybdenum, zinc, and chlorine. These micronutrients are usually replenished in the soil with the addition of organic matter such as compost.
Soil may be acid, neutral, or alkaline, depending on the amount of calcium in the soil. The measured acidity of soil is called its “pH”, (which stands for “potential of the hydrogen ion”, in case you’re wondering!) The pH scale ranges from 1.0 to 14.0. A pH of 7.0 is neutral: from 1.0 – 7.0 is acidic, from 7.0 to 14.0 is alkaline. Each number represents a ten‐fold increase or decrease in acidity from its neighbour: soil with a pH of 6.0 is ten times more acidic than soil with a pH of 7.0, for example.
An acidic soil contains too little calcium, an alkaline soil too much. With too little calcium, nutrients wash out of the soil; with too much, nutrients are trapped in the soil and can’t be absorbed by the plants.
As a general guideline, vegetables prefer a slightly acidic soil with a pH from 6.5 to 7.0. Some, however, are tolerant of, or even prefer, acid or alkaline conditions: potatoes like acid soil, whereas brassicas prefer alkaline soil, for example. See the articles for planting each type of vegetable to learn their preferences.
Creating and Maintaining Healthy Soil
In addition to adding soil amendments to improve less‐than‐perfect soil, every garden soil needs to have the nutrients absorbed by growing plants replenished every year. The primary amendments to accomplish these tasks in the organic garden are lime, compost, green manures, animal manures and organic fertilisers. For more information see the article “Soil Amendments“.
Learn to understand your soil, and you’ll have happier ‐ and healthier ‐ vegetables!