Soil Science Simplified
Care for the soil in your garden and your plants will look after themselves. This article explains the basic principles of soil science and why healthy soil is the key to healthy plants.
Soil is amazing. It may look ordinary, brown and sometimes mucky but it provides a home for a massive array of plant and animal life upon which we rely and without which we would probably not survive. It hides history, it is an ever changing product of pre-historic processes and a good soil is every keen gardener's best friend.
The Purpose of Soil
Primarily soil provides most of the food and water requirements for plants. Soil also provides a stable medium in which the roots can grow; healthy roots are able to hold a plant upright.
What is Soil?
The formation of soil is a process which happens over millions of years. Soil is primarily the product of the chemical and/or physical weathering of underlying parent rock.
The chemical weathering of rock occurs when weak acids such as carbonic acid (formed when water and carbon dioxide mix) run over the rock over a very long time period. The prime example of this is rainfall.
The main causes of physical weathering are frost, heat, water, wind and ice. Taking frost as an example, water seeps into the cracks within the rocks, freezes, expands and causes the rock to shatter.
Ingredients of Soil
The 'ingredients' of soil can be split up into 3 different groups:
Matter that has Never Lived
- Particles derived from rock erosion
Matter that Once Lived but is Now Dead
- Animal Remains
- Plant Remains
Living Organisms in the Soil
Living organisms play a vital part in the health and fertility of the soil. Their primary role is to break down organic matter which in turn improves the structure of the soil therefore increasing the ability of the soil to hold on to nutrients. Living soil organisms thrive on soil that has plenty of pores holding air.
Soils are classified according to the dominant type of soil particle that they contain, for example a clay soil is not made up of 100% clay particles, but clay is the dominant particle in the soil mix.
The particle type and the minerals that it contains is dependant upon the type of underlying parent rock.
The degradation of rock over millions of years causes the formation of 3 types of soil particle, classified by their size:
- Sand Particles: 0.06 - 2.0 mm in diameter
- Silt Particles: 0.002 - 0.06 mm in diameter
- Clay Particles: less than 0.002 mm in diameter
Types of Soil and their Characteristics
The type of parent rock and the size of the predominant soil particles determines the soil type. The particles contain the essential minerals that are required for high soil fertility.
Clay soils are usually heavy and cold. They are often difficult to dig, especially when wet. Clay soils are the most fertile and can hold a lot of water, but are usually poorly drained. When rubbed between the fingers, clay soils feel sticky.
Sandy soils are the opposite of clay soils. They are dry, light and feel gritty when rubbed between the fingertips. Sandy soils are free-draining and easy to work. They also warm up quickly in spring, which means new crops can be planted on them earlier in the season. Sandy soils are not very fertile and they have a structure that does not hold on to moisture and nutrients very well
Chalky soils are moderately fertile. They are white-grey in appearance and often contain large amounts of stones or flints. They are free draining and also have a structure that allows free drainage of water and leaching of nutrients. Chalky soils contain a large amount of lime that makes them unsuitable for a large variety of plants.
Silty soils are moisture retentive and fertile. Silty soils are neither gritty or sticky when rubbed between the fingertips, they are smooth and silky. When wet, silty soils take on some of the characteristics of clay, becoming heavy and cold.
Dark in colour, peaty soils have a spongy texture. They are rich in organic matter and usually acidic with a pH lower than 7. Peaty soils are not very fertile, they are devoid of some trace elements and their low pH prevents the uptake of some other nutrients.
The term 'loam' refers to a mixture of different types of soil particles, and a 'good loam' is the desired soil for most gardeners, it combines the fertility and water holding capacity of a clay soil with the good drainage and aeration of soils that predominantly consist of sand, silt or chalk particles.
Observing the type of surrounding vegetation can sometimes give clues to the type of underlying soil present. Heathers for example usually grow on acidic soil with a low pH value, stinging nettles thrive on soils that are very fertile.
The texture of a soil is a reference to the different proportions of sand, silt and clay particles that it is made up of. Some examples of the different textures that gardeners refer to are 'loamy sand', 'silty clay loam' and 'sandy clay'.
Soil acidity is a measure of the number of Hydrogen ions in a soil or 'potential Hydrogen' abbreviated to pH. The pH scale used to measure acidity runs from 0 to 14, zero being extremely acidic, 14 being extremely alkaline. Most soils fall in the range 3.5 to 8. The majority of plants prefer to grow on soil that is neutral (7) or slightly acidic. The higher the pH value, the lower the number of Hydrogen ions present.
Adding lime to a soil reduces it's acidity and increases the pH value. On chalky, alkaline soils adding organic matter regularly will lower the pH and increase the range of plants that can be grown.
Soil pH affects the solubility of minerals that are in the soil. Therefore as pH changes it increases or decreases a plant's ability to take up certain nutrients.
The most important nutrients in any soil are Nitrogen, Phosphorus, Potassium, Magnesium, Calcium and Sulphur.
Nitrogen, Potassium and Phosphorus are the three primary nutrients in any soil:
- Nitrogen (N) is important for vigorous, healthy foliage growth.
- Phosphorus (P) is important for strong root growth.
- Potassium (K) promotes flowering and fruit development.
Almost as important, but only required in small amounts are Iron, Manganese, Copper, Zinc, Boron, Molybdenum and Chlorine.
Soil structure can be defined as the arrangement of the particles within the soil. A soil with a good structure has particles that are grouped into 'crumbs'. There is a good blend of large pores between the crumbs and small pores actually in the crumbs. The pores in and between the soil crumbs are filled with water or air. For good plant growth, the majority are filled with water and the remainder filled with air.
Crumbs form over a very long time period, often thousands or millions of years. The formation of crumbs is aided by the constant addition of dead plant and animal material, which has a binding effect. Simplified, you could compare it to producing a crumble mix at home in the kitchen, the rubbing in of butter to flour forms a mix with a crumb like structure.
Using sand as an example of a poor soil structure, it is a collection of particles that are not bound together into crumbs, but there are large pores between the particles for air and water. Because the particles are not bound together in sand it has a poor structure that allows water and nutrients to drain away to the detriment of the plants growing in it.
When a soil is holding the maximum amount of water possible, but drainage is also still possible then it is said that the soil is at 'field capacity'.
A good loam soil will consist of about 50% pore space, most of which will be available to hold water. Because of the varying particle size in a good loam soil, the spaces between pores are of different sizes, therefore loam soils usually contain a good balance of large and small pores which provides good water retention and drainage.
Sandy soils have mainly large pore spaces so water retention is poor.
The particles in a clay soil are so tiny that they tend to pack together very tightly and can prevent the free passage of water upwards or downwards. So although clay soils are capable of holding large amounts of water, the roots of plants are somewhat restricted when they try to suck up the water and clay soils are often poorly drained and easily become waterlogged.
Improving and Feeding the Soil in your Garden
The primary aim of adding well rotted organic matter to a soil is to improve it's structure and workability. Well rotted garden compost or animal manure can provide nutrients and maintain the fertility of the soil, but large amounts have to be applied on an annual basis to achieve this. Where large amounts of compost or manure are not available, many gardeners use commercial fertilizers to make up for the shortfall if they have nutrient deficient soils.
The regular addition of organic matter improves the structure of a soil and therefore improves it's ability to hold on to nutrients and water. The term 'humus' is used to describe the animal and plant remains in the soil that decompose over a long period of time. Humus is also formed by the regular addition of organic matter such as compost or manure.
The addition of small amounts of organic matter or humus to the soil does not directly improve the fertility of the soil, but it does start to improve the capability of the soil to hold onto the nutrients that it already has by binding the particles together to form crumbs.
Animal manures and garden compost are less concentrated than commercial fertilizers. Concentrated commercial fertilizers e.g. blood, fish and bone are easy to use and apply but they do not improve the structure of the soil.
Regular addition of organic matter to clay soils will over time force the closely packed particles to split up giving the soil a much better structure, increasing the size of the pore spaces between the particles, which allows the better passage of air and water. Such clay soils that have been improved over time with the regular addition of organic matter are some of the best soils on which to garden.
Light soils such as sandy and chalky soils need the regular addition of organic matter to improve the retention of water and prevent the run off or leaching of nutrients from the soil.
Find out more about the soil in your garden with a soil testing kit.