ALL ABOUT SOIL

WHAT IS SOIL? Soil forms just a thin covering over the surface of the land. Good soil leads to abundant animal and plant life. Soil gives plants a firm base from which to stretch out their leaves to catch the sun’s energy.

WHAT IS SOIL? Soil also contains water and minerals which plants need for healthy growth. Soil acts as a home for many living organisms. These affect the quality of the soil. The non-living ‘skeleton’ of the soil consists of rock particles, water, air and humus.

ROCK PARTICLES Rock particles are tiny pieces of stone. These are slowly broken from the solid rock by the process of weathering. The soil particles which are produced by weathering come in a variety of sizes. The largest are really small stones or gravel. The rest are sorted into 3 grades: clay, silt and sand. All fertile sands are a mixture of the 3 and are called loams.

Clay particles are very small, less than 0.002mm in diameter. SOIL TYPES Clay particles are very small, less than 0.002mm in diameter. Silt particles are middling in size, ranging in diameter from 0.002mm up to 0.02mm Sand particles are the largest in the soil, ranging in diameter from 0.02 up to 2.0mm

SOIL TYPES Clay loams are heavy and difficult to dig. They do not drain well and may be cold, wet and contain little air. Sandy loams are light. They drain well and have plenty of air. They tend to be dry and to lose minerals easily. Soil crumbs are clusters of soil particles of various sizes, stuck together by water and humus.

HUMUS Humus consists of the decaying remains of dead plants and animals and their wastes which have become mixed with the soil. Partly decomposed material is eventually broken down by soil bacteria, leaving only its valuable minerals behind in the soil. The sticky humus binds together rock particles of various sizes to make soil crumbs. Humus prevents minerals being leached out of the soil by rainwater.

WATER The soil holds water in two ways: The soil particles are covered in thin films of water. Water can fill the spaces between the particles.

After heavy rainfall not all the water is drained: The films round the soil particles are held too tightly to be removed. Also, some of the water in the soil spaces is held by capillary force which can resist the pull of gravity. A good soil must show good drainage but also have good water retention. The drainage and water retention abilities of a soil depend on its particle size, it’s capillary and its humus content.

Water present in top soil and subsoil

AIR Air is an important part of any soil. Without enough oxygen plant roots do not grow well and this effects the health of the whole plant. So good soils may have up to 25% air. Soil air is held in the spaces between the soil particles.

Sandy soils have large spaces and hold plenty of air. In Clay soils the spaces are small, hold little air and so are said to be poorly aerated. Wet soils have little room for air and few things can grow in them. Air particles between soil crumbs

MINERAL SALTS The minerals in the soil come from the rock which formed it and from the action of bacteria on the organic matter in the soil (humus breakdown). Mineral salts may give the soil a particular colour – red soils for example, contain lots of iron. Iron salts give the soil a reddish colour.

MINERAL SALTS Salts of Potassium, Iron, Phosphates, Magnesium and Nitrates are present in solution in the soil water. These salts are taken up as ions by the roots of plants and used to build up substances needed for their cells.

FERTILISERS If the soil does not contain enough nutrients, the farmer adds more as fertiliser. Chemical or Artificial fertilisers are used in huge amounts. The major elements in fertilisers are nitrogen (N), phosphorus (P) and potassium (K). Different Artificial fertilisers have different amounts of these elements and the amounts are shown on the fertiliser bag.

The farmer has to be careful how much fertiliser is used and when. Natural fertilisers include farmyard manure and compost. Some farmers prefer these fertilisers because they add humus to the soil. Natural fertiliser rots down and releases nutrients more slowly. The problem for the farmer is that the amount of each nutrient in the fertiliser is not known.

FERTILISERS Spraying inorganic fertiliser Distributing farmyard manure

CROP ROTATION After many years of intense farming using artificial fertilisers, soils can become dry and powdery and are liable to be blown away in strong winds. Minerals can be washed out of the soil by rain water. This is called leaching. To tackle these problems, farmers use Crop Rotation where a different crop is grown on the piece of land each year.

CROP ROTATION Crop rotation helps to maintain the crumb structure of the soil. This also ensures that plants which need alot of one particular mineral do not exhaust it too quickly from the field.