- THE PLACE OF SOIL SCIENCE IN CROP HUSBANDRY
Soil Science is one of the subjects that make up the subject of Crop Husbandry (See Figure 1). It is particularly important because it deals with the basis of all farming, the soil and covers the formation and properties of soils.
Figure 1: Subjects that make up crop husbandry
- THE DEFINITION OF SOIL
A soil can be:
- Any substance in which plants will grow; e.g. the surface at the bottom of a pond; a culture solution ‐ water with nutrients added and;
- An agricultural soil is the loose layers of material covering the earth’s crust supplying anchorage, water, air and nutrients to plants.
3. THE 5 MAIN SOIL TYPES AND THEIR CHARACTERISTICS
- Calcareous soils
- Peat (Organic soils)
- Sandy soils MINERAL SOILS
- Clay soils
- Loam soils
- Have high lime content;
- Usually formed directly from underlying chalk or limestone;
- Light soils, with a poor water and nutrient holding capacity; low in potassium;
- Best suited for crops which thrive in lime e.g.: Barley and Lucerne and;
- Mainly found in the U.K. and South Africa.
- These soils have a high organic matter content of up to 90%;
- Black in colour;
- Acid peats occur in high rainfall areas or badly drained areas e.g. vleis. The soil becomes waterlogged and the lack of oxygen in the soil slows up the decomposition of organic matter therefore poor fertility and;
- Fen peats are alkaline soils which are very deep and fertile and occur beside river estuaries.
SOIL SCIENCE–LEARNER GUIDE
- These soils contain 20% silt and clay and over 50% sand. Sandy soils can go up to 85% sand;
- Poor water holding capacity leads to the soils drying out very quickly;
- Lacking in plant nutrients and thus a low fertility;
- Poor nutrient holding capacity and with no residual value for nutrients. Each crop has to be fertilized;
- Poor plasticity and cohesion: the soil does not stick together but crumbles;
- They warm up and cool down quickly;
- Well aerated: the pore spaces hold more air than water;
- Easy to plough and cultivate;
- Crops are easy to establish with good germination of seeds sown and;
- Will grow most crops if well fertilized. Ideal for tobacco.
- Contains over 30% of clay and less than 50% sand;
- Good water holding capacity as they dry out slowly. Some water is held very tightly in the soil with up to 15% of the water in the soil being unavailable to the plant;
- High in plant nutrients with a high fertility;
- Good nutrient holding capacity with a good residual value for nutrients e.g. some fertilizer nutrients will carry over from one crop to the following;
- Good plasticity and cohesion. If you pick up a handful of clay soil and squeeze, it will form a ball and stick together. On drying out the shape will remain the same. Bricks are made from clay and are baked to harden them;
- Warms up and cools down slowly;
- Poor aeration: the pore spaces hold more water than air;
- Difficult to cultivate and hard to get a seedbed. The lumps become hard when they dry out;
- Crops can be difficult to establish because of poor germination due to a poor seedbed;
- Many clay soils have a capping factor. After rain the soil surface will form a crust as it dries out and;
- Clay soils will grow good crops if they are ploughed and cultivated at the right time.
- Contain up to 30% clay and over 30% sand;
- Halfway between Clay and Sandy soils;
- Holds water and nutrients well and are generally fertile soils and;
- They will grow good crops and are fairly easy to farm.
4. THE COMPOSITION OF MINERAL SOILS
MINERAL SOILS ARE COMPOSED OF:
- 45 % Mineral Matter – sand and clay
- 5 % Organic Matter – decomposing vegetable matter which is rotting in the soil
- 25 % Air
- 25 % Water