Definition of Soil
Soil is the collection of natural bodies, on the outermost part of the Earth's crust, which support plants. It is the thin cover of the earth's crust that serves as the medium plant growth: crops, weeds and natural vegetation inhabit the soil.
Soil can also be defined as a mix of varying amounts of inorganic matter, organic matter, water, and air. Properties of soil are determined by the integrated effects of climate and matter acting upon parent material, as conditioned by relief over periods of time. The earth's crust is the bedrock of the soil and could be several kilometres thick while the soil ranges from few centimetres to a few metres deep.
The crust can be thicker than 80 kilometres in some spots and less than one kilometre thick in others. Underneath the earth's crust is the mantle, a layer of silicate rock approximately 2700 kilometres thick. The mantle accounts for the bulk of the Earth. In between the soil and the earth's crust is the weathered (decomposed upper part) of the earth's crust.
The Soil Profile
A segment of the upper part of the earth's crust magnified represents a hypothetical soil profile. It is a vertical succession of horizontal layers of the soil on which plants grow and man lives and sustains.
Man uses the soil as foundation for roads, buildings and other structures and as a receptor of waste from both living and non-living things. Above this is the major medium for growing crops. The surface (top) soil has a volume composition which consists of mineral, air, water and organic matter in a proportion of 45:25:25:5, respectively.
Soil can be described in terms of its physical, chemical and biological features.
Physical Characteristics of the Soil
Soil Texture
The mineral fraction of the soil results from decomposition of hard rock to sand, silt and clay. Soil texture is the relative proportion of sand, silt and clay in the soil. An ideal soil is loam with relative proportions of 6-28% clay, 28-50% silt, and 24-52% sand. The textural triangle gives the textural class of any soil. A clayey soil has more than 55% clay while sandy soil has more than 85% sand.
Soil Structure
Organic matter component binds the texture to form particles into aggregates. This gives the soil its physical frame for good crop production. The organic matter content of tropical soils is always below 3% but good agricultural soil has a range 3-5% organic matter.
Chemical Characteristics of the Soil
The chemical composition of the soil affects soil fertility and productivity. They include pH which is the soil reaction. Soil pH is a measure of the acidity and alkalinity in soils. The soil could be acidic, neutral or alkaline. This determines the availability of nutrients in the soil for plant uptake as well as other several chemical processes in the soil. The pH levels range from 0 to 14, with 7 being neutral, below 7 acidic and above 7 alkaline.
The optimal pH range for most plants is between 5.5 and 7.0: however, many plants have adapted to thrive at pH values outside this range. While alkalinity increases from 7.1 to 14, acidity increases from 6.9 to 0, hence, a soil sample with pH of 4.5 is more acidic than that of a sample with pH of 6.5.
Soil Organism
Soil is living because a vast number of micro- and macro-organisms live in it. Large proportions of them belong to plant, and some to the animal kingdom. Examples of the large soil organisms are earthworms, insects, rodents etc. They burrow through the soil and take part in soil structuring and aeration.
They do the work of reworking the soil and incorporation of organic matter to deeper depth while stirring up soil from deeper levels also. They are therefore involved in mineralisation of organic matter, soil structure and aggregation of soil particles, nutrient recycling and ion exchange processes that sustain plants animals living in the soil. Soil micro-organisms include bacteria, viruses, fungi and nematodes.
While some of the micro-organisms are useful to plants, some of them are pathogens (disease-causing agents).
Plant Essential Nutrient Elements
There are 16 essential plant nutrient elements required by crops. These essential nutrient elements are divided into three broad categories.
Major Nutrients: These are mostly supplied by nature. They are carbon (C), hydrogen (H) and oxygen (O).
Macronutrients: Macro nutrients are required by plants in large quantities. They are further divided into two sub- groups:
Primary Macronutrients: Nitrogen (N), Phosphorus (P) and Potassium (K).
Secondary Macronutrients: Calcium (Ca), Magnesium (Mg) and Sulphur (S).
Micronutrients: These are needed in small quantities by plants. They include Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Boron (B), Molybdenum (Mo), and Chlorine (CI).
These aforementioned nutrient elements are all essential because the lack, inadequacy or surplus of any of them will result in abnormal growth of the crop, low crop yield and quality. Any of this condition can predispose crops to pests and diseases.
To correct these abnormalities, the concept of fertiliser application is therefore introduced in soil management. The three principal fertiliser elements are N, P and K and can be applied as compound or single fertilisers. Examples of compound fertilisers are NPK 15- 15-15 and 20-10-10. Other fertiliser grades like NPKMg 12-12-17- 2 have also been introduced to include magnesium which is needed by oil crops.
The current trend in fertiliser use is the introduction of compost mixture with mineral fertiliser called organomineral fertiliser. It is more stable, balanced and environmental friendly, cheap and sustainable. Since it is produced from domestic, municipal or agricultural wastes, it helps in environmental sanitation through the concept of 'waste to wealth'.
Types of Fertilisers
Mineral: Derived from rock minerals or synthetic materials; e.g. urea, muriate of potash.
Organic: Derived from plant or animal origins; e.g. compost, green manure, farmyard manure, etc.
Organo-mineral: Combination of organic and mineral fertilisers; e.g. compost + urea.
Biofertilisers: The use of some micro-organisms in improving soil fertility e.g. mycchoriza, rhizobium, blue- green algae, etc.
Sources of Principal Fertiliser Elements (NPK)
N: sources are urea, CAN (Calcium Ammonium Nitrate), ammonium nitrate.
P: the sourced are single and triple super phosphates
K: sources are KCI (Potassium Chloride) also called Muriate of potash (MOP) and potassium nitrate.
These nutrients can also be supplied through organic sources.
Soil Management for Agricultural Production
Soil is a major factor of crop production; therefore, it must be well managed and maintained. There are however a lot of problems in soil management due to the inherent nature of tropical soils of which the Nigerian soil is typical.
The soil is described as fragile due to:
Low activity clay;
Low cations exchange capacity;
Low organic matter (OM) and high loss of OM due to high temperature; and
Susceptibility to rain and wind erosion due to torrential rainfall and high wind velocity in the arid regions.
The soil degrades very fast immediately after land clearing. This has been responsible for the failure of large scale farming in Nigeria. The recipe for minimising soil degradation therefore is proper management strategy. Development of management techniques that fit the tropical soil is the main trust of agronomy. They include:
Minimum disturbance soil during land clearing and tillage by avoiding the use of heavy equipment (uncontrolled tractorisation) for land preparation;
All year round coverage of the soil surface by vegetation i.e. multiple cropping (mixed cropping farming system);
Serial land clearing. Avoid a large-scale mechanical removal of trees grass without adequate precaution for soil conservation; and
Proper fertiliser use to replenish nutrient loss and depletion.
Major Soil Types in Nigeria
There are six major soil types in Nigeria. They are:
Alfisols: They have relatively adequate nutrient components. They are dominant in the moist savannah, dry and moist rainforest. They belong to family of a Ferrugenous and Latisoic soils.
Entisols: Soils of alluvial flood plains or poorly developed shallow rock ground and unconsolidated Aeolian deposits on the edge of the Sahara Desert.
Inceptisols: Soils nearing maturity of development with strong evidence of the decomposing parent rock visible.
Oxisols: Variously called Latosolsor Ferralsols, Lateratic or Ferralitic soils. They are the oldest, most intensely weathered and nutrient deficient soils in the tropics. In Nigeria, they are most abundant in the South Eastern States and wetter parts of Nigeria.
Ultisols: These are intermediate character between Alfisol and the deep very old Nutrient-poor Oxisols. They are almost equally distributed between the humid and the moist forest Zones in Nigeria.
Vertisols: They are heavy clayed soils. They are localised and found in some depressions along the riverbanks. An example is Vertisols of Benue Basin in Taraba State.
We can now appreciate the need for management strategies to maintain and sustain productivity of such a delicate soil systems on which an exploding population depends for food and nutrition.