Soil factors or edaphic factors are those factors that are related to the soil and affect agriculture. These factors include: soil profile, soil color, soil structure, soil constituents and soil pH.
LEARNING OBJECTIVES
By the end of this topic, you should be able to:
- Explain the different edaphic or soil factors
- Explain the influence of different soil factors on crop production
Soil profile
This is the vertical and sequential arrangement of soil in different layers and horizons. Strata is the name given to an individual soil layer. The horizons that make up the soil profile are:
- O horizon (organic)
- A horizon (topsoil)
- E horizon
- B horizon (subsoil)
- C horizon or saprolite
- R horizon (parent rock)
Note that there exist a transition zone that is found between any two bordering soil layers.
Superficial layer (organic horizon)
This is the upper layer of the topsoil that is made up of organic materials like dry or decaying leaves. This soil horizon is mainly black brown or dark brown in color due to the presence of organic content.
A horizon (topsoil)
It is made up of partly decayed animal and plant matter. It is dark in color. It is rich in nutrients and it acts as a water supply to plants. Plant roots, bacteria and small organisms are found in this layer. This layer is also called the zone of eluviation since a lot of nutrients are leached from it.
E horizon
This layer is made up of nutrients leached from O and A horizons. This layer is mostly common in forested areas and it has low clay content.
B horizon (subsoil)
This layer is mainly made up of inorganic materials. It is light colored but its color can vary on the basis of the parent material. Some clay deposits may be found in this layer. It has an impermeable layer called the hardpan, it is compact and less aerated. This layer is also called the zone of illuviation since leached nutrients accumulate here. Trees that have deep roots may reach this layer.
C horizon (weathered rock)
This layer is made up of loose and partly weathered rocks. It has no living organisms and organic matter. It is the thickest layer. Trees that have deep roots may also reach this layer.
R horizon (parent rock)
It is made up of unweathered rock material. It is hard and resistant to weathering. Pond water may be found in this layer. This layer forms the raw materials for soil formation.
Influence of soil profile on crop production
Crop production is influenced by the soil profile in the following ways:
- It determines the type of crops to be grown: Tree crops need mature and well developed soil profiles. This is because they require better anchorage.
- Water infiltration: A deep profile enables water infiltration while a shallow profile promotes surface run-off.
- Soil mineral content: The nature of the bedrock determines the mineral content of the soil.
- It influences the moisture content of the soil: Deep soils with well developed profiles hold more water compared to shallow soils which have poor profiles.
- It influences the method and implements used for cultivation: Profiles with hardpans may require the use of subsoilers to break them.
- It influences the nutrient availability: Well aerated soil profiles contain more micro-organisms. These micro-organisms break down organic matter to release more nutrients into the soil.
Soil texture
This refers to the coarseness or fineness of the soil mineral particles. It is also defined as the relative proportion of the different mineral particles in a certain soil.
Influence of soil texture on crop production
Soil texture has an influence on different soil properties which then affect agricultural production. These properties include:
- Soil aeration (porosity)
- Drainage
- Cation exchange capacity, hence soil pH
- Stickiness of the soil
- Capillarity, hence water distribution
- Permeability, hence water retention capacity
Soil structure
This is an arrangement of soil particles into aggregates or groups and shapes. The shape of the soil aggregates determines the type of soil structure.
Types of soil structures
- Single-grained structure
- Crumby structure
- Granular structure
- Blocky structure
- Prismatic structure
- Columnar structure
- Platy structure
Influence of soil structure on crop production
A desirable soil structure should have the following characteristics which affect crop production.
- Drainage: A good soil structure should facilitate drainage, hence avoiding water logging as it is unsuitable for many crop growth.
- Water infiltration or penetration: A good soil structure should allow enough water penetration and retention for the growth of crops.
- Aeration: A good soil structure should be well aerated for proper root growth and soil microbes activities. It should also permit free circulation of air in order to eliminate build-up of carbon (IV) oxide, and other elements like manganese and iron to toxic levels.
- Soil erosion: A good soil structure should be able to resist soil erosion that may be caused by surface run-off.
- Root penetration: A good soil structure should facilitate good root penetration, which is very important especially in the growth of tuber crops.
- Leaching: A good soil structure should resist excessive leaching of nutrients.
- Heat transfer: A good soil structure should facilitate proper heat transfer in the soil. This improves germination, microbial activity, weathering processes and root development in the soil.
- Microbial activity: It should create favorable conditions for microbial activity by ensuring a good balance between soil, water and air. This is enhanced by proper porosity of the structure.
- Land tillage: It should be easy to conduct land tillage operations like subsoiling and harrowing.
Soil color
Color is important in soil description. Soils may have different colors on the basis of the mineral composition of the parent material. The presence of organic matter on the soil also influences its color.
Importance of soil color in crop production
- Soil color influences soil temperature. Dark colored soils absorb more heat than light colored soils. Soil micro-organisms are more active when soil temperatures are high, and the decay of organic matter is faster.
- Physical and chemical weathering is enhanced by high soil temperature.
- Optimum soil temperature is associated with improved biochemical reactions in the soil and in plants. This enhances crop growth.
Soil pH
This refers to the concentration of hydrogen ions in the soil. It can also be defined as the degree of acidity or alkalinity of the soil.
Importance of soil pH in crop production
- It determines the availability of certain soil nutrients like aluminum, iron and manganese which are not available to plants at high pH values.
- It influences soil structure
- It influences crop attack by pests, diseases and weeds.
- It determines the activity of micro-organisms in the soil.
- It influences crop growth, distribution and development.
