Soil classification and properties

Soil classification is a systematic categorisation of soil types based on their features.
Soils can be classified into three primary types: sand, silt, and clay. These are collectively referred to as soil separates.
Sandy soils have large particles and are typically light, well-drained, and dry.
Silty soils have intermediate particle sizes, feel smooth to touch, and are fertile and hold moisture well.
Clay soils have the smallest particle sizes, are heavy, poorly drained, but very fertile.
In addition, there’s loamy soil, which is a balanced mixture of sand, silt, and clay, often considered to be the ideal soil type.

The vertical layering of soil, termed as soil horizons, is significant in analysing the soil’s structure.
Soil horizons, from top to bottom, are usually referred to as O, A, B, C and R horizons.
The O horizon is mostly organic matter, including decomposing organisms and plant material.
The A horizon or topsoil is a mixture of organic material with mineral particles and is the most fertile layer.
The B horizon or subsoil typically accumulates material leached from the A horizon.
The C horizon is composed of weathered parent material.
The R horizon consists of unweathered parent material, from which the upper layers are derived.

Soil pH refers to the acidity or alkalinity of soil and is a critical factor in determining the soil’s fertility.
The pH scale ranges from 0 to 14, with a pH of 7 being neutral.
Soils with pH values less than 7 are considered acidic, whereas soils with pH values greater than 7 are alkaline or basic.
Soil pH affects the plant’s ability to absorb nutrients from the soil.

Soil permeability refers to the capacity of soil to transmit water and air.
The particle size distribution (soil texture) and structure greatly influence soil permeability.
Clay soils, with their smaller particles, have less permeability. In contrast, sandy soils, with their larger particles, have higher permeability.

Soil fertility is the soil’s capacity to supply nutrients necessary for plant growth.
Fertile soil contains a balance of all essential nutrients, along with a suitable pH, permeability, and contains a good amount of organic matter.
The nutrients essential for plant growth can be divided into macronutrients (e.g., nitrogen, phosphorus, and potassium) and micronutrients (e.g., iron, zinc, copper).
Organic matter added to soil improves its fertility by increasing its capacity to hold water and nutrients.