Healthy soil is the foundation of successful farming. Understanding key soil properties – such as particle types, texture, density, porosity, and permeability – can help you make informed decisions on your farm.
This guide will break down these concepts in simple terms.
Table of Contents
Understanding Soil Particles and Size Distribution
Particle Size Distribution (PSD) is a quantitative measure of the particles in a soil sample. The table below shows how particles within a soil sample can be characterised as either sand, silt, or clay based on their size (in mm). Gravel is not considered to be a soil material.
(1) Sand | Largest | Feels gritty when rubbed between fingers.
(2) Silt | Medium | Feels smooth and soft.
(3) Clay | Smallest | Feels sticky when wet and hard when dry.
The Soil Texture Triangle
While closely linked to PSD, soil texture is instead a qualitative classification of soil based on the proportion of sand, silt, and clay in a soil sample. The Soil Texture Triangle (shown below) is a tool used to classify soils based on particle percentages. Following soil analysis, farmers can use the soil texture triangle to identify their soil type and determine the best management practices.
The Different Soil Texture Types
Using the Soil Texture Triangle, farmers can identify if their soil is more (a) sandy, (b) clay or (c) loamy, with certain implications for each.
(a) Sandy soils – Drain quickly, warm up fast, and are easy to work with but require frequent fertilisation and irrigation. May benefit from the addition of organic matter.
(b) Clay soils – Hold water well and are nutrient-rich, but they can become hard and compacted, making it difficult for plant roots to grow. May benefit from gypsum to improve soil structure.
(c) Loamy soils – The ideal soil for farming, loam is a balanced mix of sand, silt, and clay, offering good drainage, moisture retention, and fertility.
Bulk Density vs. Particle Density: What’s the Difference?
Soil density is essential in plant growth, as it influences how easily roots can spread and how well water moves through the soil. There are 2 principle terms when it comes to soil density: bulk density & particle density.
Bulk Density
Bulk density is the mass of dry soil per unit volume, including air spaces (pores). It reflects soil compaction levels:
⇒ High bulk density (compacted soil) reduces soil aeration and water filtration, making it harder for roots to grow.
⇒ Lower bulk density means the soil has good aeration, which supports healthy root development.
The movement of heavy machinery or livestock on soil can increase a soil’s bulk density and lead to soil compaction. To reduce bulk density, farmers can use deep tillage, organic matter, or cover crops to improve soil structure.
Particle Density
Particle density is the mass of solid soil particles per unit volume, excluding air spaces. For most mineral soils, the average particle density is 2.5 – 2.7 g/cm³. While soil particle density generally does not change significantly, bulk density can be managed through farming practices to optimise soil health.
Soil Porosity & Permeability For Air & Water Flow
Porosity
Porosity refers to the percentage of pore space in the soil. It determines how much water and air the soil can hold. Ideal soil porosity is around 50%, meaning half of the soil volume consists of air and water-filled spaces. Porosity depends on the soil type:
• Sandy soils have large pores, allowing water to drain quickly, sometimes too fast for plant roots to absorb moisture.
• Clay soils have small pores, holding water for longer periods but restricting airflow to roots.
• Loamy soils offer a balance with moderate porosity that supports healthy root development.
Improving soil porosity can be achieved by adding organic matter, such as compost or manure, which increases the number of small and medium pores to enhance water retention and aeration.
Permeability
Soil permeability is the rate at which water moves through soil pores. It is closely linked to porosity, but while porosity refers to the volume of open spaces, permeability determines how easily water flows through them.
Some factors affecting soil permeability include:
• Particle size: Sandy soils have high permeability, while clay soils have low permeability.
• Pore connectivity: More connected pores allow faster water movement.
• Organic matter content: Improves water retention and infiltration.
Soil permeability is an important measure for farmers. High permeability (generally in sandy soils) means water drains quickly, requiring frequent irrigation. On the other hand, low permeability (generally in clay soils) can lead to waterlogging, affecting root oxygenation.
Test Your Soil Properties with Southern Scientific
Understanding these soil properties allows farmers to make informed decisions about irrigation, fertilisation, and soil amendments. Regular soil testing ensures optimal conditions for plant growth and improves overall farm productivity.
At Southern Scientific, we offer reliable agricultural soil testing services to help farmers analyse soil texture, density, porosity, and permeability. Our accurate and timely results support better soil management for improved crop yields.
