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Horticulture 1000
HT1032 - Soils 2 - Amount of organic matter in soil
Biological properties Soil organisms

Amount of organic matter in soil

The amount of organic matter in any  soil is determined by five main factors: the climate, the amount of water in  the soil, the pH of the soil, oxygen levels in the soil, and how the soil is  managed.


Heavy frost on grass

Organic matter is broken down slowly in a cold climate or during the winter, because micro-organisms are not very active.

Water levels

The amount of water in a soil affects leaching (draining out) of nutrients from decomposing organic matter. During regular heavy rainfall many nutrients can be lost from  the soil.

Soil pH

The pH of a soil can affect which micro-organisms are around and active. Some species or types break down organic matter faster than others.

Oxygen levels

A soil with aerobic (plenty of air) conditions encourages bacteria that turn organic matter into humus. This is a fast process.

If anaerobic (low oxygen levels) conditions are present then the process is slow.

Land management

Cultivation then removal of crops will lower the amount of potential organic matter in the soil. For example, when a crop like lettuce is harvested and sold, there is a direct loss of organic matter. The crop will not decompose in the soil it grew in. If the stubble is returned to the soil some of the organic matter will be returned.

Soil depth and organic matter

Distribution of organic matter in a soil profile

Most of the micro-organisms live in the upper part of the soil where it is warmer and there is plenty of food. This is where most of the organic matter will be found. The organic matter may be taken further down into the soil profile by worms.

The graph shows most organic material is in the topsoil. There is less organic material deeper in the soil.

Organic matter in the topsoil creates the darker colour

The finger is pointing to the bottom of the topsoil in this profile. The organic matter in the topsoil gives it this dark colour.

Activity 4A Study the graph, read the statement and then complete the sentences below.

Information on the effect of soil depth on levels of organic matter is shown as:

Effect of soil depth on organic matter

The breakdown of organic matter is slower at increasing soil depths. This could be for several reasons such as:



Key points   Key points

  • Organic matter supplies plants with nutrients, holds water, raises soil temperature and improves soil structure.
  • Levels of organic matter are determined by soil pH, climate, soil depth, oxygen and water levels. Most organic matter is near the top of the soil.

What's next?

Go to: 5 Soil organisms.

Biological properties Soil organisms