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Horticulture 1000
HT1032 - Soils 2 - Chemical properties
Soil pH

Chemical properties

Soil properties describe what the soil is made up of. Chemical properties include the soil's:

  • nutrient status – what nutrients are present in the soil
  • nutrient retention – how well the soil holds onto nutrients
  • pH.

Nutrient status

Plants need 16 elements or nutrients to grow successfully. Carbon, oxygen and hydrogen are found in air and water. All the other major elements come from the soil.


When rocks are broken down into smaller and smaller particles, chemical elements are released. Some of these elements dissolve in the water in the soil.

Chemicals that dissolve in soil water split into charged particles called ions. They produce positive ions called cations, for example Mg 2+ for the magnesium ion.

Anions are negative ions, for example NO3- for nitrate. Plants can use nutrients in ion form. Some ions sit on the outside of clay particles or on the outside of humus. They are stored there until they have dissolved into soil water. They then are available for plants to use.

Nutrient uptake by plants
Cross-section of plant root in soil
  1. Nutrients held by soil particles.
  2. Nutrients move into soil water.
  3. Nutrients move into root hairs.
  4. Nutrients move into the plant.

Nutrient availability

Different soil types have different amounts and types of nutrients available for plants to take up. The nutrients available in a particular soil depend on many factors including:

  • parent material (what type of rock the soil has come from)
  • soil texture (sand, silt or clay)
  • organic matter
  • soil water and soil pH.

Parent material

Soil is made from rocks that get broken down into  small particles

Soil is made from rocks that get broken down into  small particles. The type of rock making up the parent material will determine  the nutrients available.

In the photo is volcanic rock. Soil that is formed  from this parent material has low levels of phosphorous.

Soils that are formed by flooding are often very  fertile.

Soil texture

The available  plant nutrients are influenced by soil texture. Clay can store and retain  nutrients. Soils with clay in them have more nutrients that can be dissolved in  the soil water. Clay holds phosphorus well.
  In sandy soils the nutrients wash out or leach. Many important nutrients are soluble in water, so rain  may wash them into the deeper layers of the soil where plant roots can't reach  them. Nitrogen is more readily lost through leaching than phosphorus or  potassium are.

Organic matter

Earthworms    are important in the breakdown of organic matter

When organic    matter breaks down, nutrients are released. When the nutrients are dissolved    in soil water then plants can use them.

Soils with    lots of organic matter in them have more nutrients available for plants.

Soil water

Nutrients have  to be dissolved in water before plants can use them.

If there is too  little water in the soil it is hard for plants to take up nutrients. This may  happen in a drought. Too much water in the soil means nutrients can be leached  or washed out of the reach of plant roots.

Microbes in the  soil also need water to break down and release nutrients from organic matter.

Soil pH

Nutrient availability at different pH levels

The soil pH    tells us if something is acid, neutral or alkaline.
Soil pH also    affects the nutrients that are available to plants. The diagram shows which    nutrients are available at each pH level. A wide band shows that the nutrient    is available for plants to use. A narrow band shows that only tiny amounts of    the nutrient are available at that level of pH.

The pH of a soil also determines how fast microbes in the soil break    down organic matter. Most bacteria like to work in a pH range of 6 to 7.    Fungi can work in more acid conditions.

Activity 1 Test your understanding of how plants take up nutrients from the soil. Put the statements into their correct order in steps 1-4.

Key points   Key points

  • Plants use nutrients dissolved in soil water. Most  nutrient ions are held on the outside of clay particles or organic matter.
  • The parent material, soil texture, organic matter and  pH determine the nutrient availability in a soil.

What's next?

Go to: 2 Soil pH.

Soil pH