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Horticulture 1000
HT1032 - Soils 2 - Physical properties
Soil organisms Soil temperature
6

Physical properties

Physical  properties include water holding capacity, drainage and aeration, and soil  temperature.

Soil water and air

Soil, water and air

Soil particles have spaces between them called pores. These pores contain air and water.

Plants must have water and air to carry out the processes that help them  grow. They get this from the soil pores. The proportion of air and water in the  pores is very important.

Water and oxygen

How plants use water

The amount of  available water also controls how fast:

  • micro-organisms work
  • nutrients are available
  • plants release water (transpiration)
  • plants make food (photosynthesis) and how much they make
  • the soil warms up.

Oxygen

To grow and take up water and nutrients the plants  need a large energy supply. This comes from the process called respiration.

It takes place in all living cells. The oxygen in the soil is vital for respiration carried out by plant roots and in seed germination.

Storage of soil water

Water is stored in the pore spaces in the soil and on the surface of soil particles.

The amount of water in the soil at any one time depends on several things:

  • weather – if there has been heavy rain and not much sun or wind then the soil may be quite wet
  • amount of runoff and the drainage rate of water through the soil
  • size of the pore spaces.

Types of soil water

There are three important levels of soil water content:

  • field capacity
  • saturation
  • wilting point.
Field capacity

This diagram shows soil at field capacity.

Field capacity is the amount of water the soil can hold after it is saturated and then allowed to drain for 48 hours. A soil with an ideal amount of both air and water is at field capacity. It is desirable to have soil water at field capacity for optimum plant growth. Plant processes such as photosynthesis, respiration, transpiration, nutrient uptake and germination will be able to take place when both water and air are available to plant roots.

Saturated soil

This diagram shows a saturated soil.

In a  waterlogged soil, the pore spaces are filled with water and there is no room  for air to enter. This is called a saturated soil.

The main causes of too much water in the soil are:

  • heavy rain
  • over-watering
  • poor drainage, for example in a clay soil
  • a high water table.

A saturated soil will have poor drainage. Signs of a poorly drained soil include:

  • water collecting on the surface in puddles
  • plants turning yellow and dying
  • the soil surface is muddy and sticky when you walk on it
  • the subsoil is grey-blue and smelly
  • a foul smell.

Few plants can survive for long in these types of soil. Under these conditions the oxygen levels will be low and:

  • plants die because their roots cannot respire
  • roots rot from fungus infection
  • the activity of living soil organisms decreases
  • seed germination is poor
  • soil temperature remains low and this slows down plant growth.

Permanent wilting point is when the there is very little water in the soil and it is held so tightly that is unavailable to the plant. The levels don't match the needs of the plant; their roots can't get enough water to keep up with transpiration loss.

The plant will start looking wilted and the leaves change colour from green to yellow.  Eventually the plant dies.

Permanent wilting point

This diagram shows a soil at permanent wilting point.

The causes of too little water are:

  • lack of rain
  • a low water table
  • a soil that drains too easily, for example sand
  • forgetting to water.

The water table

Some of the  water falling on the soil surface runs off into drains or streams. A lot of it  sinks into the soil and may reach a barrier that it can't get through. This  barrier may be clay or some other substance.

This barrier is  called a hard pan. A  layer of water can collect in the ground above this barrier. The water collects  on top of the pan, forming a waterlogged layer. This is called ground water.  The top of the ground water is the water table.

The water table

The depth of  the water table varies with the season. It may be only a few centimetres from  the surface or it may be many metres down. It is higher in wet weather and  lower in dry weather.

Soil texture and water availability

Soil texture  and pore size will influence the amount of water in the soil. A clay soil has  small pores but lots of them. When it is wet lots of water will be held tightly  in these pores.

Clay particles

When the clay particles get wet they form a sticky mass, which holds on to water. It has a high water-holding capacity. The water doesn't easily drain away and the soil stays wet.

Sandy soil

Sandy soils have large pores. They have plenty of air and good drainage. Water drains quickly from these types of pores so the soil will dry out. They have poor water-holding capacity.

Loams are  usually the best for growing plants in because they have good quantities of  sand, silt and clay. The properties of these soil types contribute good  aeration, drainage and water-holding capacity.

Activity 6A Use the graph to answer the following questions.

Percentage available water in different soil types

  

Complete Activity 6B in the workbook

Key points   Key points

  • Soil needs a good air/water balance.
  • Water levels help determine the speed of many plant  and soil processes.
  • Plenty of pore spaces allow good penetration of both  plant roots and water.
  • Drainage, aeration, and water-holding  capacity of a soil  mostly depend on the soil texture. Soil water levels are described as being  saturated soil, or at permanent wilting point, or at field capacity.
  • At field capacity large pore spaces are filled with  air and small pore spaces are filled with water. At permanent wilting point  water may be in the soil but it's held tightly by soil particles so plants  can't use it.
  • Poor drainage is shown by puddles, yellowing plants,  smelly subsoil.
  • Good aeration provides roots with air for respiration  and soils containing plenty of air warm up faster in spring.

What's next?

Go to: 7 Soil temperature.

 
Soil organisms Soil temperature