The structure of soil has a great influence on factors such as drainage, water retention, nutrition, compaction and infiltration rate.The structure of soil has a great influence on factors such as drainage, water retention, nutrition, compaction and infiltration rate.
Choose a link below to investigate soil in more depth:
Soil is made up of four main components:
Water - containing the nutrients that plants take up through their root system - in pore spaces.
Air - filling the spaces in the soil not filled by water and supplying oxygen to plants - in pore spaces.
Organic matter - living plant material including roots and rhizomes plus dead and decaying plant matter.
Solid mineral particles - these particles vary greatly in their size and there can be a mixture of particle sizes within any one soil.
Any soil will be composed of a variety of particle sizes ranging from large gravel particles down to tiny grains of clay.
The texture of a soil refers to the percentage of different sized particles.
Click on each soil type in the graph below to see the percentage of sand, silt and clay in each.
Soil particles do not fit together snugly. There are spaces between particles. These spaces are called pore spaces and contain water and air.
The pore spaces provide the route for the downward movement of water and allow roots to grow into them. They also provide air space, which is essential for plant growth.
The larger the pore spaces the better the drainage of water and the less water retained in the soil. Conversely, the smaller the pore spaces the less water drains away and the more water is retained in the soil.
When water is added to dry soil, some of the water drains through as it is drawn downwards by the force of gravity. However, not all of the water passes through the soil under the influence of gravity - some water remains in the soil.
The water held in the soil against the force of gravity is held in the soil pores by capillary forces. The strength of these forces depends on the size of the pore spaces.
Soils with large pore spaces have small capillary forces and can hold only small amounts of water against the pull of gravity (e.g. sandy soils). Soils with small pore spaces have large capillary forces and can therefore retain larger amounts of water (e.g. clay soils).
Click on each tap to see how much water stays in the soil.
After a soil has been completely soaked by a downpour of rain all its pore spaces are filled with water and it is regarded as being saturated. Any air in the pore spaces is forced out and the soil is said to be waterlogged.
Once the rain stops and the water has a chance to drain away, the total amount of water that remains in the soil, against the force of gravity, is called the field capacity of the soil. Due to capillary forces, water will drain away from the large pore spaces first and remain in some of the smaller pore spaces. So, at field capacity, the larger pores will contain air whilst the smaller pores hold water.
Plant roots in the soil are able to suck the water out of many of the small pores. However, eventually water in the small pore spaces is held too tightly by capillary forces for the roots to take it up. This stage, when the turf plants are unable to extract any more water from the soil and begin to wilt, is called the permanent wilting point.
Due to differences in particle and pore space size, soils differ in their field capacity and permanent wilting point.
Have a look at the different water holding capacities of sand, loam and clay.
Compaction is the loss of pore spaces in the top 50-60 mm of soil due to the mechanical pressures caused by the operation of equipment such as rollers and mowers, and trampling by players.
Compaction problems tend to be greater on loam and clay soils, however, sandy soils can also be affected.
For more information on compaction follow these links:
Conditions that favour compaction
What actually happens to the soil?
Compaction is more likely to occur:
on heavy traffic areas (e.g. golf tees and greens, the base line on tennis courts and the mat line on bowling greens)
when the soil is wet
when the grass cover is poor.
Soil needs a balance between large pore spaces for aeration and water infiltration and small pores for a water retention. When the soil becomes compacted, soil particles are pushed together and broken down so that pore spaces are reduced and filled in by smaller particles.
Smaller pore spaces means less aeration, water infiltration and poorer drainage.
Compaction reduces the size of the pore spaces. This results in increased capillary forces and therefore more water held in the soil. More water means less air, therefore less oxygen and carbon dioxide for plant respiration.
This decreased aeration impacts on:
plant condition - the turf becomes thin and does not respond to fertilising
the plant's uptake of nutrients
weed growth - as weeds can tolerate greater compaction levels than the turf they will begin to dominate.
Using specially designed aerators to make holes in the turf can relieve compaction in turf areas.
This equipment includes corers and verti-drains that pierce the soil and remove a 'plug' of soil or move through the soil, shattering the soil profile.
These procedures should be carried out when:
the soil is in the right condition (not too dry or wet). Turf is often cored the day after irrigation so that the soil will hold in the coring tyne and not fall back into the hole.
the turf plants are growing actively in spring or autumn.
problems dictate that it is necessary.