Technical
The size of the spaces between these particles and crumbs, the peds, is the pore space, and is fundamental to the ability of a soil to either to hold water or drain freely. Clay soils with lots of very small pores are said to have a high porosity, which means it will hold a lot of water; a more loamy soil with good organic matter will have fewer and larger pore spaces, a lower porosity and will hold onto less water. The clay soils with small pore spaces also hold the water more tightly, which means they are less permeable and drain less freely. This is why they can more easily become waterlogged. Soils with larger pore spaces are more permeable and drain more freely.
The Soil Water System
The Soil Water System introduces water to soil from rainfall. When rainfall hits the ground, several things can happen to it. Some can be evaporated from the soil surface straight back up into the atmosphere. Some will run off the surface into a stream, watercourse or ditch. Some will be absorbed into the soil to be taken up by plant roots and released back to the atmosphere through transpiration of the plants be they grass, crops or trees.
Rainfall will also be absorbed into the upper layers of the soil profile to become ‘soil water’. Beneath this upper layer, water will percolate by gravity through an intermediate zone into soil at a deeper level where it becomes 'groundwater'. The top of this groundwater level is the 'water table'. The level of this water table can vary depending on the soil profile, including type and depth of top soil, type of subsoil and presence of any impermeable layers.
Soil Water
There are three types of water in soil. Hygroscopic water surrounds and is held tightly to the surface of soil particles. It is held so tightly, partly because of electrolytic action that it is unavailable to plant roots.
Capillary water is held in the spaces between particles and is available to plant roots. Gravitational water is passing between the particles draining through the soil by gravity. Eventually, in a free draining soil, this water will be replaced by air.
Before this gravitational water drains away, all the pore spaces are full of water and the soil is said to be saturated. If the only water in the soil is the hygroscopic water unavailable to plants, the soil is so dry it has reached wilting point. Between these two conditions is the ideal state and the soil is said to be at or close to field capacity.
Thus, at field capacity in a well-structured, organic, well aerated soil, we have the necessary conditions for healthy plant growth.
Sadly, in the case of sportsturf, in spite of the best efforts of groundsmen and greenkeepers to create and maintain these perfect conditions, we go and spoil it all by playing sport on it!
For example, golf course drainage, especially of greens, would be much easier if people didn’t play golf on it! As soon as a golfer sets foot on a golf green, it is compacted a little bit. The more people play golf on it, the more it is compacted. When soil is compacted, all the particles get squashed together so there are no spaces between them, particularly since most soil particles are irregular or angular in shape, they nestle into each other. It is these spaces which allow water to move through the soil so, if there are no spaces, there is no drainage. Even if a good piped drainage system has been installed, if the water can’t move through the soil to get to the drainage pipes, it will not drain. This is why greenkeepers put so much effort into keeping the soil aerated to maintain a more open soil texture.
That is why all golf greens should have a sign saying “Please Keep off the Grass”!
PC JUNE/JULY 2015 I 121
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