WHY SANDY SOILS DON’T DRAIN
comparison of the new and the existing dressing. It is potentially less problematical if a coarser sand is the new target dressing, but essential with finer sands. Following a change, a period of treatments should follow to key in the new dressing and to increase the longer term layer depth.
Critical tension
This essentially is a concept that describes the depth of a particular particle sized soil/rootzone required to allow water to freely drain through. This will be determined by the pore space within the soil, which is generally determined by the particle size distribution of the soil. This description of critical tension is very basic as the concept is somewhat more complicated and worthy of an article in itself As an example, I have seen a
rectangular sponge used to demonstrate critical tension. If the sponge is soaked in water then held up with the short side downwards, the water would fail to drain out. However, if the sponge is held with
the long side downwards then water drains out. If this analogy were applied to a medium-fine sandy soil then a depth of around 300mm would allow water to drain through without perching significantly in the profile. This is typically the depth recommended in the construction of a suspended water table golf green. This depth will allow excess water to be removed but hold some moisture for plant health. If, however, a builders sand was used (typically with a wide spread of particle sizes), a far greater depth of sand would need to be used to reach the same afore described result. Therefore, if less than or more depth is used than the desired critical tension depth then a soil could drain less or more effectively than expected. Arguably it would be potentially better to compromise on quality rather than on depth if cost were an issue. In a sports pitch environment it is common to see the use of a medium-fine sand amelioration into an intensively drained native and often heavy soil. If
undertaken correctly, this is a cost- effective method of giving good levels of playability even during periods of rainfall. The difficulty is creating the depth of amelioration with a uniform gradation from the native soil through to a sandier top. Often, a quantity of hopefully quality sand is cultivated into the surface. This can create a sand layer and the result is sometimes worse drainage than if not done. The key to success is to understand the
soil texture and calculate the amount of sand required to give the desired texture and associated drainage performance. The sand could be applied in layers with progressively less deep cultivations occurring with each application. This will help ensure the drainage is not held up by critical tension as the gradation would help avoid water perching. The final potential drainage problem
relating to critical tension is turfing. Unless you use a custom-grown turf on a similar soil as the profile, then water can perch in the turf layer. This can be alleviated to some extent by hollow
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