Thursday 3 January 2013

Why gardeners dig 6: to add things to soil

To add things to the soil to improve drainage and change soil texture

Far be it from me, in this wet year, to encourage gardeners to add gritty things to the soil to improve drainage. Very often they will do more harm than good. One only has to see many gardeners’ soil, a podgy mess, to see the effect of unnecessary cultivation used to ‘work things in’. 
This year, whether we gardeners dig or not, most of us have experienced standing water. One might ask whether the flooding would be better or worse if the soil had been dug. I argue that digging per se will only help if the soil contains impermeable layers. And where does such compaction come from? Unnecessary soil disturbance. Many gardeners start to create a vicious circle for themselves when they dig.
Water might also fail to drain away on the non-diggers garden, but when it goes, the soil will still retain its excellent structure, including numerous drainage channels made by worms and plant roots. These will provide aeration and drainage deep into the subsoil.
So you may well ask...why has the non-digger’s garden flooded as much as any other garden? The answer is that the water had nowhere to go! Let me put it this way, there is nothing so well drained as a hole in the ground but it still fills up with water if the level of the ground water is high. 
There are two main aspects to successful drainage.
  • ability of water to penetrate the soil
  • a route for water to exit the site
Adding things to the soil might help with the former, unfortunately it is the latter that is the  common cause of most drainage problems.

Gardeners might want to amend their soil texture by adding sand. One can even envisage, on extreme sandy soils, adding clay! Unfortunately the effects of adding foreign materials  to the soil might not always be what you wish for! It is only practical to change the nature of soil in small ‘special’ areas. Very large amounts of sand or grit is required to be of any use at all. Imagine what a few peas would do for the structure of porridge, this will be the effect of adding small quantities of grit or gravel to clay soil. 
It might be best to confine yourself to amending only the immediate soil surface by forking in soil additive. Rather than buy sand, it is more effective to mix in very sandy soil. The local soil where I live is very sandy and a company ‘down the road’ trades in local soil as a landscaping ‘compost’. 

Some pitfalls of adding drainage materials
  • Planting holes filled with a ‘sandy mix’ in clay soils will act as a sump and water-log  plants
  • Horizontal ‘drainage layers’ of gravel underlying thin horizons of soil -  less than a foot - has the opposite effect to the one you want. Discontinuities in the soil drainage profile lead to soil retaining more water against gravity, not less!
  • There are dangers with some additives such as fine builders sand and unweathered ashes from domestic coal fires
Circumstances where sharp sands, grits and gravels are beneficial 
  • Gravel overlying land-drains increases the speed of drainage.
  • Vertical slits filled with sharp sand in otherwise undisturbed land, for example, on sports fields. Many gardeners combat compaction in their lawns by inserting a fork - best a hollow-tine fork - and brushing in coarse sand.

Standing water. My ‘up and down’ streams were up! - and my lawn had joined in! Twenty four hours later the surplus water had gone.

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