Drainage and Aeration

We may be under-estimating and not optimising the undoubted benefits obtained by 'improving the drainage' of a soil. I propose that the removal of surplus water, and it's dissolved nutrients, is little more than a loss and would be better saved for later in the year when it's sorely needed. I further suspect that what really causes the actual benefit is not the removal of water but the improved aeration this causes. And I speculate we may dramatically improve yields by pumping air into soils and aerating water.

Water logging kills many of our crop plants, except of course those used to watery environments. Most of them expire sooner or later if their roots are continually submerged. Even in hydroponic culture the water usually has to be flushed in and out so the roots can breathe. Some plants can endure days or weeks of drowning but unless it is part of their natural life cycle most plant's roots eventually die when totally water logged. Their actual life expectancy may depend on the time of year as dormant plants may be more resilient, and on the origin of the water as heavy rainfall draining away is less dangerous than standing stagnant water. Water logging kills because although plant leaves take in carbon dioxide and convert it to oxygen almost all roots do the reverse needing oxygen and giving out carbon dioxide, water logging suffocates them.

There are other detrimental effects of water logging; the conveyance of pathogens, the constant premature dissolving away of root exudates, the build up of carbon dioxide causing changes in the water's acidity and ion level et al. Water logging also ruins fertility by leaching out nutrients which drain away. And if the soil water gets too many salts dissolved in it then they can even draw the water out of roots further damaging them. Almost all the important and useful soil organisms also need oxygen and give out carbon dioxide so water logging will kill these too and worse will encourage anaerobic mould and decay organisms to proliferate in their place.

Obviously there is a difference between free water able to fill the air gaps, pores and spaces in the soil and the absorbed water held within the humus and living fraction. If we drain off the free water the soil is still moist with absorbed water, although this is soon used up as plants take it to transpire or use as raw material. More free water may be sucked up from moister soil deeper down by capillary action in the now drier soil. But of course this is not always adequate where the water table is too low, or the soil is too sandy and porous -or impermeable, then little water may rise. Then our crops become dependent on rainfall and what water we can add.

In most soils there then occurs a water pump effect. With heavy rain or irrigation the water falling on top of the soil runs in and fills all the gaps and spaces causing the stale air to be expelled. The water is partly absorbed by the humus and living fraction and the surplus drains downwards sucking in fresh air behind it. Thus a heavy soaking by irrigation or rain does not just water our plants but also helps the soil breathe! Without this pumping action the exchange of stale for fresh air goes on anyway but at a very slow rate. And then there is the dissolved air in the rain. Rain does not just fall, apparently it goes up and down many times before it ever hits the ground. This gives it ample time to absorb air and anything else blowing around. Thus rain is not just water, it contains dust and dissolved nitrates and considerable oxygen. So rain can be of significantly more value than the same amount of 'dead' tap water. I believe many gardeners have observed this effect especially after thunderstorms and pondered on it.

Now if there seems to be some magic in rainfall why not just use a sprinkler? Well the water droplets from a sprinkler are not in the air long enough to absorb much oxygen -though there will be some improvement, and there is still the value of the water itself and it's pump effect. However if, as I postulate, this effect of the air in the water is so noticeable, it may be that soil oxygen is the limiting factor preventing significant rises in fertility and crop yields. Obviously all other resources, especially light and water need be available for our plants to flourish but we don't consider the oxygen supply to their roots. This could be the limiting factor preventing them growing more and likewise of the micro-life from making more fertility.

In soil the nutrients for plants come through several routes. Some nutrients are directly leached from the soil particles; elements such as phosphorous, potassium and calcium exist in minerals and although almost completely insoluble these minerals are ground so fine in clays that their immense surface area does dissolve minute amounts continuously. Soil organisms use some of this resource to make their bodies and on death release's it again in an altered form, and also in life as by-products. Other organisms eat either the first lot, or their dead bodies or their droppings and in so doing add their own contributions to the system. And most of these use oxygen and give off carbon dioxide so if the soil air is stale they may not flourish.

This brings me to the conundrum of mulches and membranes which may not be as advantageous as we think. Given all their plus points they may still be restricting our crops by reducing soil aeration. A thick mulch more than a thin one and impermeable plastic the more so. We may have to look at ways of improving the air transport; possibly adding more air to our water and also by direct introduction. It seems apparent that water running out of a hose will not absorb much air as it trickles into the ground but if it is simply sprinkled on it still absorbs little. It should be possible to get more air into water, possibly with carefully designed jets or a mechanical agitator. However an approach to deliver enough air via irrigation water seems likely to become expensive in energy though of some benefit especially on a small scale. I tried a simple experiment on sweet peppers which respond badly to water logging / poor soil air conditions; two containers were used to water two plants, one was made aerated water by frequent vigorous shaking and the plant receiving that did do best. Similarly it seemed to show improvement when a fish tank bubbler was used to aerate the water used for plants under cover.

On any scale though the alternative has to be to get more air directly into the soil. Raised beds, where the sides are not impermeable, expose a greater surface area to the air as do ridges, trenches and mounds. All these have been noted as improving growth for many crops and are usually most beneficial in the heavier soils. I certainly recommend ridges for all crops prone to water logging especially sweet corn. Naturally air also enters through worm tunnels, aided by the pumping action of the worms as they move up and down; so more worms means more fertility in an unexpected manner. And, if present, the very drainage pipes running underground must convey air in and out when they are empty after dry weather...

Now what I suggest is surmise but may work wonders, especially for those on really heavy soils. This is drainage piping placed not to drain but to introduce air a foot or so down. By laying multiple perforated pipes a foot or so apart and connecting these to 'chimney' pipes at the higher end you could draw a lot of air through warming, drying, and if I'm right, aerating, the soil so greatly improving it's fertility. The pipes will of course drain in winter though they could be stopped to prevent excessive draw off. Alternatively, perhaps for really valuable crops, say under cover, it may be worth pumping air with a compressor through perforated seep hose of the type used for underground watering -perhaps alternating with water!

And do not forget the same effect on plants in pots. I'm so sure peppers need more air to their roots as I've seen them do so well on raised mounds compared to huge pots beside my experiments with aerated water. Indeed I'm convinced aeration is so important because plastic pots do not suit some plants (pineapple, ginger, sweet corn, watermelon, passion flowers) which often do poorly in comparison to those in more porous terracotta containers. In particular I've made my citrus much much happier by simply drilling dozens of extra holes in the sides of their tubs.