Thursday 13 June 2013

Garden myths discussed. The dust mulch theory

There are some gardeners who still believe that hoeing conserves water because it breaks up capillary pores through which water rises to the surface. Most modern gardeners will not have heard this notion and I am a little reluctant to repeat this myth and thereby promote it!  My contention today is that cultivating the soil surface does not conserve soil water.  

Sixty years ago when growers and farmers started to use herbicides it was believed soil cultivations had values other than control weeds (and they do). What would be the consequences of completely ceasing to cultivate when weedkillers were used? A number of research stations investigated the dust mulch theory and found that its application made no improvement to water conservation at all.

I did not want to dust mulch, but the rabbit insisted
The way of the world is to find earlier findings flawed. Later research showed in some circumstances dust mulching had a small beneficial effect. Most trials with a wide range of crops and on soils of many different textures compared traditional mulching of organic material with dust mulching and a control with no cultivation between crops at all. In all cases proper mulching was best. Dust mulching sometimes showed slight improvement on the uncultivated control but more often no difference at all.

Conventional mulch
A little soil science
The soil between plants only loses water to direct evaporation when wet. With tendency to dryness, soil gradually becomes ‘self mulching’ and evaporation stops. The amount lost is not insubstantial and varies with soil type. Typically it might lose between 10 to 30mm of water before evaporation  completely ceases. (Water loss restarts when the soil is rewetted at the surface by rain). Of course, crops continue to lose water via the leaves by transpiration as long as roots can find water. In a dry season plants substantially dehydrate the soil.

This unmulched uncultivated soil is dry at the surface and water loss by evaporation will have ceased. (Unfortunately the adjacent thirsty grass will extract water instead)

This soil at the base of this dried up pond will have ceased to evaporate water. The plant roots will continue to extract water pulled up by transpiration
Dust mulching
Light cultivation does disrupt capillary rise. However capillary rise ceases even without cultivation as the rate of upward movement of water slows to zero as the soil is depleted of water and dries. In colloquial  language the rate of capillary rise does not ‘keep up’ with evaporation and stops.

In gardening terms the dust mulch theory is often expressed as ‘hoeing conserves water’. Hoeing breaks capillary contacts but on the other hand more soil surface area is exposed to evaporation. Further-more light showers of rain will have poorer  penetration into loose soil than into uncultivated ground where cracks and worm channels provide a route for water absorption. On balance it would appear that in terms of water conservation it makes little difference if you hoe or not!

Why I disapprove of dust mulching
There are several reasons why dust mulching is not good land management practice.
  • Loose soil is vulnerable to wind and water erosion.
  • Particulate pollution of the atmosphere is caused by wind blown dust.
  • Soil organic matter is degraded by cultivation which in turn reduces the soil’s future water holding capacity.
  • After heavy rain capillary contacts are restored and if cultivations are not repeated the dust mulch is lost.
  • Overgenerous loosening of surface soil seriously reduces root capacity to benefit from light showers of rain.

But hoeing does conserve water by killing weeds!
Plants dehydrate the ground. I did not convince all of you that ground cover plants dehydrate the ground in my earlier post but in the case  of weeds there can be little doubt. My own perspective on hoeing  is that it should shallowly sever weeds from the ground and should not be used to ‘fluff up’ weed-free soil. Hoeing is an excellent way of controlling weeds-from-seed but in my post ‘Ring the changes with weed control’ I urge gardeners to also use herbicides and mulching.

A small idiosyncrasy of my own
I often take plants to my cemetery gardens. When dry conditions prevail I know I will not have opportunity to water them again. I sometimes heavily water-in the plant and then mulch around them with dry soil! My gamble is that this will give them time to establish if there is no rain. The loosing bet is when rain only comes as light showers!

I was fascinated by this dry-mulching equipment apparently used in Holland sixty years ago.

I believe gravel mulch to be the very best water conserver - from immediately it stops raining. My eucomis thrives on the preserved moistureI


  1. WE're hoping our weed control fabric helps preserve moisture as well as keeping the weeds down. I does seem a bit damper underneath when I plant thingg through it.

  2. Best of both worlds Sue. Reduced water loss by action of fabric and also control of weeds.
    The roots of your plants can exploit moist soil right up to the surface.

  3. Replies
    1. Which one of the options David.
      I offered my opinions on slugs in my post last month.

  4. i never know the use of adobe shadow until i saw this post. thank you for this! this is very helpful. extendable duster


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