Q: When it rains our mulch seems to get wet but not the ground underneath.
Will the rain run off the mulch over time or if it doesn’t get through during the shower will it evaporate off?
We have about 30mm of bark chippings, fairly sandy soil and not as much organic material in it as we'd like, but we are slowly rectifying that.
Thanks for any help. Julie – St Lucia.
A: When soils become extremely dry, after a prolonged drought or long periods without any watering, they become what is known as ‘hydrophobic’.
When such hydrophobic soils are watered with a hose, or when it rains after a very long spell of dry weather, the water simply flows off and is not absorbed. The condition ‘repels’ water, and this is what has happened to your soil beneath the mulch.
Sandy soils are particularly susceptible. This is very common with potted plants, when they have not been watered regularly and sufficiently.
It is a ‘bugger of a job’ to convert hydrophobic soils to absorb water once again – by just using water. One has to water such soils (and potting mixes) a little at a time, and keep going back time and time again, sometimes as many as 10-15 times and more … watering a little at a time to get rid of that ‘hydrophobic’ condition. One can use ‘wetting agents’, and I will come to that.
I will try and explain this phenomenon in simple terms.
What happens when soils get overly dry is that the organic coating or thin layer of humus on the surface of soil particles dries up causing a waxy surface. Water then just slips by, without actually adhering to the soil particle.
It would be much like pouring some water on a piece of wood that has been waxed – it just ‘slips’ or flows along the piece of wood, without any water sticking to the wood itself. Hydrophobic soil particles act in a somewhat similar manner and cannot ‘hold’ water. Remove the wax on the piece of wood, and water will stick to it once again.
The organic waxy coating on the hydrophobic soil is more soluble than the wax on your piece of wood. It CAN be removed by using water alone, but it requires a fair bit of REPEATED watering, or constant rain over several days.
Soil wetting agents are designed to overcome water repellence in soils. They are basically the same as detergents and belong to a class of chemicals called ‘surfactants’. They reduce the ‘surface tension’ of the water, which allows water to spread thinly over the waxy surface of the soil particles thereby allowing it to move into the soil through the spaces between the particles.
Some household detergents may be used for the purpose but they do not have a lasting effect on the soil. One needs to be careful with household detergents, however, as some of them could contain chemicals which would be harmful to soils.
Commercial soil wetting agents will continue to work for a considerable period … up to 2 months or more. They will eventually be degraded by soil micro-organisms.
Soil wetting agents are available in granular and liquid form and the choice of which to use will depend on what one is trying to do. In your case, I would advise that you use the liquid form and drench your soil thoroughly, over the mulch, at the recommended rates. Rain water will then actually ‘wet’ your soil.
