Over time, the soil in the garden or in potting mixes becomes ‘hydrophobic’. This is also referred to as ‘soil repellent’.
The word hydrophobic is made up of two parts. ‘Hydro’ means water and ‘phobic’ means to have a fear of something. Of course, soil can’t be scared of water but sometimes it can put up an invisible force-field to stop the water from soaking in and this is bad for the plants because their roots can’t get the moisture that they need to live.
Soil in the garden or in pots can become hydrophobic if they have been dry for a very long time but also because each little grain of sand can get covered with a waxy coating which comes from the oily leaves of plants such as Australian natives. We can’t see this coating but it puts an invisible force-field over the whole area stopping the water from getting through; instead it just sits on the surface until it evaporates or runs off down the path.
To tell if soil is hydrophobic, pour a jug of water over it and watch what it does. If the water disappears straight in, the soil is soaking it up really well and this is great for the plants. But, if the water just sits on the top or runs into another area, the soil is hydrophobic. You can see that if you dig a few centimetres down, the soil underneath will still be dry which means that any plants grown in that soil would not get enough water to their roots.
Just digging the soil is not enough to stop this problem. The solution is to dig in lots of compost or manure which encourages the little microbes in the soil to become active making spaces for the water to flow.
A great solution is to apply a wetting agent. Wetting agents break up the waxy coating on each grain of soil which allows the water to move through the gaps between the grains and down to where the plant’s roots are. It’s important to do this at least twice a year as the waxy coating will come back again, the water will be wasted and the plants will struggle.