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I have often heard that particularly dry soil cannot absorb water well. This was often associated with flooding after a long dry drought. I couldn't really imagine it, but yesterday I saw for myself how water that I had poured on very dry ground did not seep in but remained on the surface in a thick puddle. If the same soil was moister, it absorbed the water without any problems.

  • Try adding a little detergent to the water – Graham Chiu Aug 12 '18 at 5:24
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It's due to surface tension and adsorption. If you take a beaker of water you will see how the water clings to the glass sides of the beaker in a meniscus. At a molecular level the surface is really flat and will resist any attempt to break through that surface. Take a small quantity of dry peat and toss it on the surface and surface tension will keep the pieces afloat rather like insects are able to ride on the surface without getting wet. When a tap drips, the droplets come out rounded - the surface tension is pulling the small quantities of water into a rounded form since that is the optimal response in the presence of gravity and atmospheric pressure.

Chuck a heavy stone in and of course it gets wetted pretty fast - but does it? Look closely and you can see that air bubbles can form. The stone may not be completely wetted.

One way to destroy the surface tension effect is to heat the water. When horticulturists want to wet dry peat based mixes they often do so with hot water. As the water heats up water vapour starts to rise and eventually steam which means the surface is really turbulent and cannot behave as it does when cold.

Previously wetted material allows more water penetration faster because the new water joins with the old readily. If I add more water to my beaker of cold water the two waters mix very easily.

In addition there is the effect of adsorption (as opposed to absorption). The solid materials basically refuse to be wetted and put up their own molecular surface resistance if they are really really dry. Eventually this breaks down and once wetted the phenomenon changes to hanging on very tightly to the water in its immediate presence.

  • @Kur It's a funny concept but in general everything resists change or deviation from equilibrium. Even on the molecular level change (without energy dissipation) can only occur if the resulting bond is stronger then the previous bond. Another example would be inertia which is an objects resistance to change in motion. Even people and animals resist change, life's funny that way. – Rob Aug 8 '18 at 19:15
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    Well stated! I'll add that commercially available cactus are often in a peat-based soil mix. Once it dries out it's extremely difficult to rewet. Booo peat! (as a potting media) – Tim Nevins Aug 8 '18 at 19:34
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Whilst agreeing with the answer already posted, I'd add that the problem of hydrophobic soil is exacerbated if it's low in organic matter/humus content. If the soil has regular applications of composted materials (garden compost, composted manure, spent mushroom compost, leaf mould, that sort of thing) the humus content will be higher, and less prone to becoming hydrophobic during drought conditions. Heavy clay soils particularly benefit from frequent emendment with composted materials if they are likely to be in drought quite often. https://soiltosupper.com/simple-ways-to-fix-dry-garden-soil/

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