Why is dry soil hydrophobic? Bad gardener paradox

Question

When I forget to water my plants, and their soil becomes very dry, during the next watering I can see that the soil becomes hydrophobic. I can even see pockets of air between the repelled blob of water and the soil.

On the contrary, when the soil is moist, it very quickly absorbs the water.

This goes against the intuition, that the diffusion , in order to equalize the concentration soil-water, would create the opposite effect.

What is the physical explanation of this phenomenon?

I tried to search on google, but the explanations are of low quality and very "high level" without real physics involved.

Answer 1

This effect appears like a paradox, as dry soil makes a very bad water conductor. Two effects prevent water from infiltrating:

1. Air in the soil pores cannot escape: dry soil includes lots of air bubbles in small to large pores. If you expect water to get in, how do you think the air could get out? Often it gets stuck, and no water can infiltrate anymore. This effect also leads to dangerous flash-floods even during droughts, when strong rain events hit super-dry soil.
2. Sticky capillary forces: Water forms droplets and becomes sticky due to surface tension when in contact with lots of air. The effect is even stronger in smaller pores. So it is very hard for water to create a flowing stream.

Under these circumstances, water cannot simply diffuse, because random walk is hindered by air bubbles and capillary forces. As soon as the soil is saturated, water and solutes can actually diffuse. As a consequence, water flows faster in wet soil, since it can develop a continuous stream.

The dashed line in the graph below expresses this fact by showing the hydraulic conductivity (i.e. how fast water infiltrates) as a function of soil moisture (from dry to wet).

--- EDIT: The actual behaviour of water infiltration is a superposition of the two effects described above. Very dry soil is often compacted and therefore exhibits small pores. Small pores cannot absorb huge amounts of water quickly enough, because (1) the only path for the air to escape is upwards, which is where the rainfall water influx is sustaining a surface cover, and (2) water is hesitant to fill the pores since capillary counterforces are inversally related to pore radius. One might argue that air bubbles can still rise in water, but in a small-pore domain, bubble formation and uprising is slow and inefficient. Eventually, this tragedy can lead to flash floods.

Answer 2

Dry objects may tend to not break the surface tension, or "skin" of the water. With wet or damp objects the moisture in them will tend to merge with the water's skin.

Just as if you were to spill a little water on a table, you can see it has an edge where it stops. If it meets another bit of water on the table they will merge easily, and not stay two separate spots of water. Therefore damp soil can soak up water a little faster than dried soil.

Answer 3

Diffusion and adhesion are different phenomena. Diffusion happens due to a concentration gradient, and this is how the water is slowly absorbed by the soil. This happens faster in dry soil per unit area per unit time than damp soil. On the other hand, adhesion is the ability of one material to stick to the other. Damp soil is more sticky to water than dry soil.

The main idea is that dry soil doesn't really stick to the water drops. The surface area is limited. But, damp soil literally gets surrounded by the water. This difference between surface areas result in damp soil absorbing water faster even though the process is slower per unit area.