Deserts can have a lot of different types of soils, and some can retain water better than others. Soils that have lots of holes in them, like soils with rocks with holes between the rocks, can absorb water quicker. Soils with tiny particles, such as clay, don't retain water as well. If plants are around then they can also slow water down as well.

Sometimes soils can form layers of calcite, which we call caliche, which water usually can't go through. These layers can also make it hard for water to be retained in the soil in the desert.

Thunderstorms can come quickly in the desert as well. If rain falls faster than the ground can soak the water up, then the water can form a flood!

In non-desert soils, organic matter, such as decomposed plants, plays an important role in water retention. Since there are fewer things living in desert soil to decompose, there is less organic matter to hold water. Also, desert soils tend to be sandy. Sand grains are large compared to other types of soil particles. This means that the pores between the grains are also large, which makes it easy for water to drain down to deeper levels or to evaporate back out.

This isn't true. The soil in the desert can retain water after a good rain.

Soils in deserts can be made of a variety of different things, just as soils elsewhere can. Sandy soils tend to absorb water, so sandy deserts can hold water after a heavy rain.

The main reason why you often see deserts flooding in heavy rain is because of the types of storms that drop rain in deserts. In deserts, you rarely get rain, but when you do get rain, it often rains a whole lot, which is what causes the flooding. If the same super-heavy rain happened outside of a desert, it would create floods there, also, but it's easier to notice in deserts because this type of rain happens more often there than in other forests.

There are other differences as well: deserts contain fewer plants, and plants can absorb water, which does make rain harder to absorb for some deserts, but still, a sand dune in a desert will absorb more rain than a rocky soil in a forest.

Some soils and some kinds of clay get super-dry after a long time without water. They get so dry that they don't even 'like' water, so the water just runs off them. When they're that dry, they are 'hydrophobic' - 'water-hating.' Normally the soil is 'hydrophilic' - 'water-loving'.

Once when I was teaching an elementary school science class, we were planting cutting from succulent plants in little plastic dishes for Mother's Day gifts, but the potting soil was super-dry. So when we watered the plants in the dishes, the water just ran off. I was very annoyed. I could have taught the students this interesting lesson about super-dry dirt and water, but I didn't think about that.