Capillarity, or capillary action, is how water moves through a porous substance or object, such as the soil in your garden. Understanding the capillarity of soil will help you give your plants the best growing environment possible.
Science of Water
When it rains, water soaks into the soil. This is called percolation. After the rain stops and the sun comes back out, the top layer of soil dries out. But, if you have good soil structure, only the very top of the soil will be dry -- the rest will stay damp because of capillarity. Capillarity is what happens when you put a sponge in a shallow dish of water, and the sponge soaks up all the water, leaving the dish dry. Think of the top, dryer layer of your soil as the sponge, and the lower, wetter later as the dish of water. Since young plants have shallow roots, you want the top (sponge) layer of the soil to stay nice and damp.
Importance of Water
Plants absorb many of the nutrients they need to grow through their roots. These nutrients, however, need to be dissolved in water so that the plant's roots can soak up the water and then use the nutrients and minerals. If you add compost or fertilizer to your soil, your plants still can't use it unless the soil is damp enough for the nutrients to dissolve.
Have you ever noticed that when you put a straw in a full glass of liquid that the level of liquid in the straw rises higher than the top of the glass? This is because the straw is much narrower than the glass, and this is how capillary action works. So for good capillarity, your soil needs to have lots of very small holes, like a sponge. Soil with a fine texture can hold more water than soil that has a coarse texture.
Fixing Soil Structure
You can improve soil structure by adding what's missing. If your soil is extremely sandy, then you should add compost. If your soil is very heavy with clay then you can add compost and sand. If your soil is rocky and full of gravel, you can add compost and topsoil.
Managing soil structure and irrigation can be tricky. On the one hand, you want good drainage so that the soil doesn't stay waterlogged after rain. Waterlogged soil can drown plants or lead to fungus and other diseases. On the other hand, if your soil drains too freely, you will need to water your plants frequently. Drip irrigation or soaker hoses can be a good solution.