Soils and water
Have you ever gone to the beach and found the tide was in? The sea water may have soaked the sand for several hours but as the tide goes out you see that the wet surface of the sand begins to dry. A few hours later, if you dig in the sand, you may have to go deep to find water. Sand is made of hard, rough-surfaced grains that are about a millimetre across. When sand grains pack together on a beach they have large spaces between them. As the tide goes out, sea water passes easily through these spaces and leaves the sand near the surface completely dry. The water drains away through the sand under the beach towards the sea. This easy drainage is simply due to the size of the sand grains.

After heavy rain you may see that the soil in a flowerbed is soaked. Sometimes a puddle may form and it may still be there a day or more after the wet weather has gone. If you were to dig up the soil below the puddle, you may find that it is hard to move and sticks to the spade. The soil may even be yellowish in colour. This yellow sticky substance is clay. It is made from particles that are only about five hundredths of a millimetre across. These tiny particles pack together so closely that there are only very small gaps between them and the water cannot get through.

Many plants grow best in a soil that lets water pass through yet can hold some back for the roots to find easily. The gardener and farmer treat the soils so that they have a mixture of particles that make the plants grow well. For this, they often use humus to give them the blend of particles they need.

How can a sandy soil be made to hold more water?
More humus needs to be added to the soil. This will stick the grains together to make crumbs. The humus coats the sand grains and acts like a sponge. It soaks up some of the water that passes through the soil. The humus can hold the water in the soil but when a root grows by it the humus gives up its water to the plant.
A good source of humus is manure.

How can clay soil be made to hold less water?
The clay particles have to be separated to let more water pass between them. This can be done in several ways. Sand, gravel or ash can be mixed with the clay to break it up. The clay soil can be turned over before the winter so frosts can break up the lumps of clay. Lime can be added. It makes the clay particles stick together in groups. Each group makes a large particle in the soil. These larger particles have larger spaces so more water can pass between them. Manure can be mixed with the clay particles to make soil crumbs which also have large spaces between them.

Why don't plants like a soil full of water?
Plant roots need oxygen from the air to live. The spaces between the particles normally have air in them so the roots get the oxygen supply they need. When a soil is full of water we say it is waterlogged. All the spaces in a waterlogged soil are full of water not air. The roots cannot get the oxygen they need so the plant dies.

Can water wash minerals out of the soil?
Yes, it can. The minerals come from the rock as it breaks down to form soil. Plants need many minerals for good health so it is important that the minerals are not lost. Sand lets dissolved minerals pass by as water passes through it. Clay is different. It grabs hold of the minerals and holds them in the soil. In some soils the iron minerals are washed out of the topsoil into the subsoil. They form a hard material known as an iron pan. This stops water passing through the subsoil. The top soil above becomes waterlogged and only marsh or bog plants can grow in it.

Can water wash soil away?
Yes. When soil has plants growing in it, the roots help to hold the soil in place and the leaves and stems protect the soil surface from heavy rain. If the plants are removed, water can wash away the soil because there are no roots to hold it in place. In tropical rainforests, where trees are cut down for timber, the soil is washed away. This leaves the subsoil or underlying rock exposed. Plants cannot grow in them so the forest cannot grow back. It can take 2,500 years to make a layer of soil two and a half centimetres thick but it can be washed away and destroyed in ten years.