Soil water FAQs

Why is soil moisture important?

A plant can take up foods from the soil if they are in liquid form—as when fertilisers, for instance, are dissolved in the soil moisture. It can happen that plants starve because, although sufficient fertiliser is present, the soil is too dry to make it into solution and thus make it available to the plants.

The moisture taken up by the roots enables a plant to transport foodstuffs internally. Moisture is also circulated, rather like water in the engine of a motor car, to help keep the plant cool. Water in the form of vapour is passed out through pores in the leaves. If there is not enough moisture in the soil the leaves flag or, in severe cases, shrivel. Moisture is also necessary for bacteria and other soil organisms to function properly.

How often should I water the garden?

Too much watering will wash plant foods out of the soil and will cause excessive leafiness and poor root action. Too little water will cause starvation, stunted growth, and poor crop yield.

Plants need more watering than usual during long (summer) days when they are making active growth, especially when the weather is warm. Soil moisture can be lost by evaporation from the surface as well as by drainage. The amount of evaporation will depend on the size of the plants: if leaves cover the ground, less evaporation will take place. The same applies to soil covered by a mulch .

A good rule-of-thumb test for moisture is to take a handful of soil from around the root zone of a plant and squeeze it. If it binds together well and feels as moist as a wrung-out face-flannel, it is moist enough for the plants. If it refuses to bind and is dusty or barely moist, it needs watering. Give it a really good soak by leaving a lawn sprinkler running over it for up to two hours.

What is meant by the water table?

When soil is saturated by rain or irrigation, the water drains downwards until it reaches a common level, known as the water table. This may be at a depth of several metres or only just below the surface. The level may vary from time to time, depending on the season: in winter (the wettest season) it is usually much higher.

If the level is so near the soil surface that plant roots would be immersed in water for most of the time, it would be necessary to lower the water table by installing a drainage system if you were planning to make a garden on such a site . A high water table can lead to a cold, stagnant soil unfavourable to most plants.

Is soil temperature important?

Soil temperature is vital to the health of plants at every stage of growth. Seeds sown in soil which is too cold for them will fail to germinate and in many cases will rot. The same applies to plants set out before the soil has had time to warm up. Bacteria are sluggish when the soil is too cold and so may fail to convert fertiliser into a form which can be taken up by plants; the fertiliser may then be washed out of the soil by rain before the soil has had time to warm up. On the other hand, low temperatures are required by some seeds to break their dormancy, so that germination can take place. Seldom is the soil temperature too high in the open ground for normal spring sowings. Dark-coloured soils tend to be warmest; light, sandy soils are warmer than heavy clay. A shallow mulch helps to keep the soil warmer than a bare surface in early spring. But a mulch put on ground already cold will prevent it from being warmed by the sun.

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