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Only pure water is transpired through the leaves. The soil moisture taken up by the roots is of lower concentration than cell-sap. It must be supplied in quantities sufficient to offset the loss of water from the foliage. All this time, with the absorption of soil moisture, more and more food is being taken into the plant tissues, resulting in growth. Rapid growth cannot be made by a plant without ample water supply, ample heat to draw water from the leaves, and ample light to help the chlorophyll to work. Retention of foliage well into the winter indicates good health.

Heavy applications of soluble chemical manures produce a soil moisture of unduly high concentration. The roots shrivel and are said to have been "burnt". The only remedy is copious waterings, commenced before the damage is irreparable and repeated several times at short intervals to wash away, from the reach of the roots, most of the highly soluble salts. Common salt, sodium chloride, is not a plant poison, but it can do great harm by raising the concentration of soil moisture too high. This may occur if seaweed is used as a mulch and not heavily watered before or after being spread.

The function of leaves is equally vital. Each is covered with a layer of flattened cells. The upper surface is glossy, and impermeable to water from without or within. The under-surface is dull, due to its continuity being broken by small apertures, called stomata, which are the all-important breathing pores of the leaf. At the edge of each stoma are sensitive guard-cells that regulate its opening and closing. If the rate of water transpiration exceeds the rate of absorption of soil moisture, these guard-cells collapse over the opening of each stoma, closing it and so reducing the loss of water.

The total moisture-holding capacity of heavy soil is much greater than that of light soil, but the amount of moisture available for plant use is very much the same. For example, plants grown on a heavy soil with a moisture-holding capacity of about forty-five per cent will wilt when the soil moisture falls to about thirty per cent. The available moisture is only about fifteen per cent. By contrast, plants grown on a very light soil, with a moisture-holding capacity of about eighteen per cent, will not wilt until the soil moisture falls to two or three per cent, so that the sandy soil actually holds as much available moisture as the clay. In very many instances a clay holds more total water than a sand, but the available water is much less.