Water,
Warmth,
Food,
Fresh air,
Light.
We may add a sixth: there must be no harmful substance present in the soil.
Wild plants growing in their native haunts get no attention and yet their wants are supplied. We will try and find out how this is done.
Fig. 29. Loam and sand both retain water, but loam better than sand
Water comes from the rain, but the rain does not fall every day. How do the plants manage to get water on dry days? A simple experiment will show you one way. Put about four tablespoonsful of dry soil on to the funnel shown in Fig. 29 and then pour on two tablespoonsful of water. Measure what runs through. You will find it very little; most of the water sticks to the soil. Even after several days the soil was still rather moist. Soil has the power of keeping a certain amount of water in reserve for the plant, it only allows a small part of the rain to run through. Do the experiment also with sand, powdered clay, and leaf mould. Some water always remains behind, but less in the case of sand than in the others. In one experiment 30 cubic centimetres of water were poured on to 50 grains of soil but only 10 cubic centimetres passed through, but when an equal amount was poured on to 50 grains of sand no less than 20 cubic centimetres passed through. Very sandy soils, therefore, possess less power of storing water than do soils with more clay or mould in them, such as loams, clays or black soils.
