In drying, soils shrink. Those which shrink least are sandy and chalky soils. Humus soils, on the other hand, shrink most. _Most favourable Amount of Water in a Soil._ The amount of water in a soil most favourable for plant-growth is a question of considerable difficulty. Too great an amount of moisture renders the land cold; air cannot obtain access to the soil-particles, and the plants sicken and die. Hellriegel has found that as much as 80 per cent of what the soil can hold is hurtful to plants, and that from 50 to 60 per cent is the best amount.[40] _Hygroscopic Power._ A property possessed by soils in relation to water, which is quite distinct from absorptive power, is their hygroscopic power. By this is meant their power of absorbing water from the air where it is present in the gaseous form. This property is identical with the property which will be adverted to immediately--viz., capacity for absorbing gases. The extent to which soils possess this hygroscopic property seems to be regulated very much by the same conditions as regulate their ordinary absorptive power.[41] This property is considered to be of great importance in the case of soils in hot climates, where their agricultural value may be said to depend to a large extent upon it. The amount of water, however, absorbed in this way is, comparatively speaking, insignificant. Lastly, it may be observed that there are certain methods of drying soils afflicted with too much moisture. These consist in making open ditches, and thus relieving them of their superabundance of water, or in planting certain kinds of trees, such as willows and poplars. The amount of green surface presented by the large number of leaves of trees, from which the constant evaporation of water goes on, is very great. The consequence is that trees may be regarded as pumping-engines. It is from this cause that foresters have noticed that clay lands are apt to become wetter after the trees growing upon them have been cut down.[42] _Capacity for Heat in Soils._ A property which depends largely on those we have just been considering is the capacity soils possess of absorbing and retaining heat.[43] The temperature of a soil, of course, largely depends on the temperature of the air; but this, we must not forget, depends also on the soil itself. The heat given forth by the sun's rays strikes the soil, with the result that, while so much of its heat is absorbed, a certain portion--and this will vary according to the nature of the soil--of its heat is radiated into the air. The changes in the temperature of the soil naturally take place more slowly than the changes in the temperature of the air; the depth of soil thus affected by those changes varies also in different climes. It has been calculated that in temperate climes the changes of temperature occurring from day to night are not felt much below three feet down. _The Explanation of Dew._