During dry weather plants require a soil which is absorptive and retentive of atmospheric moisture. The amount of this retention is generally in direct ratio to two factors, _viz._, the amount of organic matter and its state of division. The capillary water of the soil is very closely related to its percolating power, since all waters in the soil are governed in their movements by what is known as capillary force. Liebenberg has shown that this movement may be either upwards or downwards, according as the atmosphere is dry or supplies soil-saturating rain. The water absorbed by the roots passes into the plant circulation, and the greater part is evaporated from the leaves. Where the supply of water is insufficient, the plant wilts, and if the evaporation long continue in excess of the supply obtained from the soil, the plant must die. The experiments of Hellriegel have shown that any soil can supply plants with all the water they need, and as fast as they need it, so long as the moisture within the soil is not reduced below one-third of the whole amount that it can hold. The quantity of water required and evaporated by different agricultural plants during the period of growth has been found to be as follows: One acre of wheat exhales 409,832 pounds of water. „ „ „ clover „ 1,096,234 „ „ „ „ „ „ sunflowers „ 12,585,994 „ „ „ „ „ „ cabbage „ 5,049,194 „ „ „ „ „ „ grape-vines „ 730,733 „ „ „ „ „ „ hops „ 4,445,021 „ „ „ Dietrich estimates the amount of water exhaled by the foliage of plants to be from 250 to 400 times the weight of dry organic matter formed during the same time. Cultivation conserves soil moisture. It must be remembered that this water contains soil ingredients in solution. Hoffmann has estimated that the quantity of matter dissolved from the soil by water varies from 0.242 to 0.0205 per cent of the dried earth. The experiments of Humphrey and Abbott have shown that about one-sixth of the total sediment of the Mississippi river is soluble in water. =142. Determination of the Porosity of the Soil.=—The porosity of the soil is fixed by the relative volume of the solid particles as compared with the interstitial space. It is most easily determined by dividing the apparent by the real specific gravity. Let the real specific gravity of a soil be 2.5445 and the apparent specific gravity of the same soil be 1.0990. The porosity is then calculated according to the following ratios, _viz._: 2.5445 : 1.099 = 100 : X Whence X = 43.2 = per cent volume occupied by the solid particles of the soil. The per cent volume occupied by the interstitial space is therefore 56.8.