While these properties are undoubtedly necessary for fertile soils, it is needless to add that they may be possessed by a soil to too great an extent. The soil that is unable to throw off any excess of water becomes cold and damp, and does not admit of proper tillage. Its pores become entirely choked up, and the circulation of air, which, as we shall see, is of so much importance, is rendered impossible. Plants in such a soil are apt to sicken and die, the water becomes stagnant, and certain chemical actions are caused which give rise to poisonous gases, such as sulphuretted hydrogen, &c. A stiff clayey soil offers a good example of the disadvantage of over-retentiveness. Owing to the difficulty such soils experience in throwing off their excessive water, they are extremely difficult to till; and sowing operations are on that account apt to be delayed. _Power Plants have of absorbing Water from a Soil._ It is a strange fact, and one worth noticing in this connection, that the power plant-roots have of drawing their moisture from a soil, seems to depend on the retentive power of the soil. By this is meant that plants have not the means of exhausting the water in a retentive soil to such an extent as in a non-retentive soil. In some extremely interesting experiments, carried out by the well-known German botanist Sachs, it was found that plants wilted in a loamy soil, whose water-holding capacity was 52 per cent, when its moisture reached 8 per cent; while in a sandy soil--water-holding capacity 21 per cent--the same species of plant did not wilt until its moisture reached 1-1/2 per cent. Here, then, we see that on one kind of soil the plant was able to live, and obtain sufficient water for its needs, while it died of thirst in another soil, although that soil contained quite as much moisture. Speaking generally, we may say that Hellriegel's experiments have shown that any soil can supply plants with all the water they need so long as its moisture is not reduced below one-third of the whole amount it can hold.[38] _How to increase Absorptive Power of Soils._ The absence or presence, in excess, of the above properties, suggests a word or two on how these natural defects may, to a certain extent, be remedied artificially. It stands to reason, that if organic matter in a soil renders its absorptive power greater, a simple method of improving a soil defective in this property is by the addition of organic matter. One of the benefits of ploughing-in green crops on sandy soils is undoubtedly due to this fact; the addition of farmyard manure having also a similar effect. The absence of a sufficient amount of retentiveness, such as is found in sandy soils, in the same way suggests, as a remedy, the addition of clay; and, _vice versa_, where the soil is too clayey, the natural method of improvement will be the addition of sand.[39] _Shrinkage of Soils._