214 THE BOOK OF NATURE STUDY 



rise to the surface, and the consequent evaporation and cooling. 

 Their coldness causes clay soils to be late and slow growing in the 

 spring ; they are therefore unsuited to market gardening where a 

 quick succession of crops is aimed at. As a set-off they hold 

 their growth well into the summer, and do not suffer from short 

 droughts ; a long-continued drought, however, punishes the plants 

 on a clay soil very severely, because when once the original stock 

 of water is exhausted its renewal by capillarity is very slow. The 

 shrinkage of the clay on drying also results in deep and wide cracks, 

 which further aggravate the loss of water. If a field section of a 

 heavy clay soil be examined it will be seen that the plants are 

 comparatively shallow rooted, partly owing to the difficulty of 

 penetration, and partly to the abundance of water that is usually 

 present. This shallowness of the root system contributes to the 

 injurious effect of a long drought on a clay soil. Just as a clay 

 soil is late to start, in the autumn its cooling is correspondingly 

 slow, and in consequence plants continue to grow longer on clay 

 than on sandy soils ; in some cases this is an advantage, in others 

 maturity is apt to be deferred until bad weather sets in. Owing 

 to the expense and uncertainty of spring cultivation on clay 

 soils they have very largely been laid down to grass since the 

 fall in agricultural prices, but very often they will not grow 

 good grass, because of the deficient aeration brought about by 

 the setting together of the soil that occurs a few years after 

 the arable cultivation has ceased. It is not until a stock of 

 humus has been built up and the soil has become thereby 

 once more open and friable that grass really does well on 

 these heavy soils. The typical crops of strong land are Wheat, 

 Beans, and Mangolds ; of these large crops can be grown, and 

 of good quality. Certain weeds are troublesome " Black 

 Bent Grass " and "Field Mint " in the arable land, but as a 

 rule weeds are not abundant on cultivated clay land. In the 

 pastures " Dyers Weed/' Buttercups, and the thorny form of the 

 " Rest-Harrow " are common weeds ; with many plants spines and 

 thorns become strongly developed on clay land. Owing to the 

 lack of aeration in the soil the grasses are apt to develop a stoloni- 

 ferous surface-rooting habit, and "Bent Grass " the stoloni- 

 ferous creeping-rooted form of " Agrostis alba " often usurps 



