SOILS AND THEIR PROPERTIES 63 



tend to reduce the colloidal condition of the soil. A certain 

 amount of colloid is certainly valuable in light soils. At 

 Woburn it has been observed that nitrate of soda removes 

 colloids from the surface soils, and deposits them again 

 deeper down, so that the surface soil loses its adhesive 

 properties, and becomes too dry and sandy. On heavy soils 

 too much colloidal matter makes the clay almost unworkable. 

 It should be noted that fertilizers, in addition to their purely 

 chemical value, have a powerful influence upon the colloidal 

 character of the soil. It is doubtless perfectly possible that 

 in a few special places this influence of the fertilizers on the 

 colloids may help to overwhelm the influence of the chemical 

 elements, but in most situations it will be found that the 

 considerations given to the fertilizers in Part I. will be a 

 fairly correct method of assessing the increment of plant 

 production. Nevertheless, the secondary influence of the 

 fertilizers upon the physical properties of the soil must never 

 be overlooked, since it may produce some profound changes. 



Personal observation shows that, on clay lands, basic 

 slag produces an abundance of deep fibrous root, sulphate 

 of ammonia a shallow black humus, and muriate of potash 

 a black humus a few inches deep, with a sticky subsoil. 

 On light soils, nitrate of soda gives a surface saiid with hard 

 pan subsoil. 



Much depends upon the ability for growth of the surface 

 vegetation, and this is illustrated in a striking manner in 

 experiments on pasture land. At Cockle Park, in North- 

 umberland, basic slag has been continuously applied to grass 

 land, with the result that the soil has been steadily deepened, 

 so that the active part of the soil on the surface has invaded 

 the inactive subsoil underneath (see p. 29). No person to- 

 day, who did not know the history, and was shown slices 

 of the two soils, would imagine that they ever could have 

 been the same. This marked change in the soil has been 

 brought about by the increased root development of the 

 natural vegetation, which has been encouraged to grow by 

 the application of an appropriate fertilizer, in this case basic 

 slag. It must not be imagined, however, that for any and 



