88 SCIENCE AND FRUIT GROWING 



Broadbalk field at Rothamsted, a condition of equilibrium be- 

 tween the soluble salts removed and those becoming soluble 

 is not attained till after about fifty years of continuous cropping. 



Another element of uncertainty lies in the different depths to 

 which plant-roots extend. This must affect the question in more 

 ways than one. A plant which draws any considerable portion of 

 its nourishment from the subsoil must be comparatively insensible 

 to the richness of the surface soil, and will respond less readily 

 to any dressing applied to this latter; whilst, at the same time, 

 the tapping of the resources of the soil at lower depths cannot fail 

 to influence the progress of the changes occurring in the con- 

 stituents of the soil. Not only do different kinds of trees and 

 plants extend their roots to different depths below the surface, 

 but trees of the same kind will differ materially in this respect in 

 different soils, the chief factor conditioning the level at which the 

 roots flourish appearing to be more dependent on the aeration 

 of the soil than on the food supply in it. A striking instance of 

 this has already been adduced from the planting of paradise 

 stocks with their roots 2 feet below the surface (p. 39). 



Contrasting fruit trees with ordinary farm or vegetable crops, 

 there is a marked difference in the mass of soil from which their 

 nourishment is drawn. With farm crops there is considerable 

 diversity as to the depth of rooting, but it is probable that in most 

 cases the bulk of nourishment is drawn by them from the top 

 nine inches of soil. With fruit trees, in most instances, the case is 

 very different : the bulk of the roots are not in the surface soil 

 at all, and many of them will extend to several feet below the 

 surface. This difference, however, does not apply with so much 

 force to plant-growth in the soil of the Fruit Farm at Ridgmont, 

 for there we are dealing with a shallow surface soil overlying the 

 Oxford Clay, and in it all fruit trees assume a remarkably shallow- 

 rooting habit (p. 309). But the extent to which roots penetrate 

 into the subsoil is not the only way in which they may, by their 

 habit of growth, affect the manurial requirements of the soil; for 

 roots which are concentrated within a small area must soon ex- 

 haust the soil immediately surrounding them, and, as the motion 

 of water in the soil is often a slow process, time must elapse before 

 supplies from a greater distance can reach the plant. In such a 

 case the plant would require a greater concentration of food- 

 material in the soil than in other cases where the roots were more 

 openly distributed through the ground, even though the actual 

 amount of food required was the same in both cases. Naturally, 

 too, the rapidity of root-action, as determined by the physical 



