HORTICULTURAL SOILS. 



81 



How Plants abe Dependent upon the Food Supply in 

 the Soil. 



The invaluable investigations of Rotham,sted just referred 

 to, and others of a similar kind, illustrate this fact among others 

 — that the crop or particular plant we grow has to do not only 

 with the supply of food in the soil as a whole, but also with each 

 of its ingredients separately. The total productive power of a 

 soil cannot exceed its power to supply to the growing plant each 

 and all the necessary food constituents. Every plant we culti- 

 vate must have a certain amount of each of the nutritive 

 elements — potash, phosphoric acid, and nitrogen — or it cannot 

 grow satisfactorily. Thus the plant cannot rise above the level 

 of the lowest ingredient in the food supply. If each description 

 of food comes up to the required standard, and other conditions 

 of heat and moisture are favourable, a good result may be 

 expected; but if any one element falls below this standard, the 

 growth of the crop must suffer. 



We have seen in the various illustrations brought forward 

 that the food supply available to plants varies greatly in different 

 soils. Sometimes one constituent and sometimes several may 

 be lacking. An horticultural soil may have a proper texture, 

 with a suitable amount of moisture, and, in fact, a full supply 

 of everything the plant needs, except phosphoric acid ; if so, it 

 cannot yield a full crop. Add phosphate in an available form 

 and the growing plants will be benefited. Another soil may be 

 deficient in potash, another in lime, another in nitrogen, still 

 another in two or three of these substances. This same variation, 

 as shown in Table VII. (6), may run through inherent fertility 

 of the soil and in the solubility of its constituents. Therefore 

 an horticultural soil may be deficient in available mineral 

 ingredients or in available nitrogen. Or it may be so compact 

 that air and moisture cannot get into it to work over the crude 

 material it contains, nor the plant roots make their way through 

 to obtain the food that has been made soluble. Again, it may 

 be so loose and non-retentive that the food constituents will 

 escape by drainage. Or, on the other hand, it may be so dry 

 that fertilisers will be useless, and plants wither for lack of 

 moisture ; or so wet and cold as to prevent plant growth. In these 

 several cases proper tillage operations will assist in amending the 



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