THE CHEMISTRY OF THE SOIL. 83 



Crop Requirements. 



From the preceding considerations, which I have endeavoured to make as 

 clear and as genera] as possible without going into precise details, we shall 

 have realised that a definite relationship exists between the requirements of 

 the plant and the nature of the food with which we have to supply it. All 

 plants do not take up the constituents presented by the soil in the same 

 proportion, and are consequently benefited differently by different manures. 

 Whilst all crops will benefit from the application of a complete manure 

 containing all the elements of plant food, there is in each case a special ingre- 

 dient or ingredients which the crop more particularly requires. Some require 

 potash, others nitrogen, whilst others do not thrive unless there is abundance 

 of phosphoric acid. 



What these special requirements are, ami the nature of the different 

 manures by means of which they are satisfied, we are now in a position to 

 inquire into. 



We know that in the selection of the proper manure for use in any special 

 case we have, in the first place, to consider the requirements of the crop in 

 question, and, secondly, how far the soil can supply those requirements. 



In connection with the rational application of artificial manures, we have 

 therefore the following points to consider : — 



1 . What are the chemical requirements of the different crops 1 



2. How far does the soil supply these requirements 1 



3. In what direction is jt to be assisted by manure ? 



I do not propose to touch upon the question of tillage or of general farming 

 operations, except in so far as they may produce chemical changes in the soil 

 constituents. The action of the different manures will be regarded entirely 

 from a chemical point of view as plant-food, 



In the first place, with regard to the requirements of the different cultivated 

 crops, a very superficial observation is enough to show us that the food 

 required by different plants must vary considerably. Plants vary enormously 

 in character, and every plant requires one or more special sorts of food, and 

 exercises a power of selection in regard to such food. Some crops cannot 

 make use of the soil-ingredients to the same extent as others do. 



Some crops also occupy the ground for a longer period than others ; those 

 that come rapidly to maturity will require a larger store of food which is 

 immediately available. Again, their requirements vary in many cases at 

 different stages of their growth. What these special requirements are we 

 will proceed to examine more in detail for each class of crop. 



The Requirements of Wheat. 



Wheat thrives best in free loamy soil, with a dry subsoil about 9 to 12 

 inches below the surface. A good subsoil is of great importance in the dry 

 districts, as it retains the moisture and enables good crops to be grown 

 where otherwise the crop would fail through the drying out of the soil. 



The quantities of the different fertilising ingredients removed from the 

 soil by the wheat crop are as follows : — 



A wheat crop of 30 bushels grain to the acre weighs about 5,000 lb., or 

 something over 2 tons of grain and straw, and removes from the soil during the 

 period of its growth 48 lb. nitrogen, 21 lb. phosphoric acid, and 29 lb. potash. 



