26 AGRICULTURE. [CH. 



table growth. The roots of plants may be unable to 

 penetrate a soil because of its hardness, or the 

 presence of stagnant water may have the same effect. 

 A plant in sending its roots into the soil, requires 

 not only that the roots shall be able to extend 

 through the soil in search of food, but that the soil 

 shall also be in a healthy condition. A supply of water is 

 necessary for the roots, but a supply of aiFTs equally 

 necessary. When the soil is charged with an accumu- 

 lation of stagnant water, the roots which come within 

 its influence are unable to discharge their func- 

 tions in a healthy manner, and the growth of vegetation 

 is consequently very slow and imperfect. 



46. Another condition of fertility is the presence in 

 the soil of all the food which the crop requires. An 

 abundance in the supply of one portion of the food 

 does not compensate for a short supply of another 

 equally important portion of the food. Hence the 

 fertility of a soil is determined by the quantity of that 

 essential food which is present in the least propor- 

 tion, and not by that which is in great abundance. 

 To illustrate this by a familiar example, a builder may 

 liave plenty of stone for the construction he intends to 

 erect, but if he has little mortar his progress is soon 

 stopped for want of a further supply. It would not 

 assist him if you increased his supply of stone ; he wants 

 something else, and until this is ready for his use he 

 can make no progress. It is the short supply of 

 mortar which regulates his work, and not the abun- 

 dance of stone. It is just the same with vegetable 

 growth ; the plant requires a variety of materials, and 

 that essential material which is present in the least 

 abundance regulates the crop, and not those which are 

 more plentifully supplied. 



47. The terms good and poor land have reference 

 to the relative productive powers of land. In a good 

 soil we have a combination of conditions favourable 

 for the production of large crops we have a soil with 



