FERTILIZERS 111 



protozoa. The protozoa are one-celled animals that feed upon 

 the helpful bacteria, often to such an extent as to effect the 

 fertility of the soil. These may be killed or reduced in 

 numbers by burning or boiling the soil, or by treating it with 

 disinfectants. Such treatment does not appear to materially 

 harm the bacteria. The increase in fertility in soils burned 

 over is attributed to the fact that the burning killed the 

 protozoa. Florists usually bake the soil in which young 

 seeds are to be sown, or they may pour boiling water over it, 

 and in this way get rid of the harmful organisms in it. 



Limiting factors in plant growth. The production of the 

 maximum crop is thus seen to depend on many things besides 

 a sufficient amount of the necessary chemical elements in the 

 soil. The temperature may be too high or too low, there 

 may be too little sunshine at some critical period of plant 

 growth, or the soil itself may contain too much or too little 

 moisture. Any unfavorable condition at once becomes the 

 limiting factor in plant growth, and changing this condition 

 frequently results in doubling or trebling the crop. In the 

 West water is often the limiting factor, but under irrigation 

 or in regions of sufficient rainfall the lack of some mineral 

 constituent of the soil is likely to prevent the maximum yield. 

 The presence of insects or plant diseases may also affect the 

 crop, and thus the limiting factor may even change from year 

 to year, with favorable or unfavorable seasons. By supplying 

 the soil with sufficient fertilizers and regulating by irriga- 

 tion, drainage, and cultivation the amount of moisture in it, 

 the farmer renders favorable such conditions as can be con- 

 trolled, which fortunately are among the most important. The 

 photographs on pages 102 and 103 illustrate very clearly the 

 change that may result from adding a single chemical element 

 to the soil. Here the application of a fertilizer containing 

 phosphorus had the effect of immediately adding nearly twelve 

 bushels an acre to the crop. 



