144 Bulletin 189. 



2d. Fertilizers do not take the place of humus in soils. 



The commercial fertilizers consist principally of phosphates of 

 lime (mineral), salts of potash (mineral), and some form of com- 

 bined nitrogen. The two chief sources for available nitrogen are 

 nitrate of soda (mineral) and dried blood (animal). This dried 

 blood, and frequently tankage, are the only substances that would 

 tend to keep up the humus or mold in the soil. 



The virgin soil was rich in humus from the fallen leaves of cen- 

 turies. Under the influences of air, heat and moisture this humus 

 decayed. The products of this decay furnished not only available 

 nitrogen to the plant, but also part of the necessary mineral sub- 

 stances. The rock part of the soil also gave up some of its mineral 

 plant-food through the decay of the humus. The j^resence of 

 humus is necessary for the best physical condition or tilth. The 

 soil on which the yields tabulated in Experiment 9 were obtained 

 contained very little humus. This deficiency of humus, and not the 

 quality of the fertilizer, accounts for the relatively low yield. 



The amount of moisture stored in the soil, or brought to it by 

 rains, is a more potent factor than the mere adding of plant-food. 

 This is well illustrated by Experiment 10, where the snov/ protected 

 Plats 1 and 2, and stored more water in the soil. 



On account of these various influences there is not necessarily a 

 direct relation between the composition of the soil and some par- 

 ticular substance the application of which will increase fertility. , 

 The chemical composition is only one of several important factors 

 that are concerned in productivity. It is the factor least under 

 man's control, and the one to which he should last resort ; yet, the 

 addition of commercial fertilizer may sometimes give most profit- 

 able results. 



