4 oo POPULAR SCIENCE MONTHLY. 



readily than would be possible if lie bad to reduce tbe metal from the 

 ore; and Nature uses this principle over and over again. The im- 

 portance of nitrogen to plants and its abundance in Nature have been 

 mentioned, but it has also been said that plants can not use it directly, 

 as most animals do with oxygen. The tiny bacteria intervene, and 

 this they do in two ways: first, by causing decay of animal or vege- 

 table matter containing nitrogen, and by this decay producing sub- 

 stances that plants can absorb; and, secondly, by producing little 

 nodules or " tubercles " on the rootlets, through which the plant can 

 take up nitrogen.* Now, when a plant is sated with nitrogen, it 

 ceases to form these tubercles, and their formation is a sure sign that 

 the plant is craving this article of food. When it is supplied, and its 

 own life is ended, these form reservoirs from which other plants may 

 be supplied, as new castings may be made from broken wheels. The 

 great value of " green manuring " depends on the store of available 

 nitrogen so laid up, but it is open to failure in one direction. The 

 liability of fermentation to go to the acid stage from contamination 

 with acid-forming ferments has been mentioned, an accident the pos- 

 sibility of which is impressed on us from time to time by sour bread ; 

 and similarly the organic matter turned under may undergo acid 

 fermentation, rendering the ground " sour " and unfit for cultivation. 

 The limits of this paper forbid the consideration of special fer- 

 tilizers, but from the general principles laid down the rules for any 

 special case may be deduced. A soil should contain a sufficient 

 amount of potash, soda, lime, iron, and a few other minerals; phos- 

 phoric acid, nitrogen, organic matter, and, for some special crops, 

 some other ingredients may be needed. When the soil needs re- 

 newing, there are two ways of accomplishing it. One way is to 

 guess at what is needed; to buy fertilizers at high prices, without 

 inquiring whether the soil needs the substances in that particular 

 brand or not. Though very common, this is not a good plan. It is as 

 though a physician were to give a patient any drug that was conven- 

 ient, without inquiring into the disorder or the needs of the system, 

 and it is followed by much the same result. That acid phosphate 

 gave Farmer A a good crop, is no reason that Farmer B's land is also 

 deficient in phosphorus. The same reasoning would teach that a 

 heart stimulant that rouses a patient from shock would benefit one in 

 danger of apoplexy, where the least increase in heart force might be 

 fatal. A physician using such reasoning as the basis of his practice 

 would not be considered a master of his art; and were he to attribute 

 the fatal outcome of his logic to the poor quality of his stimulant, he 

 would display criminal ignorance of drugs as well as disease; yet it 



* Leguminous Plants for Green Manuring and for Feeding. E. W. Allen, Ph. D. United 

 States Department of Agriculture. Farmers' Bulletin, No. 16. 



