572 



SCIENCE 



[N. S. Vol. XXXVI. No. 931 



The chief process by which plant food 

 becomes "available" is weathering, in 

 which water and carbon dioxide play the 

 chief roles. The plant's supply of mineral 

 salts as well as of carbon thus depends on 

 the supply of carbon dioxide. The amount 

 of this substance in the air is kept rela- 

 tively constant not only by the production 

 of carbon dioxide on the earth's surface, 

 but by large amounts constantly escaping 

 from its interior: and the ocean acts as a 

 reservoir of carbon dioxide and a regulator 

 of the amount in the air. 



While the action of carbonic and other 

 dilute acids benefits the plant by making 

 plant food available, higher concentrations 

 of acid may be injurious. As is well known, 

 the acidity of the soil is a large factor in 

 productiveness and some plants prefer acid 

 soils while others show the opposite tend- 

 ency. We are now in possession of a 

 series of indicators which make it possible 

 to ascertain various degrees of acidity 

 found in soils without the labor of gas 

 chain determinations, and these indicators 

 may be used in the field. 



An analysis of the factors of soil acidity 

 is difficult on account of the many com- 

 plications involved. Much aid may be ex- 

 pected from the application and extension 

 of such careful quantitative studies as have 

 recently been made on the ability of cer- 

 tain substances in solution to preserve the 

 neutrality of the solution despite the addi- 

 tion of a considerable amount of acid or 

 alkali to it. 



To what extent the plant itself renders 

 substances "available" by excretions from 

 its roots is an unsolved problem. While it 

 is generally agreed that the carbon dioxide 

 excreted by the root is of importance in 

 this respect, it is not certain whether other 

 substances given out by the plant have a 

 similar action. Investigation should be 

 directed to the fatty acids which have been 



said by various observers to be excreted by 

 the root. It is important to determine 

 whether these substances are given off by 

 the living or by the dead cells. 



There can be no doubt that the bacteria 

 of the soil help to render mineral food 

 available by producing carbon dioxide. 

 According to recent investigations the bac- 

 teria in one hectare of soil to the depth 

 of 40 cm. produce yearly several million 

 liters of carbon dioxide. Hence it becomes 

 important in judging the fertility of a soil 

 to test the amount of carbon dioxide which 

 it produces under natural conditions. 



The supply of available mineral salts is 

 commonly augmented by the application of 

 mineral fertilizers upon the theory that a 

 deficiency in the supply of any necessary 

 substance constitutes a "limiting factor" 

 which retards the development of the crop. 

 The addition of the deficient substance 

 produces a great increase in the crop: the 

 relation between this increase and the 

 amount of salt added has been expressed 

 by Mitscherlich in a simple formula having 

 a constant which is independent of the 

 yield. The constant for any given salt is 

 called its efficiency value. 



It is not my province to discuss this sub- 

 ject further and I will only mention that 

 the application of one substance may set 

 free a different one; for example, where 

 magnesium is applied to the surface of the 

 soil the roots at a lower level may receive, 

 not magnesium, but calcium in soluble 

 form (Hilgard). Sodium has been used 

 in this way to set free potassium. Further 

 study of this subject will doubtless bring 

 to light facts of importance. 



Some of the substances thus set free may 

 exist in a loose combination to which the 

 term adsorption has been applied. But it is 

 clear that a variety of processes are in- 

 cluded under this term. In the first place 

 there are reversible processes, as when a 



