APPENDIX A XXXV 
available is relatively small. Much the larger portion of the elements 
of fertility are stored in the soil in insoluble forms. Hence only a small 
proportion of the store laid up for the use of the husbandman can in 
any given period be wasted, no matter how ignorant the farmer may be 
or how unwise the treatment to which he subjects his land. ‘The process 
by which such insoluble plant food is changed to soluble forms, is a 
gradual one, in which bacteria are said to play an important part. This 
process is also accelerated by frequent and thorough cultivation of" the soil, 
so as to expose its particles to the action of the sun and air. ‘Thus 
industry and energy on the part of the farmer bring their reward in 
increased supplies of available plant food, when, under favourable cli- 
matic conditions, increased crops are realized. 
Recent investigators have found that when soils are exposed to the 
air and rain for considerable periods they show marked increases in the 
quantities of nitrogen they contain and that these additions of nitrogen 
have been brought about by minute living organisms in the soil, which 
have the power of taking nitrogen from the air and converting this into 
nitrogenous compounds which are retained in the land. 
These nitrogen-fixing bacteria consist of a number of different 
species, varying in the degree of their activity according to the conditions 
of light, temperature, moisture and the porosity of the soil in which 
they are working. Soil bacteria and other lower organisms are also 
said to produce acids in the soil, which aid in dissolving and rendering 
assimilable other important elements of plant food. 
Thus through the agency of soil bacteria some portion of the nitro- 
gen lost through cropping is brought back directly to the soil. This 
will partially explain why land which has been impoverished by over- 
cropping does recover some portion of its fertility if it is allowed to 
remain long unused. Nitrogen is all about us. It constitutes about 
fourt-fifths of the ocean of air in which we live. The supply is inex- 
haustible and if means could be devised for readily producing soluble 
compounds containing this element at a low cost, the possibility of sup- 
plying this important ingredient to the soil in quantities sufficient to 
induce luxuriant vegetation would be permanently assurec. 
Tt has long been known that many farm crops are greatly improved 
by the ploughing under of a previous crop of clover. Within recent 
years it has been shown that this result is largely due to the fact that 
clovers in common with other leguminous plants have the power of taking 
nitrogen from the air, and that this is done through the agency of 
colonies of bacteria which inhabit small nodules or swellings on the roots 
of these legumes. Experiments conducted for a series of years at the 
