SOIL FERTILITY. 625 



however, should determine for himself whether his soil needs inocula- 

 tion. The test is the formation of nodules on the roots. Xodules are 

 more likely to occur on poor soil, especially if it he of a sandy character. 

 If the roots of clover are found to possess nodules, then there is no need 

 of inoculation for that form which grows upon clover. It is similarly 

 true with peas, vetches, alfalfa and the like. The farmer can decide 

 the matter for himself hefore he makes anv outlav for material for 

 inoculation purposes. 



It should be said, however, that there is much experimental work 

 yet to be done to show in how far such plants as soy beans enrich the 

 ground for succeeding crops. Some contend that the results are dis- 

 appointing, especially in the case of the soy bean which produces so large 

 tubercles and in such quantity. 



With respect to the ecological aspect of the question it may be said 

 that the growing of mixed crops, cover crops, rotation of crops, and 

 the problem of weeds enter largely into the subject. This involves the 

 relationship existing among different species and the soil conditions. 

 This is comparatively a new aspect of crop conditions, for very little 

 has been done to establish any definite results, although for many years 

 it has been suspected that one plant might exert some injurious or bene- 

 ficial effects upon others in the same soil, apart from the physical condi- 

 tions arising out of the relationship. It is quite commonly known that 

 on the ground under trees certain plants will not thrive, for some reason 

 or other, aside from the injury produced by the shading. The tree 

 seems to produce an effect upon the soil injurious to that particular 

 plant. Just how different species of plants react upon one another 

 when growing in the same soil, whether in an injurious manner or not, 

 and just how they affect the soil for a succeeding crop are questions 

 full of promise. 



It has recently been asserted that certain parasitic Mucors (molds) 

 are capable of doing what the bacteria of the root-tubercles do for those 

 leguminous plants upon which they work. And the suggestion has 

 also been offered that some Mycorrhizas (root fungi) may serve the 

 office of adding to the available nitrogen supply of the soil. And this 

 may account for the immense growth of pine on sandy barren soil. 

 The Mycorrhizas do serve the function of root hairs, and they may also 

 take the place of the bacteria of the root-tubercles in supplying nitrogen 

 to the host plant. 



Soil fertility, then, is not so much a chemical as it is a physical 

 bacteriological and ecological problem. So, whether the soil contain 

 phosphorus, nitrogen, sulphur, potassium, calcium and other elements 

 is not of so much importance as how it contains them, and what rela- 

 tionship they have to water, air, to one another, to the soil particles and 

 to the bacterial content of the soil. 



vol. lxvti. — 39. 



