MO LIVING ORGANISMS OF THE SOIL [chap. 



quantity, half a ton per acre or so, of soil taken from a 

 field in which the crop in question had before grown suc- 

 cessfully, or by sprinkling on the seed an artificial culture 

 of the organism. A number of artificial cultures of the 

 organisms appropriate to such crops as clover, lucerne, 

 sainfoin, beans, peas, etc., have been prepared and can 

 now be purchased commercially in an active state. 

 The practice recommended has been to add the small 

 pure culture that is purchased to a fair bulk of water 

 containing sugar and nutrient salts, or to diluted 

 separated milk, and to keep the mixture for a day or 

 two in order to allow the bacteria to increase, then the 

 seeds are dipped in the liquid for a moment immedi- 

 ately before sowing. The results of such inoculation 

 trials have, however, been disappointing from a practical 

 point of view ; in most cases the soil is already so well 

 stocked with the usual nodule bacteria that the introduc- 

 tion of a few more in the inoculating liquid does not 

 affect the growth of the leguminous plant. In the much 

 rarer cases where the soil is entirely without the 

 organism, its artificial introduction may prove of 

 enormous benefit and enable the crop to grow where it 

 would otherwise have starved. But very often the 

 absence of the organism is only a sign that the soil is in 

 some way unfit to carry it, and some improvements 

 must be effected in the soil before either organism or 

 leguminous plant can be established. Even when the soil 

 is not actually in a condition to prevent the organism 

 growing, the introduction of a particular bacterium, 

 such as that associated with lucerne, cannot be done all at 

 once, or with the rapidity and certainty of growth that is 

 seen when even a single bacterium is added to a sterile 

 medium in the laboratory. The strongest inoculation 

 that is practical only introduces a minute new element 

 into an active flora already in possession of the soil ; 



