34 
The Colorado Experiment Station. 
surest and most economical method of inoculation is by means of 
soil collected from a locality where the desired bacteria are abun¬ 
dant as indicated by the nodules on the roots of plants. One to two 
hundred pounds of such soil per acre scattered over the field will 
almost always produce a satisfactory inoculation, other things 
being favorable. 
Although the nodule forming bacteria are cross-inoculable to 
a certain extent on different legumes, a higher per cent of successful 
inoculations has been obtained when the culture employed has been 
isolated from the same species as the legume to be inoculated. For 
this reason, when ordering either cultures or soil, the specific legume 
for which the material is desired should be stated. For example, an 
alfalfa culture should be purchased for alfalfa, and not a bean or 
a cow pea culture; the same applies to soils. 
In using soil for inoculating land, the danger of introducing 
obnoxious weeds and plant diseases must not be overlooked. In this 
connection may be mentioned the recent discovery that the organ¬ 
ism of crown-gall, Pseudomonas tumefaciens, is widely distributed 
in cultivated soils, and that its attacks are not confined to fruit trees 
only, but that they extend to the sugar beet, salsify, tomatoes, and 
many other plants, including the legumes. 
The chemical reaction of the soil has been shown to be a limit¬ 
ing factor to successful inoculation, neutral and alkaline soils being 
preferred. Another investigator has observed that seeds in swell¬ 
ing, previous to germination, secrete a substance which tends to pre¬ 
vent the formation of nodules. The character of the soil, the sub¬ 
soil, and the drainage are additional factors which must be taken 
into consideration. 
CHANGES IN MINERAL SUBSTANCES. 
In addition to the carbon and nitrogen which enter into the 
composition of plant tissues, there are several other elements equal¬ 
ly indispensable to their growth. Phosphorus, potassium, sulfur, 
calcium, magnesium, iron and silicon, all share in the make-up of 
green plants. Inasmuch as these elements occur in all soils, more 
abundantly in some than in others, we have given ourselves little 
concern over the time when they shall cease to be plentiful. Already, 
the potash, phosphoric acid, and lime (calcium) have become prac¬ 
tically exhausted in some soils, and they are being supplied in the 
form of commercial fertilizers as superphosphate of lime, sulphate 
of potash and limestone respectively. In some soils, it is not so much 
a question of the supply being depleted as it is a matter of convert- 
