148 STATE BOARD OF AGRICULTURE. 



MOISTURE. 



As it is impossible for animals and hisher plants to exist for any length of time 

 without water, so it is impossible for bacteria of any kind to exist and be active 

 without water. In the soil there must be moisture present, or the bacteria fail 

 to grow and carry on their functions, in fact many of them will die; hence the 

 necessity of conservation of the soil moisture by frequent and thorough culti- 

 vation, not only for the benefit of the crop, but also for the benefit of the soil 

 bacteria. 



BEACTION. 



A third essential point in furnishing a proper environment for soil bacteria 

 is to have the soil exhibit a proper reaction. Laboratory experience has shown 

 that soil micro-organisms will not develop in an acid medium. It must be about 

 neutral or slightly alkaline to litmus. If too much humus is present in the soil 

 the decomposition of the same may result in the formation of various organic 

 acids which prevent further growth of bacteria. Such soil we say is "sour," and 

 vegetation is scarce because the bacteria are checked and are unable to change 

 the plant food to a form in which it may be assimilated. Such a condition may 

 be remedied by the addition of lime to the soil in amounts of 1,500 to 2,000 pounds 

 per acre. Considerable discretion should be observed, however, in the liming of 

 soils, as actual tests often show that soils seemingly sour are, in reality, not acid, 

 but rather are very perceptibly alkaline. On such lands the addition of lime 

 would only aggravate rather than alleviate the trouble. 



BESPIRATIOX. 



One of the first essentials for the vigorous growth of soil bacteria is a bountiful 

 supply of oxygen, as most of them are checked in their growth in proportion as 



the oxygen supply is reduced. About one-half the volume 

 of average soil under ordinary conditions is "pore 

 space." The soil water is gathered in films round the 

 soil granules, thus leaving air spaces of greater or less 

 size according to the amount of moisture in the soil. 

 This may be represented by a diagram such as is shown 

 in Figure I. The soil granules are represented by the 

 ruled spheres, the water films by the dotted areas sur- 

 rounding them, and the air spaces by the clear areas. 

 P, P, P, P, connected with each other, forming an intri- 

 cate and complex system. It is through these openings 

 and channels that the bacteria get the necessary supply 

 of air for their growth. Here is further demonstrated 

 FiKiire |. the value of tillage in keeping the soil well aerated as 



soil iKini^lTs'^''surroum^^^^^ as to conserve the moisture present, 



capillary water films, air spa- 

 ces, p, p, p, p, after Chester. 



FOOD SUPPLY. 



Before discussing this requirement, it will be necessary to classify the soil 

 organisms, as each 'class requires different food materials for its development. 

 They may be broadly divided into four groups: First, the common saprophytic 

 germs acting upon organic matter, which may be designated as the ammonifiers. 

 Second, that group whose especial function is the destruction of nitrates, or the 

 denitrifying organisms. Third, those which build up nitrates or the nitri- 

 fiers. Fourth, those whose sole function is to extract free nitrogen from the at- 

 mosphere. 



AMMOXIFICATION. 



It will be remem1;prpd from the statements of the amount of plant food stored 

 up in the soil, that there is enough nitrogen to last only 90 years. Hence the all 

 important question is that of the nitrogen supply. 



When dead organic matter falls upon the ground, it immediately begins to 

 undergo a process of decay. Dead animals left upon the ground or buried in it 



