THE NEW SOIL SCIENCE. 



73 



on the other hand, is inimical to their life ; also certain physical and 

 chemical soil conditions. 



Surface soils and those rich in organic matter are the home of soil 

 bacteria, but layers of the same soil at different depths exhibit marked 

 differences in respect of its bacterial inhabitants both quantitatively and 

 qualitatively. The uppermost layer of soil, however, does not contain 

 the greatest percentage of organisms, as it is a layer exposed to too rapid 

 alternations of excessive moisture and dryness, and heat and cold, as well 

 as to the influence of the sun's rays. The largest bacterial content of 

 the soil is always found at a depth of about 10 to 20 inches below the 

 surface. As we go down below two feet bacteria become fewer, and below a 

 depth of five or six feet only a few anaerobes — germs capable of development 

 in the absence of oxygen — are found. The condition of the soil in respect 

 to heat, aeration, and moisture therefore influences its bacterial population, 

 and these are the conditions that go to make " tilth." The percentage of 

 germs is also higher in summer than in winter, while it falls in dry 

 weather and increases in wet ; but the condition which above all controls 

 the quantity and quality of bacteria in a soil is the degree and quality of 

 the organic matter in the soil. Virgin soils, however, contain much fewer 

 bacteria than cultivated lands, and in cultivated land the number of 

 organisms increases with the amount of cultivation given and nature of 

 manures used. 



Assuming that a soil is in a healthy state in respect to its germ life, 

 let us see what work the economic bacteria perform. In most soils there 

 is an average amount of organic matter derived from animal or vegetable 

 sources. In order that this complex material should be of service and its 

 constituents not lost, it is necessary that it should be broken down into 

 simpler constituents. This is accomplished in a degree by the decom- 

 position bacteria (and some of them denitrify as well as break down organic 

 compounds) ; but the most important, from the cultivator's point of view, 

 are the denitrifying bacteria which reduce the nitrates present to 

 nitrites, or to nitric or nitrous oxide gas, or to nitrogen itself, and in all 

 of these processes a loss of nitrogen is involved. The conditions requisite 

 for bringing about denitrification are : (1) the specific micro-organism ; 

 (2) the presence of nitrate and suitable organic matter ; (3) the usual 

 essential conditions of bacterial growth ; and (4) a supply of atmospheric 

 oxygen in the soil relatively not in excess of the supply of organic matter. 

 The entire process either of decomposition or denitrification is not 

 accomplished by one species. There seems to be a remarkable division of 

 labour between the decomposition and denitrification groups, and also 

 between different species of the same group, and by their action an 

 enormous quantity of combined nitrogen is daily set at liberty in the soil. 

 To replace this loss as far as possible is the work of the nitrifying bacteria. 

 The results of the action of these bacteria, i.e. those that bring about 

 decomposition and denitrification, are free nitrogen, carbonic acid gas, 

 ammonia bodies, and nitrites. The nitrogen liberated passes into the 

 atmosphere and is " lost " as far as the cultivator is concerned ; the carbonic 

 acid is used by vegetation, and the ammonia and nitrites await further 

 changes, as plants require their nitrogen to be in the form of nitrates in 

 order to use it. Nitrates contain a considerable amount of oxygen, nitrites 



