The Science of the Soil 125 



It should not be forgotten that, apart from the benefits of retaining 

 moisture in the soil during a hot summer by loosening the ground with 

 the hoe, other advantages are also secured. The loose and more finely 

 powdered soil absorbs the dews more readily, inorganic foods are liberated, 

 and the grubs or pupse of various insect pests are brought to the surface 

 where they are readily pounced upon by the birds. 



10. LIVING ORGANISMS IN THE SOIL 



It has already been shown (p. 108) that mineral foods alone are 

 insufficient to supply all that is needed for plant life. Scientific investi- 

 gation has proved that the soil, if properly cultivated, is teeming with 

 life. Millions of minute organisms or bacteria are working away in the 

 dark converting the minerals and metals of the earth into plant food, 

 with the aid of the fresh air and the organic manures. This explains 

 why tons of stable or farmyard manure and vegetable refuse of all 

 kinds vanish after a time in the soil. It has been eaten away by the 

 bacteria, and in the process a certain amount of heat and fermentation 

 have been generated. This heat (more or less according to the condition 

 of the manure applied), in conjunction with a proper amount of moisture 

 and fresh air, dissolves some of the mineral substances, and brings them 



o 



into such a condition that they are readily absorbed by the roots. So 

 long as all the necessary factors are present the work goes on steadily; 

 but if one or the other is absent or in a poor state the whole work is 

 impeded. If lime, or manure, or both be absent no bacteria come into 

 existence and no changes take place. Hence a soil without them would 

 be as sterile as a heap of cinders or as the material in the roadway. 



Nitrification. The most important work done by these soil bacteria 

 is to bring about the production of ammonium salts and convert them 

 into nitrates. These bacteria are most active at a temperature of 86 F. 

 (30 C.) according to some authorities, and at 98 F. (37 C.) according 

 to others, and become less active as the temperature rises above or falls 

 below these points. It follows from this that nitrate-producing bacteria 

 manufacture more plant food during the summer months, when the 

 temperature of the soil is about 20 degrees warmer than in winter. 

 This is just as it should be, for plants require far more nourishment 

 when in an actively growing state in summer than when in a compara- 

 tively inactive condition in winter. 



This wonderful nitrifying process is perhaps brought to the highest 

 state of activity under glass at almost any season of the year. The 

 temperature is raised by artificial means, and plants are "forced" into 

 growth. This simply means that the bacteria respond to the higher 

 temperature and the moisture, and proceed to attack the substances in 

 the soil and place them more readily at the disposal of the crop. If the 

 temperature is suddenly reduced the plants are said to "catch a chill" 



