no BACTERIA AND SOIL FERTILITY 



from twenty-four to seventy-two hours sooner than in the other 

 parts of the tray. The temperature of the soil may be even 2 

 degrees higher in the colored portion than in the uncolored. 



2. The decay of the organic manure in the soil generates heat. 

 This may increase the temperature of the soil. Practical use is 

 made of this in the preparation of the cold-frame for the forcing 

 of young plants in the spring. The truck gardener also uses it 

 when adding horse manure to his early truck gardens. 



3. Organic matter changes the structure of the soils. When 

 added to a tight or clayey soil, it tends to spread the particles, thus 

 causing the soil to puff up. This increases the pore space and 

 hence the quantity of oxygen entering. This accelerates bene- 

 ficial bacterial activity. Partly decayed organic manures have a 

 tendency to stick together; hence, when applied to a loose sandy 

 soil they improve its tilth. The addition of large quantities of 

 fresh organic manures may loosen up some soils to such an extent 

 that they are injurious. 



4. Organic matter acts like a sponge when brought in contact 

 with water. Hence, when applied to a soil it increases its water- 

 holding capacity. It also makes drainage possible in a tight soil. 



K. It is the ffreat reservoir in which is held the nitroe^en of 

 the soil as well as considerable phosphorus and potassium. 



6. As the organic material of the soil decays there are pro- 

 duced various acids which act upon the insoluble plant- food of 

 the soil and make it available. The more economic systems of 

 maintaining soil fertility make use of this factor. The plant- 

 food is added to the soil in the cheapest insoluble form and then 

 rendered available as needed by the aeration of the soil, the addi- 

 tion of plant residues, and manure. It is the fresh organic 

 manures and not the old decayed manures which are valuable in 

 this respect. 



7. The nitrogen-gathering bacteria of the soil must have en- 

 ergy in order to change the inert atmospheric nitrogen into com- 

 pounds available to higher plants. This energy is obtained 

 through the oxidation of the organic matter of the soil. 



Decay. — Plants and animals including man are permitted to 



