WATER FILM ON SOIL PARTICLES 311 



substances for the support of plant life. Many of these 

 minerals are needed in such small quantities that most 

 soils have an abundance of them. Nitrogen, phosphorus, 

 and potassium are the soil elements that are used most 

 freely by the growing plant. 



Plants also require a great deal of water. Yet few plants 

 thrive if they are submerged in it, or even if their roots are 

 submerged. Air is also necessary to the growth of plants. 

 Air must reach not only the part of the plant growing above 

 ground but the underground portion as well. 



But if a soil had all necessary substances for plant growth 

 in it, it would still lack fertility if it were not for the micro- 

 scopic life of the soil. Some germs increase the fertility 

 of the soil and some decrease it. If those which increase 

 fertility are to thrive, certain conditions must be main- 

 tained. It is the skill of the agriculturist in maintaining 

 and increasing these favorable conditions which largely de- 

 termines his success or failure. 



Water Film on Soil Particles. Experiment 91. Take 

 about a quart of soil from a few inches below the surface of the 

 ground and after sifting out the large chunks, put it in a sheet iron 

 pan and carefully weigh it to the fraction of a centigram. Place the 

 pan containing the soil in a drying oven or ordinary oven, the tem- 

 perature of which is but little above 100 C. The soil should be 

 spread out as thin as possible. Allow it to remain in the oven for 

 some time, until it is perfectly dry throughout. Weigh again. The 

 loss of weight will be the weight of water contained in the soil. 

 As there was no free water in the soil how was this water held ? 

 Dip your hand into water and notice how the water clings to it 

 after it is withdrawn. Examine with the eye and the lens several 

 particles of the original soil as taken from the ground and see if 

 there is a water film on each of these as there was on the wet hand. 



Experiment 92. Take the soil that has been dried and weighed 

 in the previous experiment and heat it throughout to a red heat 



