166 SOILS: PROPERTIES AND MANAGEMENT 



soils the presence of such material is extremely necessary, 

 as it tends to bind the soil together, facilitates granula- 

 tion, and prevents loss of plant food by leaching. It is 

 only in heavy soils in which such material is excessive 

 that a detrimental condition is likely to exist. This occurs 

 because of a high cohesion and plasticity, because of the 

 competition for food that is likely to arise with the crop, 

 and because of the tendencies toward acidity. Where 

 lime is low or lacking, the situation has a tendency to 

 become still more aggravated by further colloidal devel- 

 opment. 



In general, the practice of underdrainage by allowing 

 the wetting and drying of the soil to proceed, is the first 

 step not only for the curbing of excessive and improper 

 colloidal influence, but also for the encouragement of 

 just the right development thereof. The freezing of 

 winter, tillage at proper times, the addition of humus, 

 and the application of lime are all practices that aid 

 in the control of colloidal conditions. Since this control 

 and utilization of colloidal influences is only a phase of 

 soil structure as related to tilth and granulation, a further 

 discussion of the subject will be reserved for later consid- 

 eration. 



111. Estimation of colloidal content. The colloids 

 in the soil are so complex, so numerous, so variable in 

 function, and so susceptible to change, that an exact 

 determination of their amount is impossible. The knowl- 

 edge of colloidal material in general is so meager that it 

 is not surprising that such slight advances have been 

 made in fully and clearly determining their character in 

 a complicated material, as the soil undoubtedly is. The 

 important methods of estimating the colloid content of 

 the soil depend for their expression on the intensity of 



