THE SOIL 43 



temperature, the water-holding capacity and the porosity 

 (air-space). 



31. Physical Properties of Soils. The above classifi- 

 cation depends upon the size of the soil particles and 

 upon the organic matter content, these factors determin- 

 ing its physical properties. Sandy soils have large 

 particles predominating and this results in their loose- 

 ness and openness, their non-adhesiveness. Water drains 

 readily through such soils, and less is retained for crops. 

 Air circulates more freely, the pore spaces are larger 

 and because of this, and the fact that they hold 

 less water, sandy soils are "warmer", "earlier", and 

 therefore best adapted in the humid sections to truck 

 farming and early vegetable gardening. 



Clays, on the other hand, are plastic, difficult to work 

 when wet. This is due mainly to the extremely small 

 soil particles and in part to the plastic nature of the 

 material, kaolin or fire-clay, which makes up a con- 

 siderable part of such soils. Clays retain more moisture, 

 the particles being smaller and hence more numerous 

 per given volume; the water film surrounding the 

 particles is more extensive in area, hence the ability to 

 hold more water. Air circulates less freely. The total 

 air space is greater in clays (porosity is inversely pro- 

 portional to the size of the particles), but the movement 

 of air finds greater resistance. A further fact about 

 clays is that though they retain more moisture, they give 

 it up with less ease to plants than do sands. Plants 

 will wilt in a clay soil, whereas in a sand of equal total 

 moisture content, they would show no signs of lack of 

 moisture. Thus in general clay soils are "colder", 

 "later", more retentive of moisture and more difficult to 

 work. 



