OTHER FARM CROPS 533 



also that these different types of soil maintain very different con- 

 ditions of moisture for the plants. It may be said, in fact, that 

 while climatic conditions determine the general distribution of 

 plants make it impracticable, for example, to grow cotton in a 

 northern latitude the texture of the soils, or the relative amount of 

 sand and clay they contain, and the relation of these differently tex- 

 tured soils to moisture largely determine the local distribution of 

 plants and explain why some soils are adapted to cotton, wheat, to- 

 bacco, or to truck farming, and why other soils are not so well 

 adapted to these crops. 



As a rule, the relative amount of moisture maintained by dif- 

 ferent soils for crops under normal conditions depends upon the 

 resistance the soil offers to the descent of the rainfall. The actual 

 resistance, and therefore the relative amount of moisture, main- 

 tained by different soils depends upon the amount of space in the 

 soils for the water to enter ; upon the number of grains of sand, silt, 

 and clay, for this will determine how much the space is divided up ; 

 upon how these grains are arranged, for this will have an influence 

 upon the resistance or the friction the soil offers to the descent of the 

 rainfall; upon the amount of organic matter in the soil, and upon 

 the depth of the soil. 



Soils contain, as a rule, about 50 per cent by volume of empty 

 space ; that is, in 1 cubic foot of soil there will be one-half cubic foot 

 of space into which water or air can enter. In a sandy soil this space 

 will not be divided up so much as in a clay soil; the sand having 

 fewer grains the spaces between the grains are larger, so there is less 

 friction and the water moves downward more quickly. These sandy 

 soils will not, therefore, maintain so much moisture for the plants. 

 The particles of clay soils, on the other hand, are so exceedingly 

 minute, and there are such a vast number of them in the soil, that 

 the spaces between them are exceedingly small and offer a great re- 

 sistance to the descent of rain, so that the water moves very slowly 

 and a large amount is maintained for the plants. A strong clay soil 

 will usually contain three or four times as much water as a sandy 

 soil, and this has a very important effect upon the growth of cotton. 



Advantage is taken of this fact in greenhouses, but in nature, 

 of course, the conditions cannot be controlled. In the latter case 

 the amount of rain which falls must be taken, but different soils 

 being so different in texture and in the resistance which they offer 

 to the descent of the rainfall, maintain quite as different conditions 

 of moisture as are supplied in practice under artificial conditions of 

 greenhouse culture. Light sandy soils, being moderately dry, force 

 plants to an early maturity, and these soils are used at present for 

 the very profitable truck farming which has developed into such 

 an important industry in recent years. Such soils are not adapted 

 to wheat or grass, not necessarily because they are deficient in any 

 particular food required for these crops, but because the dry condi- 

 tions force the plants to an early maturity and the yield per acre is 

 small. Clay soils, on the other hand, are adapted to wheat and grass, 

 because they maintain uniformly moist conditions and the plants 



