38 THE FERTILITY OF THE SOIL [CH. 



that the other three conditions are also favourable 

 to plant growth. 



Soil Types. 



In order to increase the soil fertility it is necessary 

 first to seek out the factor limiting plant growth and 

 then to remove it. As the different soil types have 

 certain characteristic limiting factors we can now 

 advantageously turn to them for a time. 



Sandy soils consist chiefly of inert silica with only 

 about 6 per cent, or even less of clay, and are con- 

 stitutionally poor in those mineral compounds that 

 give rise to plant food. Any stock that might 

 originally have been present is constantly being 

 reduced by solution in the rain water that drains 

 through. They are therefore poor in plant food. 

 But on the other hand, if they are free from such 

 obstructions as layers of rock, hardpan, or stagnant 

 water, they allow of very copious and deep develop- 

 ment of plant roots. The actual volume of soil upon 

 which the plant may draw for food and water is con- 

 siderable, and in consequence sandy soils yield better 

 crops than might at first be supposed. Generally 

 also the crops ripen well and give early produce of 

 good quality. Only small amounts of calcium car- 

 bonate appear to be necessary to prevent sourness. 



The deficiency in food is readily made good 

 by frequent small additions of manure. The water 



