CH. VlJ 



Uncontrolled Factor, 



107 



Each improvement has consisted in removing some 

 factor that was keeping down the yield to a certain 

 level. But there still remain two sets of factors that 



POTATO SOILS 



Vm& Gravel, larger 

 than 1 ram. 



Coarse Sand, 1 to 0-2 mm. 



Silt, Ot to -01 mm. 

 Fine Silt, 01 to 002 mm. 



Fine Sand, i)-2 to 0-04 mm. 



Clay, smaller than -002 mm. 



Fig. 28 h. Mechanical composition of soils well adapted for 

 certain crops. 



cannot yet be controlled : the climate and the soil type. 

 The difficulty is met by growing crops suited to the 

 coniitions, and this explains why certain crops tend to 



