may generally be said that very deep root development is charac- 

 teristic of the weeds and crops of dry lands and dry seasons every- 

 where. 



Under such conditions it is essential to conserve what soil 

 moisture is retained at the end of the rainy season. Of course 

 the retentive power of the soil itself varies very much. Other 

 things being equal, a sandy soil will retain least and a peat 

 soil the most, the amount retained increasing on the one hand 

 with the fineness of the soil, and on the other with the amount of 

 organic matter (or decaying vegetable matter) it contains. Thus to 

 take an American example among tobacco soils given by Whitney 

 (U. S. A. Department of Soils, Bulletin 3), we have 



These figures may however in themselves be somewhat illu- 

 sive, because a clay soil, though it naturally retains a much greater 

 proportion of water than a sandy soil, possesses part of it in such 

 a form (termed hygroscopic), that it is not available for vegetation. 

 Thus, for instance, taking again some American results (R. H. 

 Loughridge. Report, California Experiment Station, 1897-1898), 

 we have for some soils growing oranges 



TOTAL HYGROSCOPIC 



MOISTURE. MOISTURE. 



Yellow Loam Soil ... 6-3 per cent. ... 3-1 per cent. 



Dark Red Clay .. 15-4 8 ' 6 



Black Clay (adobe) .. io'o ,, .. 10-0 



Here 'it is evident that the black clay soil is, from a plant's 

 point of view, perfectly dry, though really it contains far more 



USEFUL 

 MOISTORE. 

 3'2 per cent. 



6'8 

 none. 



