WATER CAPACITY OF SOILS 69 



results when soils of different volume weights are compared. 

 It is the quantity of plant food in a given volume of soil 

 which determines its poverty or richness. The roots are dis- 

 tributed through a certain space : it is the quantity of water 

 and of plant food in that space which is important. We 

 have already seen (pp. 42 and 47) that the volume weights, 

 or apparent specific gravities of different soils may vary 

 greatly, the extreme differences being shown by sand and 

 humus. In Table VIII the proportion of water has been 

 calculated both in respect of the volume and of the weight of 

 the soil; the results by weight are also expressed both as 

 percentages of the wet soil, which is the usual English mode, 

 and also as per hundred of dry soi], which is a mode frequently 

 adopted by American and foreign writers. It will be seen at 

 once that the relative value of the different soils as storehouses 

 of water for the plant appears wholly different according to 

 the particular mode of statement we regard. Thus, reckoned 

 by volume, the peat subsoil is seen to supply rather more 

 than twice as much water as the coarse sand, and this is the 

 relation between them perceived by a plant. If, however, 

 we look at the percentage by weight reckoned on the wet 

 soil, we should conclude that the peat supplied four times 

 as much water as the sand ; and looking at the figures per 

 ico dry soil, we should conclude that it supplied fifteen times 

 as much water as the sand. The unfairness of these com- 

 parisons by weight is at once apparent if we recollect that 

 in the last case we are really comparing the water in seven 

 volumes of wet peat with the water in one volume of wet sand. 

 Experiments were made by King ( Wisconsin 6th Rep., 196) 

 on the amount of water required to saturate the soil and sub- 

 soil of his station when these were in their natural con- 



