AGRONOMY 23 



possess a selective power for certain mineral in- 

 gredients of soils — hence the varied nature of 

 their ash constituents (see chapter iv.), and a 

 proper system of manuring. A soil destitute of 

 any mineral constituent requisite for the growth 

 of trees, or the ripening of fruit, or the pro- 

 duction of flowers, may become more or less 

 barren, since it is the minimum of any one essential 

 ingredient, and not the maximum of others, which 

 is the measure of fertility. The law of mini- 

 mum should be the basis of all sound husbandry. 

 It is erroneous to suppose " that the more humus 

 (organic matter) a soil contains the more fertile 

 that soil becomes, for it is possible for a soil to 

 be overladen with humus. Indeed, by the ex- 

 cessive application of farmyard manure, or by 

 the undue accumulation of crop residues within 

 the soil, it may become disadvantageous to culti- 

 vate it." ' 



The tables on the two following pages give the 

 composition of various soils (percentages) : — 



" The high percentage of iron oxide present in 

 the Californian soil is probably the chief contri- 

 buting cause of the brightness in colour of the 

 Californian fruits" (C^ea^). 



1 Fream's Soils and their Prop6rUes, p. 92. 



