Soils of the South-Western Districts of the Cape Colony. 159 
the proper fertilisers to apply to particular soils than was the case in 
years gone by. None the less there still is a very extraordinary rush 
on guano as the hoped-for saviour of the land from all its ills, and 
people will not recognise that on poor lands this method of treatment 
results in all the more speedy depletion of the soil, for guano, by 
virtue mainly of its nitrogen, is a stimulant, and the usual results of 
stimulants follow its use. What is known to agricultural chemists 
as the law of the minimum should be borne in mind. Briefly stated, 
it is this: The growth and development of plant material is regulated 
by the amount of that particular form of plant food which is present 
in smallest proportion. If one particular substance required by the 
plant is deficient in the soil, no excess, however great, of other 
varieties of plant food will cover the deficiency. The soil may 
contain abundance of nitrogen, lime, and potash, for instance, but 
if phosphates are absent, or present only in small amount, no crop 
can reach perfection; for one reason, because the quantities of the 
former taken up are proportionate to the quantity of the latter 
available ; hence, if only one of the plant-food constituents is deficient, 
the crop suffers as much as if all were wanting. 
Now to supply stimulating manures in such cases is worse than 
useless, as the reaction is sure to follow. Under the influence of the 
stimulant the plant makes, as it were, a special effort to get sufficient 
phosphates as an adjustment to the other nutritive constituents, and 
the result is a more rapid impoverishment than would otherwise 
have been the case, inevitably bringing on a collapse from which no 
amount of stimulants will be able to rouse the land again. 
Moreover, the lack of one constituent is sometimes not only the 
indirect, but the immediate cause of others being deficient. Research 
has shown that nitrifying bacteria need phosphates for their develop- 
ment: hence lack of phosphates goes hand in hand with retarded 
nitrification. This latter process, besides, cannot go on except in a 
soil sufficiently alkaline, and it is therefore also retarded by a 
defective lime supply, for the lime neutralises acidity in the soil and 
renders it ready for the reception of the nitric acid formed in the 
process of nitrification. Thus we see that the supply of nitrogen is 
also dependent upon the presence of a sufficiency of lime and phos- 
phates in the soil. 
From a utilitarian point of view one cannot help regarding it as in 
the highest degree unfortunate that we should spend millions upon 
millions in fouling our rivers and other sources of water supply and 
in casting into the sea what nature meant to be restored to the earth 
whence it came. Every sewer we construct is in a sense an additional 
step towards the impoverishment of the land, and all the refuse we 
