174 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



the current ideas of his time. This discourse might have been the 

 starting-point of a separate soil science. But unfortunately it was 

 not sufficiently experimental to stimulate other investigators, and at 

 the end of the eighteenth century, when the foundations of most of the 

 modern sciences were being laid, there was no one of outstanding 

 genius who took any special interest in the soil. In consequence it 

 has never fallen into any of the conventional divisions of science, 

 but lies in the borderland where the chemist and botanist meet the 

 farmer and the gardener. 



It is now recognized that the plant takes up something of every- 

 thing which is dissolved in soil water, quite regardless of whether the 

 effect is going to be good, bad, or indifferent. Even the most unlikely 

 elements — gold itself and still rarer elements — have been detected 

 in plants and have come in from the soil. 



By long custom gardeners and farmers give the name plant food 

 to those substances in the soil which help the plant to grow. I am 

 retaining the expression in this sense, though it could be severely 

 criticized from the physiological standpoint. Strictly spealdng, 

 the substances about which I am going to speak are not plant foods 

 at all, but only the raw material out of which the plant builds up 

 its food by a process infinitely wonderful and complex. But we 

 shall avoid all ambiguity by giving our definition at the outset, and 

 we can further disarm the criticism of the physiologist by inviting 

 him to give us a better term that is equally simple. 



We all admit the principle that we must be prepared to alter 

 our vocabulary whenever the old terms would cause us to lose touch 

 with the pure chemists and plant physiologists. But the changes 

 in vocabulary of any one branch of science are now so rapid that one 

 shudders to think what would happen to a borderland subject that 

 tried to keep pace with two or three sciences. We may therefore 

 be allowed to keep to our old words provided they convey a definite 

 meaning to us, and that we make this meaning perfectly clear. 



By plant food from the soil we shall understand those sub- 

 stances which the plant takes from the soil and which it utilizes in 

 building up its tissues. 



The plant food obtained from the soil is, roughly speaking, of two 

 kinds : 



(1) Substances already formed in the soil, which were part and 

 parcel of the minerals from which it was derived. 



(2) Substances not originally present, but which have come in since 

 the soil was laid down as the result of the changes produced by 



vegetation. 



Both of these are equally important, and both have given rise 

 to a vast amount of research work. But each requires a different 

 type of investigation, and so it has happened that each has been 

 studied in different laboratories and under somewhat different con- 

 ditions. The substances derived from the rock minerals, often called 



