l66 CHEMISTRY AND CROPS. 



What do these facts point to if not to the great importance of 

 the mineral constituents of foods in the maintenance of the 

 health of man and beast? How is it possible that our food 

 should contam sufficient quantities of these substances if our 

 soils be lacking-? Is it not folly to continue to ignore this aspect 

 of soil investigation? 



In turning to insect pests we find that our wheat crops are 

 frequently ruined by the wheat louse; that the depredations of 

 "gieen fly" make us wish we had never planted peach trees in 

 our orchards. Of course the biologist tells us that at such times 

 as the insect gains the upper hand its natural enemies have been 

 quiescent. This is certainly only a part of the truth, and no 

 doubt exists in my mind that it is not the most important part. 

 Judging from the analogies of nature, one would think that a 

 plant-sap pre-eminently suitable as food for these little creatures 

 is the main cause of their occasional alarming multiplication. If 

 this be the correct interpretation, the devastation wrought by 

 them will ultimately be traced to soil conditions which need not be. 

 and probably are not, of a permanent character. In Bloemfon- 

 tein sweet peas sown after October almost invariably receive 

 much damage from infestations by an aphis. As a matter of 

 routine, in connection with some experiments I made with a view 

 to promoting the development of bacterial nodules on the roots 

 of legumes, I examine the roots of all legumes grown in my 

 garden. In those cases in which sweet peas have been most 

 damaged by the aphis I have frequently noticed that there has 

 been a very considerable development of the bacterial nodules, 

 which suggests that the plant has received more nitrogen than 

 was good for it. This may be only a coincidence, but at the same 

 time it appears to be of sufficient interest to warrant its being 

 placed on record. 



Very much akin to the depredations by aphides are the 

 attacks on grasses and cereals by rust, which, by Hall, is said 

 to be due to a sudden excessive nitrate production in the soil. 

 The same writer mentions a very interesting fact about mangel 

 leaf-spot fungus. This fungus invariably attacks crops grown 

 on plots that have been manured excessively as regards nitrogen. 

 Further, it would appear that whereas this fungus thrives well 

 on a pulp made from healthy leaves grown on such plots, it does 

 not do so on a pulp made from leaves grown on plots receiving a 

 normal plant-food ration. It would therefore seem that soil com- 

 position may have much to do with the causation of plant dis- 

 eases and insect depredations. It behoves us to find these things 

 out. 



I must now conclude ; but before doing so let me say that 

 I am aware of many shortcomings in my paper, for which I 

 would ask indulgence. Further, may I hope that I have suc- 

 ceeded in accomplishing that which I set out to do, i.e., to show 

 the great importance of chemistry as the hand-maiden of agricul- 

 ture, and to enlist active sympathy in the removal of those ob- 

 stacles that are impeding the progress of its successful applica- 

 tion to the problems connected with the soils of this country. 



