254 JouJiiNAj- OF Till- J)i;r AHTMKM ov Agkicultuke. — 8kpt., 1922. 



A.PPARENT INFERTILITY OF THE SOIL 

 AROUND TREES. 



By C. (). Williams. li.Sc, A.R.C.S., Clieiuist, Scliool ot A<Jii«ulturp. 



Cedara . 



liT is a conuuc)]! practice iu Natal and other parts ot the Uiiioii to 

 plant belts and clumps of trees in order to provide shade and shelter 

 for the farm animals, and iiicideiitally as a direct source of revenue 

 as well. It is very noticeable that little in the way of crops can be 

 grown contig-uous to these trees. In fact, a zone may be observed 07i 

 each side of a shelter-belt of wattle trees in which absolutely nothing 

 grows within the first few feet from the trees, and tlie crop improves 

 progressively as we recede from the shelter-belt, until at tlie maximum 

 distance of roughly tweiity yards, on the average, we find that the croj) 

 reaches the ordiuary standard of growth observed in that field. 



Supply of Light and Air. 



Several theories have been advanced to explain this apparent 

 infertility of the soil near trees, and as the effect varies with the 

 distance from the trees, up to a particular point, it is obvious that 

 the deleterious effect is due in some Avay to the presence of the trees. 

 Some put it down to the shade of the trees cutting off the supply of 

 light and air, but this cannot be the true reason, or at least not the 

 chief one, for the barren strips are observed on all sides of a shelter- 

 belt or cluni]) of trees, and extend to a greater distance from the trees 

 than where the shadow usually falls. Also, there is generally 

 sufficient light, even in the. shade of the trees, for assimila- 

 tion in tlie leases to go on vigorously; in fact, it has been shown by 

 experiment that ordinary daylight can be reduced to one-twelfth its 

 intensity without any ill-effect. Pickering, the late Director of the 

 Woburn Experimental Fruit Farm, by erecting canvas screens to 

 simulate, and even exaggerate, the shading effect of trees, proved that 

 a crop under them gave exactly the same values as on the unshaded 

 ground. As for the question of supply of air, the shelter-belts as a 

 rule are not so thick as to prevent free circulation of air, and the fact 

 that the barren strips occur on the side facing the permanent winds 

 to the same extent as on the sheltered side contradicts this theory. 



Two Other Thkokies. 



Another cause of the barrenness advanced by some people is the 

 probability that the trees have deprived the soil iu their vicinity to 

 a very large extent of the available plant-food. A third supposed 

 cause is based on the assumption that the soil solution near the trees 

 contains an excess of mineral salts, owing to the ground having been 

 unduly deprived of its moisture, thus bringing about too great a 

 concentration of saline matter in the soil-water in that neighbour- 

 hood for the good of the crops. These last two assumed causes are, 

 however, rather contradictory, for if the trees have drawn exhaustively 



