66 



THE AGRICULTURAL IfEWS. 



Februaey 26, 1916. 



-«)ut another indispensable substance. This is very well 

 illustrated by the injurious effect of too heavy a dose 

 of a particular artificial manure. In short, all the 

 requirements of the plant must be satisfied. Anyone 

 left unsatisfied constitutes a limiting factor, preventing 

 -further growth. Increases in any one i'actor give 

 increases in growth nntil something else proves 

 insufficient and becomes a limiting factor. 



It is quite possible that futnre investigators may 



•establish other principles besides these. Indeed it has 

 already been suggested that there are certain directive 



agents in the soil in the nature of hormones found 

 in animals, which may regulate plant nutrition To 

 these the name of auximones has been given. Little 



-can be said definitely about them at present until the 



■subject has been more fully investigated. 



A very important aspect of plant nutrition is its 

 ■physical side. The study of the higher physics of the 

 •soil is only in its infancy, but some important principles 

 have become established already. For example, there 

 is the (juestion of adsorption relationships. The 

 discovery — and it may be noted that this again is an 

 outside one — that charcoal can take up and keep in 

 spite of the most thorough washing small (quantities 

 of dilute acids by virtue of the so-called surface tension, 

 led to a consideration of this phenomenon in relation 

 to the soil. It has been found that a similar relation- 

 ship exists between phosphates and soil particles. Clay 

 soils adsorb more phosphates than sandy soils, which 

 explains what has long been observed, namely that clay 

 .soils in spite of a high phosphate content are often 

 unable to meet the re(juirement of plants in this 

 respect. In other words, there is a competition between 

 the plant and the soil for phosphate. 



Another principle of soil physics, again the result 

 4)f an extraneous observation, concerns what is known 

 as 'soil climate'. Whitney at the time of his con- 

 nexion with the United States Weather Bureau 

 •observed that both the ipiality and (juantity of 

 tobacco crops were connected closely with climatic 

 conditions, and further with certain soils irrespec- 

 tive of manuring. This led him to bring the 

 production of (luality into line with 'soil climate', by 

 which is meant soil temperature, moisture content, air 

 supply, etc., and Whitney was able to show that these 

 factors can be correlated with the size of the soil 

 particles. In other words, th(> mechanical analysis of 

 n tobacco soil is of far greater importance than the 

 chemical. High grade tobacco can only be grown on 

 .soils having a low proportion of clay particles. 



The foregoing principles are general in their 

 application, that is to say, we have no reason to believe 

 that that they do not hold good for tropical soils. lb 

 is only in regard to tlie extent to which the different 

 factors are present that variation exists. The chief 

 limiting factor in the tropics is dryness, whereas in 

 temperate regions it is generally lack of warmth. 



In concluding this review of Dr. Russell's lecture. 

 it should be pointed out that the speaker limited 

 himself to a consideration of the principles of pi-oduc- 

 tion solely in relation to the .soil. In the tropics we 

 have attacked the subject of crop production through 

 the plant. The tendency has been to improve pro- 

 dtiction by means of varietal selection, to make the 

 plant fit the soil rather than the soil tit the plant: and 

 very considerable success has attended activities in 

 this dii-ection. Particularly has this been so with sugar- 

 cane and with cotton in the West Indies. The study 

 of tropical soils has been somewhat neglected chiefly on 

 account of the moi'e elaborate equipment required for 

 such work, and also becatise plant selection leads 

 quicker to results of practical value. Again we have 

 come to realize as the result of experience that climatic 

 conditions — and chiefly the climate of the air rather 

 than W^hitney's climate of the soil — are of paramount 

 importance, determining as they do the distribution 

 and productivity of crops. In this connexion humidity 

 of the air is of . first importance, and is one of 

 the first principles of crop production in the tropics. 

 At the same time tlie principles of soil chemistry 

 hold good in their particular realm, and especially 

 fche biological changes referred to in the second 

 of Dr. Russell's three principles. In considering 

 crop production, however, we must always renieniber 

 that there are two main aspects: the root and its 

 environment, the soil; the foliage and its environment, 

 the air; and further, that there is a close relationship 

 between the air and the soil just as there is between 

 the foliage of a pl.uit and its roots. 



In rejjard to the recent examinations in practical 

 agriculture conducted by the Imperial Department, informa- 

 tion has been received to the etfect that the certificate g-ainod 

 by Mr. Stanley E. Iladley, of (Irenada, was presented to him 

 by the (Jovernor at a meeting of the Legislative Council held 

 on February 4. We may also .state that at a meeting of the 

 Antigua .\gricultural and Commercial Society held on 

 February 1, a similar certificate was presenteti to Mr. Arnold 

 Oalhvy, by His l'',.vcellency the Acting Oovernor of the 

 Leeward Islands. Dr. Watts, C.M.G., the Imperial Com- 

 missioner of Agriculture was present on this latter occ;usion. 



