46 



[January, 1910. 



SCIENTIFIC AGRICULTURE. 



SOIL FERTILITY AND SOIL 

 EXHAUSTION. 



(From the Gardeners' Chronicle, Vol. 

 XLV., 1, 171, Jane, 1909.) 



The theories as to the cause of soil 

 fertility are in general vogue. Accord- 

 ing to one, which may be called the 

 chemical theory, a soil is fertile which 

 possesses the chemical compounds such 

 as nitrates, phosphates, and salts of 

 potash, necessary for the growth of 

 plants, in sufficient quantity and proper 

 condition of solubility. The second or 

 physical theory, holds that the essential 

 factor in soil fertility is its relation to 

 water. Roots of plants require air as 

 well as water. The root has to supply 

 the leaves with large quantities of 

 water. Only when the physical condi- 

 tion of the soil admits of the root of a 

 plant obtaining adequate supplies of 

 water and also of air, can the plant 

 grow properly. On the physical theory 

 alone such soils are fertile in which these 

 conditions obtain. 



These two theories are not necessarily 

 mutually exclusive. We may combine 

 them into a chemico-physical theory, 

 and attribute fertility, in part, to the 

 presence in the soil of the essential 

 mineral substances, and, in part, to the 

 proper relations of soil to water. 



The extreme adherents of the physical 

 theory go further than this, and are apt 

 to maintain that a soil does not beeome 

 exhausted by plants in consequence of 

 the removal by the latter of the avail- 

 able chemical food materials. They 

 urge that as such materials held in 

 solution in the soil- water are taken up 

 by the roots of plants, corresponding 

 quantities of similar substances pass into 

 solution and thus replace in the water of 

 the soil those absorbed by the plant. 



On this view it is not easy to under- 

 stand how the addition of definite 

 chemical fertilisers produce their well- 

 marked effects on soil fertility. If a 

 soil slacks phosphates, for example, it is 

 easy to understand the beneficial result 

 following on the addition of phosphatic 

 fertilisers. But if a soil does not lack 

 phosphates, how can the addition of 

 these substances produce, as in certain 

 soils, and on some crops it indubitably 

 does produce, an improvement in ferti- 

 lity ? The workers in the Bureau of 

 Soils of the United States Department 

 of Agriculture propose to explain such 

 facts as these on a new hypothesis of 

 soil fertility and soil exhaustion. Accor- 

 ding to this hypothesis, soil fertility is 



not reduced because of the removal by 

 the crop of mineral food materials, but 

 because the crop forms a definite, 

 chemical poison, which is liberated in 

 the soil and acts adversely on the follow- 

 ing crop. The role of artificial, chemical 

 manures is to neutralise the poisonous 

 effects of the toxic root excretions. The 

 supporters of the toxic theory, as it may 

 be called, have brought forward a con- 

 siderable body of evidence in support of 

 the suggestion that plants excrete 

 definite poisonous substances, and they 

 urge that the common practice of rota- 

 tion of crops lends support to their view. 

 It is too early yet to pronounce defi- 

 nitely either for or against the toxic 

 theory, though that it will replace 

 altogether the chemical theory would 

 seem improbable. It is not unlikely 

 that the toxic substances produced in 

 the soil represent not the excretion of 

 plants but the by-products of the acti- 

 vity of certain races of soil bacteria. A 

 vast and almost un tilled field of investi- 

 gation is presented by the bacteria of 

 the soil, and it is probable that soil 

 fertility will be found to depend on 

 chemical, physical and biological factors, 

 not solely on one of these, and to be the 

 consequence of complex, diverse condi- 

 tions rather than of one condition only. 



THE EXPERIMENTAL ERROR IN 

 FIELD TRIALS. 



By A, D. Hall, m,a., p.r.s. 



(From the Journal of the Board of 'Agri- 

 culture, Vol. XVI., No. 5, August, 1909.) 



In all experimental work some error is 

 inevitable ; it is only on paper that 

 results come out exactly, but when deal- 

 ing with things, even the simplest 

 measurement involve an error, the 

 magnitude of which depends on the 

 methods employed. A carpenter measur- 

 ing a table with a foot-rule can with care 

 be exact to within an eighth of an inch ; 

 the maker of fine machinery will only 

 allow himself a margin of about a 

 thousandth of an inch ; while it is pos- 

 sible with the utmost refinement to 

 make sure of the length of a small piece 

 of polished metal to within about a 

 millionth of an inch. Granting, then, 

 that it is impossible to eliminate error 

 and that absolute correctness is unattain- 

 able, the scientific method is to ascertain 

 how large the error is likely to be and 

 decide whether it is such as will vitiate 

 the conclusions drawn from the experi- 

 ment. 



