164 



isms, Mild tho (|U;intity of nitroccMi (■onsunicd l).v the phiDt will ho nieasnred by 

 tlu' .•miouiit of ;iviiil;il)li' ])lios])lionis. |iot;issiuni, cmIcIuiu. and other mineral 

 elements which the soil may furnish. (Jiven a soil deficient in one or more of 

 these elements, hut havin.ii a jihysical condition favorahle to husbandry, and 

 subject it to a system of croppin.u and tilla.u:e calculated to stinuilate the growth 

 of the soil organisms, and we may expect to find the jiroduction of nitric acid 

 exceeding its consiunption. If the soil be well supplied with lime or some other 

 salifiable base, the excess of nitric acid may be stored as nitrates, which will be 

 held in the soil until used by the plant or washed out by the rain; but in the 

 absence of such a base, and with a rainfall not sutlicient to i)ass through the soil 

 into the drainage, there may be a sulticient accunndation of nitric acid in the 

 soil, together with the organic acids formed as a secondary result of nitrifica- 

 tion, to produce a temi»orary acid reaction. This accunuilation of acid will have 

 a toxic action upon the nitrifying organisms which would finally result in lim- 

 iting nitrification. Let. now. an alkaline base be supplied. The excess of acid 

 will be neutralized, and nitrification will again commence, but unless we furnish 

 not only the base necessary to neutralize acidit.v. but also such mineral elements 

 as may be necessary to restore the suppl.v of plant food in the soil to equilibrium 

 for the ])articular crops grown, there will still be a limit to production below 

 that of the normal caiiacit.v of those crops. 



Therefore, when we add acid phosphate, potassium chlorid, and sodium 

 nitrate to a lime-hungry soil, we may have furnished all the phosphorus, 

 potassium, and lime which the wheat crop reipiires. while still leaving the 

 lime supply insutticient to feed the clover ; and if we omit the sodium the 

 base may be so far insufficient that the toxic effect of the excess of acid will 

 still be manifested in the clover crop, the wheat having consumed all the 

 lime furnished in the acid phosi)hate. Certainly, under such conditions, the 

 acids carried in the fertilizers would intensify the toxic effect, unless they 

 were so closely adsorl)ed by the film water of the soil grains as to be com- 

 pletely inaccessible to the plant roots and to bacterial action. It is easy to 

 conceive that such close association nught take place as to jn-oduce the results 

 which Doctoi' Whitney has noted in his litnnis-i)aper tests, but m.v concejition 

 of ])lant root and bacterial action is that it is more vigorous than mere water 

 solution. 



Dr. ('. (J. Hopkins has suggested that future investigation may reveal whether 

 the large accunuilation of acidity, especially in some difiicultly pervious sub- 

 soils, is in the form of organic acids or acid salts, or in the form of inorganic 

 acid salts, such as might result from the reaction of nitric and organic acids 

 upon neutral silicates forming acid silicates; also, whether this a<adity is 

 retained in soils because of its ver.v low solubility, or because of its absorption 

 in the film water surrounding the soil grains. 



An interesting jxiint in this matter is that this season, when the wheat was 

 taken off the fourth section of land which has come under the exi)eriment with 

 lime, there was no such difference between the limed and unlinied ends of 

 the plats as we had noticed on the three preceding sections. The si»ring and 

 early summer had been marked by excessive rainfall, a condition which con- 

 tinued until August. After the rains began to diminish, however, the unlimed 

 clover practically ceas«'d to grow, while that which had been limed continued, 

 so that now the contrast is nearl.v as striking as it has l»een at any previous 

 period. 



Another effect of Ihe excessive rainfall has been the production of the 

 greatest crop of clover, on the second .vear"s growth, which has been har- 

 vested in this I'egioii for maii.v years. The lii)eral water suppl.v has apparently 

 accomi)lished one or both of two effects. It has removed excessive acidit.v or 

 it has brought into solution a larger supi)ly of mineral plant food in the soil. 



In conclusion : 



(1) The Woburn and Ohio exiierimeiits luive shown that the geological his- 

 tory of a soil is a matter that can not be neglected if we would intelligently 

 ])rescribe for its treatment. 



(2) I'.otli exiteriments show that chemical i>roblems must occujiy a position 

 of first inijiortance in any effective study of the soil. 



('A) The Woinirn ex]»eriments have shown that the physical effect of fer- 

 tilizers on the soil is a factor to be leckoned with. 



(4) Both experiments support the belief that no investigation of the soil 

 can be coinidete which fails to recognize the vast importance of bacterial action. 



Finally, my object in ]»resenting this jiajier has been to suggest some of the 

 problems pertaining to the maintenance of soil fertility which are urgently 



