December, 1909.] 



529 



Scientific Agriculture. 



When we come to the sulphate the 

 evidence is clearer. There are definite 

 bacteria which feed on sulphur and 

 separate in both in the form of oily 

 globules of the element and a9 sulphur- 

 reted hydrogen. The effect of feeding 

 the bacteria of the soil with gypsum 

 (hydrated sulphate of lime) i-- most 

 marked. Pichard states the fact in the 

 following way : the nitrification in the 

 soil by bacteria is stimulated by: — 



Magnesium carbonate 12'3 times proportionately * 



Calcium carbonate 13 '3 ,, ,, 



Potassium sulphate 35 '8 ,, ,, 

 Sodium sulphate 47 - 9 ,, ,, 



Gypsum 100 „ ,, 



In the case of the carbonates the action 

 is probably simply due to the neutralisa- 

 tion of acids which act deleteriously on 

 bacteria; but actions of sulphates is 

 certainly direct and is due to their 

 forming food-stuff for the organisms. 



Though no direct evidence is as yet 

 available as to the action of sulphur 

 bacteria in the soil, there are the 

 researches of Zelinsky and Brussilovsky 

 on the bacteria in the Black Sea, which 

 leave very little doubt that the reaction 

 on land is a similar one to that in the sea. 

 The surface waters of the Black Sea con- 

 tain free oxygen and support an abund- 

 ance of organic life ; but the deeper and 

 denser waters are charged with sulphur- 

 etted hydrogen, and the only organisms 

 present are the bacteria. The amount of 

 sulpheretted hydrogen increases with 

 depth. At 100 fathoms there are 33 

 cubic centimetres in 100 litres ; at 200 

 fathoms, 222 c.c, and at 1185 fathoms, 655 

 c.c. Several species of bacteria have been 

 observed, but only one, the Bacterium 

 hydrosulphuricum ponticum, has been 

 studied in detail. This bacterium 

 possesses the power of liberating sulphur- 

 etted hydrogen, not only from organic 

 matter containing sulphur, but also 

 directly from sulphates and sulphites. 

 All authors are agreed that the sulphates 

 of the sea- water are acted upon, but there 

 is some divergence of views as to whether 

 the changes are due solely to bacteria, or 

 whether they are in part purely chemical. 

 Changes of an opposite kind take place 

 in the zone where water containing sul- 

 phuretted hydrogen comes into contact 

 with that containing oxygen. This zone 

 occurs at a depth of about 200 fathoms. 

 According to Yegunov and Vinogradski, 

 there is at this depth a race of sulphur 

 bacteria which derive the energy neces- 

 sary for their existence from the sulphur 

 of the sulphuretted hydrogen. This 

 sulphuretted hydrogen is separated in 

 their cells in the form of soft, oily 



* E. W. Hilgard, Soils in Humid and Arid 

 Regions, New York, 1906, p. 147, 



67 



globules, and the oxidation of this 

 sulphur gives them the necessary 

 vital energy in precisely the same 

 manner as the oxidation of carbon in 

 other organisms supplies it. I must 

 express my indebtedness to the presi- 

 dential address to the Geological Society 

 by Dr. Teall for the above facts, es- 

 pecially as it has led to the train of 

 reasoning adopted in the present article.* 

 The organic substance of plants and 

 animals, the protoplasm, consists essen- 

 tially of carbon, hydrogen, oxygen, 

 nitrogen, sulphur and phosphorus. We 

 have dealt with the separation of all 

 these by micro-organisms except the 

 last. We know of no phosphorus-eating 

 bacteria, but phosphorus exists plenti- 

 fully in the soil, being derived from the 

 mineral apatite, a calcium phosphate, 

 which is an accessory mineral in all 

 igneous rocks, The natural phosphates 

 of lime and magnesia are readily soluble 

 in all weak acids and are available 

 directly to the plant roots ; but if there 

 is any hydrated oxide of iron present 

 in the soil, there is formed a phosphate 

 ot iron which is totally insoluble. 

 Basic slag, for instance, which contains 

 large quantities of phosphorus and iron, 

 lies unaltered on fields with damp soil 

 showing au acid reaction. As pre- 

 viously stated, Kunze has proved that 

 in many of the higher plants the roots 

 do not secrete organic acids, the only 

 secretion being water containing carbon 

 dioxide. Prianischnikoff found that on 

 growing peas, lupines, mustard and 

 buckwheat, in sand containing alumi- 

 num phosphate, which, like phosphate 

 of iron, is insoluble in carbonic acid, 

 there was a vigorous absorption of the 

 phosphates by the plants, and the con- 

 clusion he arrived at was that the 

 breaking down of the insoluble sub- 

 stances had been accomplished by 

 bacteria, t 



In South Africa the central parts of 

 the country are dry; along the river 

 courses there are magnificent stretches 

 of alluvium, which, however, can only 

 be occasionally watered by floods or 

 artificial irrigation. The soil is usually 

 rich in phosphate, but on taking samples 

 of the soil after a spell of drought 

 they are found to be in the insoluble 

 form. When, however, the fields are 

 watered and brought under cultivation, 

 then on analysis the soil shows a high 

 percentage of soluble phosphates. There 

 may be other means of explaining this 

 phenomenon, but the one which sug- 



* J. J. H. Teall, Quart. Journ. Gcol. Soc, Vol 

 lviii. p. lxvii, 



t D. Prianischnikoff, Berichi. deutsch, hot, 

 Ges. Vol. xxii. 1904, p. 134. 



