366 



The Supplement to the Tropical Agriculturist 



ROTHAMSTED EXPERIMENTS. 



THE FERTILITY OF THE SOIL. 



Mr. A. D. Hall's British Association Address. 



Mr A D Hall, the president of Subsection B. — 

 Agriculture, gave in his address at the British 

 Association at Sheffield on September I, an ac- 

 count of the ebb and flow of ideas as to the 

 causes of the fertility of the soil, and described 

 most interesting experiments at Rothamsted. 



Having described earlier experiments, he said 

 it was Daubeny, Professor of Botany and 

 Rural Economy at Oxford, and the real founder 

 of a science of agriculture in this country, who 

 first pointed out the enormous difference be- 

 tween the amount of plant food in the soil and 

 that taken out by the crop. Daubeny obtained 

 by his experiments the results with which they 

 were now familiar, that any normal soil con- 

 tained the material for from 50 to 100 field crops. 

 If, then, the growth of the plant depended upon 

 the amount of this material it could get from 

 the soil, why was that growth so limited, and 

 why should it be increased by the supply of 

 manure, which only added a trifle to the vast 

 stores of plant food already in the soil ? For ex- 

 ample, a turnip crop would only tak6 away about 

 30 lb. per acre of phosphoric acid from a soil 

 which might contain about 3,000 lb. an acre ; 

 yet unless to the soil about 50 lb. of phosphoric 

 acid in the shape of manure was added, hardly 

 any turnips at all would be grown. Daubeny 

 then arrived at the idea of a distinction bet ween 

 the active and dormant plant food in the soil. 

 The chief stock of these materials, he concluded, 

 was combined in the soil in some form that kept 

 it from the plant, and only a small proportion 

 from time to time became soluble and available 

 for food. He took a further step and attempted 

 to determine the proportion of the plant food 

 which could bo regarded as active. He argued 

 that since plants only took in materials in a dis- 

 solved form, and as the great natural solvent 

 was water percolating through the soil more or 

 less charged with carbon dioxide, therefore in 

 water charged with carbon dioxide he would 

 find a solvent which would extract out of a soil 

 just that material which could be regarded as 

 active and available for the plant. 



The Production of Nitrates. 

 When the systematic study of plant-nutrition 

 began, it was demonstrated that plants could 

 only obtain their supply of the indispensable 

 * element nitrogen when it was presented in the 

 form of a nitrate, but it was not until within 

 the last thirty years that they obtained an idea 

 as to how the nitre came to be found. The 

 oxidation of ammonia and other organic com- 

 pounds of nitrogen to the state of nitrate was 

 one of the first actions in the soil which was 

 proved to be brought about by bacteria, and by 

 the work of Schloesing and Miintz, Warington 

 and Winogradsky they learnt that in all culti- 

 vated soils two groups of bacteria existed which 

 successively oxidised ammonia to nitrites and 

 nitrates, in which latter state the nitrogen was 

 available for the plant. These same investi- 



tators showed that the rate at which nitri- 

 cation took place was largely dependent upon 



operations under the control of the farmer; 

 the more thorough the cultivation the better the 

 drainage and aeration, and the higher the tem- 

 perature of the soil, the more rapidly would the 

 nitrates be produced. As it was then considered 

 that the plant could only assimilate nitrogen 

 in the form of nitrates, and as nitrogen was the 

 prime element necessary to nutrition, it was 

 then an easy step to regard the fertility of the 

 soil as determined by the rate at which it would 

 give rise to nitrates. Thus the bacteria of nitri- 

 fication became regarded as a factor, and a very 

 large factor, in fertility. This new view of the 

 importance of the living organisms contained in 

 the soil further explained the value of the sur- 

 face soil, and demolished the fallacy which led 

 people instinctively to regard the good soil as 

 lying deep and requiring to be brought to the 

 surface by the labour of the cultivator. 



The discovery of nitrification was only the first 

 step in the elucidation of many actions in the 

 soil depending upon bacteria — for example, the 

 fixation of nitrogen itself. As the world must 

 have started with all its nitrogen in the form of 

 gas, it was difficult to see how the initial stock 

 of combined nitrogen could have arisen ; for that 

 reason many earlier investigators laboured to 

 demonstrate that plants themselves were cap- 

 able of fixing and bringing into combination the 

 free gas in the atmosphere. In this demonstration 

 they failed, though they brought to light a num- 

 ber of facts which were impossible to explain 

 and only became cleared up when, in 1886, Hell- 

 reigel and Wilfarth showed that certain bac- 

 teria which existed upon the roots of leguminous 

 plants, like clover land beans, were capable of 

 drawing nitrogen from the atmosphere. Thus 

 they not only fed the plant on which they lived, 

 but they actually enriched the soil for future 

 crops by the nitrogeu they left behind in the 

 roots and stubble of the leguminous crop. Long 

 before this discovery experience had taught 

 farmers the very special value of these legumi- 

 nous crops , the Roman farmer was well aware of 

 their enriching action, which was enshrined in 

 the well-known words in the Georgics beginning, 

 "Aut ibi flavaseres, : ' where Virgil said that the 

 wheat grows best where before the bean, the 

 slender vetch, or the bitter lupin had been most 

 luxuriant. Since the discovery of the nitrogen- 

 fixing organisms associated with leguminous 

 plants other species had been found resident in 

 the soil which were capable of gathering com- 

 bined nitrogen without the assistance of any 

 host plant, provided only they were supplied 

 with carbonaceous material as a source of energy 

 whereby to effect th e co m bination of the nitrogen. 



The Influence of Bacteria. 



Though numerous attempts had been made to 

 correlate the fertility of the soil with the num- 

 bers of this or that bacterium existing therein, 

 no general success had been attained, because 

 probably they measured a factor which was only 

 on occasion the determining factor in the pro- 

 duction of the crop. Ever since the existence of 

 bacteria had been recognised attempts had been 

 made to obtain soils in a sterile condition, and 

 observations had been from time to time re- 

 corded to the efiect that soil which had been 

 heated to the temperature of boiling water, in 



