November, 1909.] 



434 



Scientific Agriculture. 



to be a biological process, the soil is rich 

 in certain moulds and other microfungi 

 which rapidly attack solutions of ammo- 

 nium salts, and by withdrawing am- 

 monia from their own nutrition, set free 

 the acid. It was shown that the degree 

 of acidity thus produced was approxi- 

 mately equal to the soluble acidity of 

 the Rothamsted plots soon after the 

 ammonium salts had been applied as 

 manure. At the same time, in the soils 

 there was also a very much larger 

 quantity of comparatively insoluble 

 humic acid, which had accumulated year 

 by year as a result of the attack of the 

 mineral acids split off from the ammo- 

 nium salts upon the neutral humus of 

 the soil. 



The first consequence of the acidity 

 of the soil on these plots has been the 

 cessation of the nitrification process, 

 because the bacteria bringing about that 

 change will only work in a neutral 

 medium. Some of the falling off in the 

 yield of these acid plots is thus due to the 

 fact that the grass is driven to obtain its 

 necessary nitrogen from ammonia instead 

 of from the more usual nitrates : at the 

 same time, the mass of micro-fungi with 

 which the soil is permeated competes 

 successfully with the grass for the 

 manure. Whether these fungi also 

 excrete substances more or less poisonous 

 to the grass has not yet been definitely 

 settled. The remedy for this acid con- 

 dition of the soil lies in the use of lime, 

 which, applied at the rate of 2,000 lb. per 

 acre to portions of the Rothamsted 

 grass plots, has effected a great im- 

 provement both in the yield and the 

 character of the herbage. 



Another problem of the same order — 

 the secondary effects of certain ferti- 

 lisers upon the soil — is afforded by the 

 well-known fact that the use of large 

 quantities of nitrate of soda upon heavy 

 soils always makes them very wet and 

 sticky after rain and causes them to dry 

 with a hard, intractable crust. This has 

 been attributed to the attiaction of 

 nitrate of soda for moisture, but the 

 amount of water absorbed by the few 

 hundredweight per acre of nitrate of 

 soda which are ever applied is insigni- 

 ficant and could not cause the effects ob- 

 served. Some of the Rothamsted plots, 

 which have been receiving nitrate of soda 

 every year for the last half century, 

 show these effects to a marked degree, 

 and on examination the clay on these 

 plots was found to be in its most " de- 

 flocculated " condition. Clay consists 

 essentially of excessively fine particles, 

 and when a clay soil is in good tilth 

 these particles are largely bound to- 

 gether in loose aggregates, thus giving 

 the soil as a whole a coarser texture. 



Any working of the soil when wet, or 

 the " puddling " which a potter or brick- 

 maker gives to his clay, breaks down 

 these loose aggregates, and, by giving 

 the clay its most finely-grained condition 

 markedly increases its typical properties 

 of impermeability to water, and shrink- 

 age of drying. It is also found that a 

 trace of any soluble alkali, such as car- 

 bonate of soda, will loosen these aggre- 

 gates and deflocculate the clay. 



By further experiments it has been 

 shown that a growing plant fed with 

 nitrate of soda gives rise to a little 

 carbonate of soda, because it takes up 

 more of the nitric acid than the soda 

 base with which it was combined, leav- 

 ing the latter in the soil combined with 

 the carbonic acid excreted from the 

 roots. It was found possible to extract 

 free carbonate of soda from the plots 

 which had long received nitrate of soda 

 as a manure ; one of the grass plots 

 yielded as much as 175 lbs. per acre 

 dcwn to a depth of 3 feet. This alkali 

 then, by defloeculating the clay, is the 

 source of the bad tilth resulting from 

 the use of nitrate of soda. 



The bad tilth, which is a serious 

 trouble to many market gardeners who 

 manure heavily with nitrate of soda, 

 cannot be rectified by use of lime, 

 which, beine: itself an alkali, only ex- 

 aggerates the trouble. The use of acid 

 manures like superphosphates, and 

 liberal application of soot, will improve 

 matters, but the best plan is to use, 

 instead of nitrate of soda alone as a 

 nitrogenous fertiliser, a mixture of it 

 with sulphate of ammonia. Since the 

 one tends to set free acid and other 

 alkali in the soil, jointly they would 

 leave it unchanged, and they would 

 also come into action successively as 

 sources of nitrogen. 



These and other cases of the same 

 character go to show that we must 

 study more closely the chemical and 

 biological actions of fertilisers upon our 

 soils if we are to obtain full value from 

 them, and avoid some of the disadvan- 

 tages long recognised by farmers as 

 attending their use. 



