^5^ 



NATURE 



[October 8, 1914 



and contain lo to 20 per cent, of organic matter and 

 3 to 5 parts per thousand of nitrogen. These soils are 

 all calcareous, they occur in regions of a moderate 

 rainfall inducing grass-steppe or bush conditions, and 

 the annual fall of vegetation provides the organic 

 matter which the Azotobacter requires as a source of 

 energy in order to fix nitrogen. Non-calcareous soils 

 under similar climatic conditions do not accumulate 

 nitrogen and become rich ; in the absence of carbonate 

 of lime the nitrogen-fixing organisms are not active, 

 and the soil only receives from the annual fall of 

 vegetation the nitrogen that was originally taken from 

 it. There is but a cyclic movement of nitrogen from 

 the soil to the plant and back again, whereas in the 

 calcareous soils there is also continuous addition of 

 fresh nitrogen derived from the atmosphere, in which 

 process the carbonaceous part of the annual crop 

 supplies the motive power. 



The other leading case to be found at Rothamsted is 

 that of certain grass-plots which have artificially been 

 brought into an acid condition by the continued applica- 

 tion of sulphate of ammonia. In these soils nitrifica- 

 tion is suspended, the nitrification organisms have 

 even disappeared, though the herbage still obtains 

 nitrogen because most plants are able to utilise am- 

 moniacal nitrogen as well as nitrates. The interesting 

 feature, however, is that the decaying grass on these 

 acid soils passes into the form of peat, a layer of which 

 is forming upon the surface of the soil, though nothing 

 of the kind is found on adjacent plots where the use 

 of lime or of alkaline manures has prevented the 

 development of acidity. From this we may learn that 

 the development of a surface layer of peat, independent 

 of waterlogging (when another kind of peat forms 

 even under alkaline conditions), is determined by 

 the acidity of the soil, when certain of the bacterial 

 processes of decay are replaced by changes due to 

 micro-fungi which do not carry the breaking-down of 

 organic matter to the destructive stage. This affords 

 us a clue to the origin of many areas of upland peat 

 in the British Isles, where the remains of ancient forest 

 roots and stumps of trees are found on the true soil 

 surface below the layer of peat, but where there is no 

 water-logging to bring about the death of the trees 

 and the formation of peat. We may suppose that 

 when the land-surface became fit for vegetation at the 

 close of the glacial epoch it covered itself with a 

 normal vegetation, chiefly dwarf forest, because of 

 the rainfall and temperature. The soil, however, 

 being without carbonate of lime, would in time become 

 acid with the products of decay of the vegetable matter 

 falling to the ground, and as soon as this acid con- 

 dition was set up peat would begin to form from the 

 grassv surface vegetation. The process would con- 

 tinue until the acid conditions and the depth of the 

 accumulating layer of peat would kill the trees, the 

 stumps of which would remain sealed up below the 

 peat. I am far from thinking that this explanation is 

 complete, but at least we have facts in sight which 

 could lead one to suppose that a non-calcareous soil 

 originally neutral and carrying a normal vegetation 

 can naturally become acid, alter the character of its 

 vegetation, and clothe itself with a layer of peat. The 

 point of economic importance is that these peaty acid 

 soils are of very little value as long as they are acid, 

 though they take on a quite different aspect if they are 

 limed and made neutral. 



Of all the soil factors making for fertility I should 

 put lime the first; upon its presence depend both the 

 processes which produce available plant food in quanti- 

 ties adequate for crop-production at a high level and 

 those which naturally regenerate and maintain the 

 resources of the soil ; it is, moreover, the factor which 

 is most easily under the control of the agriculturist. 



NO. 2345, VOL. 94] 



I need say little about those cases in which infer- 

 tility is due to the presence in the soil of some sub- 

 stance which is actually injurious to plant-growth, 

 because such substances are nearly always due to the 

 physical environment of the soil, to too much or too 

 little water. In waterlogged situations we may find 

 in the soil peaty acids, iron salts, sulphides, etc., in- 

 hibiting the growth of plants ; in arid regions the soil 

 may still be charged with an excess of soluble com- 

 pounds of the alkalis and alkaline earths, resulting 

 from the decomposition of the rocks that have been 

 broken down to form the soil, but which through the 

 inadequate rainfall have never been washed out. The 

 establishment of normal conditions of growth, irriga- 

 tion in one case, drainage in the other, will speedily 

 result in the removal of the deleterious substances. 

 Practically, only bodies that are soluble can get into ,B 

 a plant to injure it, hence such bodies can be removed -■ 

 from the soil by water, provided that the wster can 

 find its way through the soil and escape. 



Let us now consider the various methods by which 

 land suffering from one or other of the disabilities we 

 have just discussed is nowadays being brought into 

 cultivatipn. The most important, if we consider the 

 area affected, is the extension of cropping into regions 

 of a deficient rainfall by means of what has been 

 termed dry-farming. So far as its immediate methods 

 go, dry-farming consists in nothing more than the 

 application of the principles of husbandry worked out 

 by English farmers in the east and south-east of 

 England, principles first expounded by Jethro Tull, 

 though a complete explanation was not then possible, 

 even if it is now. In the first place, the tilth must 

 be made both deep and fine, thus whatever rain falls 

 will be absorbed and the conditions favouring a deep 

 and full root range will have been established. Next, 

 the soil below the surface, though finely worked, must 

 be compact, because only thus can the water present 

 travel to the roots of the plant. Lastly, a loose layer 

 must be maintained on the surface, which, though dry 

 itself, acts as a screen and a barrier to prevent loss 

 of water from the effective soil below by any other 

 channel than that of the plant. Granted these methods 

 of cultivation, the new feature about "dry-farming," 

 which has been introduced by settlers in the arid 

 districts of Australia and North America, is the use 

 of a year of bare fallow in which to accumulate a 

 supply of water for the next year's or two years' 

 crop. This raises the fundamental question of how 

 much water Is necessary for the growth of an ordin- 

 ary crop. The first Investigation that Lawes and 

 Gilbert carried out at Rothamsted dealt with this 

 very point; they grew the usual field crops in pots, 

 protected the surface of the soil from evaporation so 

 that all the loss of water proceeded through the plant, 

 weighed the water that was supplied from time to 

 time, and finally weighed the produce, expressing 

 their results as a ratio between the dry matter pro- 

 duced and the water transpired by the plant. These 

 experiments have been repeated under different 

 climatic conditions by Hellriegel in Heidelberg, by 

 WoUny in Vienna, by King and others in America. 

 Now the two processes in the plant, carbon assimila- 

 tion and transpiration, are not causally connected, 

 though, as both are carried out In the leaf and 

 have some factors in common, they are found to 

 show some constancy In their relative magnitudes. 

 Lawes and Gilbert obtained a ratio of about 300 lb. 

 of water transpired for each pound of dry matter 

 harvested, but the other Investigators under more arid 

 conditions found much higher figures, up to 500, and 

 even 700 to i. Now, a crop yielding 20 bushels of 

 wheat per acre will contain alaout a ton of dry matter 

 per acre, so that, taking the high ratio of 500 to i. 



