194 



THE AGKICULTURAL NEWS. 



June 25, 1910. 



soil fertility is the content of nitrogen. This fact is well 

 brought out in a recent paper* by A. D. Hail, M.A., 

 F.R.S., Director of the Rothamsted Experiment Station, 

 which, although it deals with conditions in a country 

 situated in temperate latitudes, is worthy of special 

 attention. Kere, it is pointed out that the changes in 

 the amount of nitrogen in the soil are brought about 

 as follows : by the removal of crops; by the action of 

 certain bacteria, which break down organic matter and 

 liberate nitrogen in the free state: by the removal of 

 nitrogen in drainage waters, as nitrates which have 

 been formed by bacterial action: by the addition of 

 nitrogen from the air, through the agency of bacteria 

 which may live free in the soil, or in symbiosis with 

 plants, these plants being generally leguminous, as 

 far as is known; and by the addition of nitrogen 

 brought down by rain. It is seen that the first 

 three of these influences tend to decrease the amount 

 of nitrogen in the soil, while the others conduce to 

 effect its increase. In considering the latter, the 

 action of rain is negligible; figures given in the article 

 show that the average amount of nitrogen added by 

 rain, at Rothamsted, is only 3'84> lb. per acre, per annum. 

 One of the chief conclusions reached, in this paper, is 

 that where no special provision is made for bringing 

 manure on to the farm, or for increasing the nitrogen 

 content of the soil by other means, the fertility of the 

 soil rapidly decreases until a state of equilibrium is 

 reached, in which the losses of nitrogen are so far 

 balanced by the gains from bacterial activity that the 

 soil attains a level of productivity which, though lower 

 than it was originally, remains practically constant. 



Under natural conditions, the amount of phos- 

 phates in the soil is increased by the weathering of 

 mineral particles and of the underlying rocks. It is 

 decreased by the removal of plants and, to a certain 

 extent, in drainage water. In cultivated soils, this 

 decrease is likely to be larger, because of the special 

 removal of crops, as harvest. There is, however, another 

 cause of the loss of phosphates, to which perhaps, suffi- 

 cient attention has not been given in the past. This is 

 the application of large quantities of manure: though 

 the exact manner of this is not yet understood. There 

 is, therefore, a necessity, when it is designed to subject 

 land to heavy dressings of manure, to consider the 

 possible effect on the phosphate content of the soil. 



Potash is added to, and taken away from, the soil 

 in much the same ways as the phosphates; it is, how- 

 ever, likely to be lost to a greater degree in the drain- 

 age water. Experiments have shown that, when the 



♦ Tlie Jiiurniil nf the Bond 'if Jgiicidture, Vol. XVIl, 

 p. 114. 



extent to which potash salts are to be added, as manure 

 is being considered, allowance must be made for the 

 irifluence of phosphates on the potash compounds 

 already existing in the soil. Soluble phosphates have 

 been found to make these compounds soluble, and there- 

 fore more available for the use of plants. This effect 

 has been proved to be increased by small applications 

 of lime, on account of the liberation of potash brought 

 about by it: lime in large amounts, however, has the 

 opposite effect, in that it renders the phosphates 

 insoluble, and therefore able no longer to act on the 

 potash compounds. 



The amount of lime in the soil is also subject to 

 fluctuations, though these are not likely to be as great 

 as those of the constituents that have been dealt with 

 already. The importance of this substance may be 

 summed up in its effect in preserving a good state of 

 tilth, its use in preventing acidity, and the changes 

 that it produces, in conjunction with other manures. 

 The loss of this constituent from the soil is increased 

 by the use of sulphate of ammonia; it is decreased when 

 pen manure or nitrate of soda is employed. + 



There is now left the examination of the conditions 

 foi the best existence of those living inhabitants of the 

 soil which produce beneficial changes in it. Acidity 

 and alkalinity have a great influence in determining 

 what, among these, shall continue to exist and show 

 the greatest effect. In a soil which is alkaline to the 

 ordinary extent, the changes are such as lead to increased 

 fertility; in aciil soils, as is well known, changes take 

 place which result in a decrease of productiveness, and 

 the medium becomes unfitted for the support of plants. 

 Among the chief causes which tend to increase this 

 acidity are insufficient drainage, together with the 

 presence of large amounts of organic matter, and the 

 too great employment of ammonium sulphate as 

 a manure. + Acidity is a condition that is most likely 

 to arise in heavy soils containing little lime. 



It is a comparatively rare circumstance, however 

 that cultivated soils are allowed to become so acid as 

 to prevent them from bearing adequate crops. The 

 conditions for the reduction in numbers of the favourable 

 organisms are far oftener allowed to come into existence 

 on account of insufficient tillage, or even because of 

 the careless application of manures. The extent to 

 which such organisms are present, in comparison with 

 that of the harmful ones, is dependent upon the state 

 of tilth, or texture, of the soil, so that a sufficient 

 argument is provided for the exercise of thorough 



t See Journal <if the Royal HorticaUnrol Society of 

 EnyhiwU Vol. LXX, p. 12, and Agrknltnml N(\n, Vol. IX, 

 p. 187. 



