Scientific Agriculture. 



230 



[September, 1910. 



CHANGES IN THE SOIL AND ITS 

 FERTILITY, 



(Prom the Agricultural Neivs, Vol. IX., 

 No. 213, June, 1910.) 



The attention of agriculturists is being 

 drawn continually to the fact that the 

 soil is in a state of constant change. 

 Formerly, the ideas concerning this 

 change were restricted to the more 

 obvious sources of loss or gain in avail- 

 able plant food, and these were attri- 

 buted to purely physical or chemical 

 causes, brought about by natural means 

 alone. This is no longer the case. Full 

 recognition is being attained of the 

 great importance and extent of the 

 action of living organisms in the soil, 

 and there is no longer the general opi- 

 nion that the operation of manuring the 

 soil results simply in the addition of 

 plant food in a more or less available 

 condition. The consequence of these 

 matters seems to justify their review in 

 the form of a summary. 



The conditions which surround any 

 given portion of soil will, first of all, 

 determine if it is to continue to exist as 

 such. Where the "wash" from rain- 

 storms, especially on hillsides, will 

 probably be great, the loss is lessened 

 by the construction of contour drains. 

 Plants possessing strong, binding roots 

 are also used for the same purpose, and 

 have been found especially useful where 

 the soil is likely to be blown away by 

 wind- The conservation of the soil by 

 means of plants has, however, a far 

 wider importance than this. It is a 

 subject which requires due recognition 

 when the reforestation of a district or 

 country is being considered. It was the 

 want of knowledge of this that led to 

 the destruction of forests that has taken 

 place in some parts of the world in the 

 past, and which has caused all the evils 

 that have arisen from such destruction. 



The importance of the changes in the 

 water content of the soil is evident. 

 These are likely to be of greater propor- 

 tionate magnitude than any of the 

 others, and are of special consequence 

 to the agriculturist because of the neces- 

 sity of water to the plant. It suffices to 

 draw attention to the progress that has 

 been made in the development of tillage 

 methods that are designed to conserve 

 the water that the soil contains for the 

 uses of plants— methods that have turned 

 semi-arid regions into districts of large 

 agricultural usefulness. 



Next to water the most potent factor 

 in influencing soil fertility is the content 

 of nitrogen. This fact is well brought 

 out in a recent paper by A. D, Hall, M. A., 



P.R.S-, Director of the Rothamsted Ex- 

 periment Station, which, although it 

 deals with conditions in a country situ- 

 ated in temperate latitudes, is worthy 

 of special attention. Here, 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 nitro- 

 gen 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 soil rapidly de- 

 creases 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 phosphates 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 re- 

 moval of crops as harvest. There is, 

 however, another cause of the loss of 

 phosphates, to which perhaps sufficient 

 attention has not been given in the past 

 This is the application of large quan- 

 tities 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 way 

 as the phosphates ; it is, however, likely 

 to be lost to a greater degree in the 

 drainage water. Experiments have 



