POTASH AND PHOSPHORIC ACID 29 



As will be seen later, continuous cropping without manure 

 soon reduces such materials in the soil to a low ebb, below 

 which they do not fall appreciably in succeeding years ; the crop 

 production becomes very nearly stationary and is accompanied 

 by a very small reduction in the original stock of carbon and 

 nitrogen, even if there are not compensating influences at 

 work maintaining the store at a constant low level. Similarly, 

 when very large amounts of organic matter are added every 

 year as when plots are continuously dunged, after a time there 

 is but little increase in the proportions of carbon and nitrogen 

 present in the soil, because the bacterial agencies which 

 generate carbon and nitrogen compounds of a gaseous nature 

 are so stimulated by the abundant food-supply as to keep 

 pace with the annual additions. 



Of the other important constituents of plant food the soil 

 carries an abundant stock of potash; a complete mineral 

 analysis, in which the Broadbalk soil was completely broken 

 up by hydrofluoric acid, yielded as much as 2 '26 per cent, of 

 potash, quite four times the amount that can be extracted by 

 long digestion with hydrochloric acid. Though this vast stock 

 of potash is in the main dormant, it slowly becomes available 

 for crops through the weathering agencies which are brought 

 into play by cultivation. 



In phosphoric acid the soil is by no means so rich ; the 

 unmanured plots contain now rather less than O'l per cent., 

 the highest limit reached on some of the very heavily manured 

 plots being about 0'25 per cent. ; under ordinary farming 

 conditions, however, the soil shows no particular need of 

 phosphoric acid, as do many clay soils. 



Magnesia is fairly abundant in the Kothamsted soils ; in 

 the subsoil, indeed, it is present in almost the same proportions 

 as the lime, it is only in the artificially chalked surface soil 

 that the ratio of lime to magnesia is a high one. 



Soda is present in small quantities, partly combined with 

 chlorine as common salt derived from rain, and partly in the 

 double silicates of the clay. 



