112 
INVESTIGATIONS ON KOTHAMSTED SOILS. 
plats. This may be partly accounted for by the supposition that the 
phosphoric acid contained in the dung, at any rate over a portion of 
the time, may have been overestimated, but there appears reason for 
supposing ihat there has been a greater descent into the subsoil in the 
case of the plat dunged for fifty years than in the case of the chem- 
ically manured plats. On the plat dunged for nine years only we 
find the estimated accumulation fairly represented in the upper 
depths — for the most part in the first 9 inches. 
As in the case of nitrogen, the soil possesses naturally a very large 
reserve of phosphoric acid, but again, as in the case of nitrogen, the 
quantity annually rendered available for the use of plants by natural 
soil processes is insufficient to suffice for the needs of anything like 
an average crop. 
Potash. 
General Discussion and Statement of Analytical Results on Broad- 
balk Soils and Subsoils of 1893. 
The total potash has not been determined in the various samples. 
The Broadbalk soil, like most soils containing much clay, contains a 
great quantity of potash in the form of silicates decomposable only 
by fusion or* by treatment with hydrofluoric acid. The quantity of 
potash, including this, is between 1 and 2 per cent in the surface soil 
and probably more in the subsoil. This means that in the first 9 
inches of soil there are probably 15 tons of potash per acre in a 
dormant form, while the quantity in each succeeding depth is proba- 
bly greater. Such potash forms a reserve stock for the distant future, 
and is no doubt very gradually rendered available for plant use by 
the natural processes going on within the soil. But, even with this 
great reserve of total potash, the soil is unable to furnish a sufficient 
annual supply to the wheat crop under a system of continuous 
cropping. 
It is usual in soil analysis to neglect to take into account potash, 
existing in so insoluble a form as to be separable only by means of 
fusion or treatment with hydrofluoric acid, for such potash is obvi- 
ously Par removed from the range of present utility, and only to take 
into account such potash as is soluble in hydrochloric acid. It is by 
this lime well recognized, however, that even this solvent extracts, at 
all events in many cases, far more potash than can be in any sense 
regarded as of present utility. Unfortunately, this is not the only 
unsatisfactory aspect of the determination of potash by hydrochloric- 
acid ext ract ion. The quantity of potash extracted varies greatly, not 
only with the strength and quantity of acid used, but also with the 
duration of the process and with the temperature. 
The potash dissolved by hydrochloric acid was, however, deter- 
mined in a large number of samples, the quantity of soil taken for 
the determination being LO grams, and the extraction being made by 
