142 TALKS ON MANURES. 



taken, was in a condition to yield 14 bushels more wheat, per acre, 

 than that upon which wheat had been previously grown ; the yield 

 of wheat, after clover, in these experiments, being fully equal to 

 that in another field, where large quantities of manure were used. 

 " Taking all these circumstances into account, is there not pre- 

 sumptive evidence, that, notwithstanding the removal of a large 

 amount of nitrogen in the clover-hay, an abundant store of availa- 

 ble nitrogen is left in the soil, and also that in its relations towards 

 nitrogen in the soil, clover differs essentially from wheat ? The 

 results of our experience in the growth of the two crops, appear 

 to indicate that, whereas the growth of the wheat rapidly ex- 

 hausts the land of its available nitrogen, that of clover, on the 

 contrary, tends somehow or other to accumulate nitrogen within 

 the soil itself. If this can be shown to be the case, an intelligible 

 explanation of the fact that clover is so useful as a preparatory crop 

 for wheat, will be found in the circumstance, that, during the 

 growth of clover, nitrogenous food, for which wheat is particularly 

 grateful, is either stored up or rendered available in the soil. 



" An explanation, however plausible, can hardly be accepted as 

 correct, if based mainly on data, which, although highly probable, 

 are not proved to be based on fact. In chemical inquiries, 

 especially, nothing must be taken for granted, that has not been 

 proved by direct experiment. The following questions naturally 

 suggest themselves in reference to this subject: What is the 

 amount of nitrogen in soils of different characters ? What is the 

 amount more particularly after a good, and after an indifferent crop 

 of clover? Why is the amount of nitrogen in soils, larger after 

 clover, than after wheat and other crops ? Is the nitrogen present 

 in a condition in which it is available and useful to wheat ? And 

 lastly, are there any other circumstances, apart from the supply cf 

 nitrogenous matter in the soil, which help to account for the bene- 

 ficial effects of clover as a preparatory crop for wheat ? 



" In order to throw some light on these questions, and, if pos- 

 sible, to give distinct answers to at least some of them, I, years 

 ago, when residing at Cirenccster, began a series of experiments ; 

 and more recently, I have been fortunate enough to obtain the co- 

 operation of Mr. Robert Valentine, of Leighton Buzzard, who 

 kindly undertook to supply me with materials for my analysis. 



" My first experiments were made on a thin, calcareous, clay soil, 

 resting on oolitic limestone, and producing generally a fair crop of 

 red-clover. The clover-field formed the slope of a rather steep 

 hillock, and varied much in depth. At the top of the hill, the soil 

 became very stony at a depth of four inches, so that it could only 



