I. AGRICULTURE AND RURAL ECONOMY. 401 



crops, with very little grain, make up the chief produce of 

 the soil. But little dung has been applied, nor has the cus- 

 tom of returning straw to the soil ever prevailed as in Eu- 

 rope. Clearing land by burning wood has probably aided 

 the exhaustion. "No doubt other matters besides potash 

 have been removed from the land by these practices, nor 

 that, in many instances, phosphates are needed also; but the 

 evidence would seem to show that, in the present case, the 

 supply of potash originally contained in the land has given 

 out first. It is no great matter of surprise that this thing 

 should have occurred in a country mainly devoted to grazing 

 and the growth of forage. If New England had been a grain- 

 growing country, phosphoric acid might perhaps have been 

 its weakest point. 



In the field experiments of Messrs. Lawes and Gilbert, in 

 England, alongside of heavy crops that have been raised, 

 year after year, on manured plots of land, smaller yet not in- 

 considerable yields had been obtained in successive crop- 

 pings, on similar plots, without manure. This has served to 

 bring out very forcibly the fact that soils have a certain 

 capability of re-supplying the plant-food removed in crop- 

 ping by the working over of materials present in greater or 

 less quantity in every soil, into forms fit for the nourishment 

 of the plant. To this restoring power the term " natural 

 strength" has been applied. 



In Storer's experiments, crops of some, though limited, size 

 were obtained without manure. By adding moderate quan- 

 tities of appropriate manure a much greater yield was ob- 

 tained. But an increase in the manure above this amount 

 was not followed by a corresponding increase of crop. 

 Very heavy manuring was not economical. 



In the causes of this are to be found some principles of 

 great practical importance. 



From the fact that only very small crops were obtained 

 without manure, Storer concluded that the natural strength 

 of his soil, in the sense above referred to, is not great. 



But there is another sense in which this term may be used, 

 and another condition of the capability of a soil for pro- 

 ducing crops, besides its capacity for working over into 

 available forms the stores of plant-food it may contain. It 

 is important that it should be able to utilize, economically, 



