CLOVER AS A PREPARATORY CROP FOR WHEAT. 453 



for a supply of it in a soluble state. Silica, indeed, if at all capa- 

 ble of producing a beneficial efi'ect, ought to be useful to these 

 crops, either by strengthening the straw or stems of graminaceous 

 plants, or otherwise benefiting them ; but after deducting the 

 amount of silica from th'e total amount of mineral matters in the 

 wheat produce from one acre, only a trifling quantity of other and 

 more valuable fertilizing ash-constituent of plants will be left. On 

 comparing the relative amounts of phosphoric acid and potash in 

 an average crop of wheat and a good crop of clover hay, it will be 

 seen that one acre of clover hay contains as much phosphoric acid 

 as two and a half acres of wheat, and as much potash as the pro- 

 duce from five acres of the same crop. Clover thus unquestionably 

 removes from the land very much more mineral matter than is done 

 by wheat ; clover carries ofi' the land at least three times as much 

 of the more valuable mineral constituents as that abstracted by the 

 wheat. Wheat notwithstanding succeeds remarkably well after 

 clover. ■ . 



Four tons of clover hay, or the produce of an acre, contains as 

 already stated, 224 pounds of nitrogen, or calculated as ammonia, 

 272 pounds. 



Assuming the grain of wheat to furnish 1.18 per cent, of nitro- 

 gen, and wheat straw .64 per cent., and assuming also that 1500 

 pounds of corn and 3000 pounds of straw represent the average 

 produce per acre, there will be in the grain of wheat per acre 26.7 

 pounds of nitrogen, and in the straw 19.2 pounds, or in both to- 

 gether 46 pounds of nitrogen ; in round numbers, equal to about 

 55 pounds of ammonia, which is only about one-fifth the quantity 

 of nitrogen in the produce of an acre of clover. Wheat, it is well 

 known, is specially benefited by the application of nitrogenous 

 manures, and as clover carries off" so large a quantity of nitrogen, 

 it is natural to expect the yield of wheat after clover to fall short 

 of what the land might be presumed to produce without manure 

 before a crop of clover was taken from it. Experience, however, 

 has proved the fallacy of this presumption, for the result is exactly 

 the opposite, inasmuch as a better and heavier crop of wheat is 

 produced than without the intercalation of clover. What, it may 

 be asked, is the explanation of this apparent anomaly? 



In taking up this inquiry I was led to pass in review the cele- 

 brated and higlily important experiments undertaken by Mr. Lawes 

 and Dr. Gilbert, on the continued growth of wheat on the same 

 soil for a long succession of years, and to examine likewise care- 



