EFFECT OF A THREE TEAUS' COURSE OF CROPPING. 221 



I. Lei us consider the effect upon the soil of a still too common three 

 years' course of cropping— /aZZoi^;, wheat, oals."^ If the produce of such 

 a course be 25 bushels of wheat and 50 bushels of oats, there would be 

 carried from the soil every three years in pounds — 



WHEAT. OATS. 



, ^ , , • <• Total. 



Grain. Straw. Grain. Straw. 



Potash .... 3-3 0-6 3-75 32-7 40-35 



Soda 3-5 0-9 3-3 — 7-7 



Lime 1-5 7-2 2-5 57 16-9 



Magnesia. ... 1-5 1-0 1-7 0-8 5-0 



Oxide of Iron . . — — 1-0 — 1-0 



Silica 6-0 86-0 500 172-0 314-0 



Sulphuric Acid . . 0-75 1-0 0-9 3-0 5-65 



Phosphoric Acid . 0-6 5-0 1-43 0-5 7-53 



398-13 

 The gross weight carried off in these crops is large — amounting to 

 about 400 lbs. It will vary, however, with the kind of wheat and oats 

 which are grown, and may often be greater than this. — [See the follow- 

 ing section (§ 5) of the present Lecture.] The greatest portion of the 

 matter carried off, however — upwards of three-fourths of the whole- 

 consists of silica; the rest of the materials are equal to 

 60 lbs. of dry pearl-ash, 

 36 lbs. of the common soda of the shops, 

 28 lbs. of bone-dust, 

 12 lbs. of gypsum, 

 5 lbs. of quick-lime, 



5 lbs. of magnesia, — or for the last three may be substi- 

 tuted 33 lbs. of common Epsom salts and 17 lbs. of quick-lime. 



The form in which the silica may be restored to the soil in a state in 

 which the plant can absorb it, will be considered hereafter. 



Though large as a whole, the weight of each of the ingredients, taken 

 singly, is not great; and yet it is not difficult to understand that if a 

 constant drain be kept up on the soil year after year, and the practical 

 farming adopted is of such a kind as not to restore to the soil a due pro- 

 portion of each of the substances carried off— the time must come when, 

 under ordinary circumstances, the soil will no longer be able to supply 

 the demands of a healthy and luxuriant vegetation. 



II. Let us next consider the effect of a four-years' course system in 

 withdrawing these inorganic substances from the soil. And for this 

 purpose let us adopt one suited to the lighter soils — as to that of Norfolk — 

 turnips, barley, clover and rye grass, wheat. 



Let the crop of turnips arnount to 25 tons of roots per acre, of barley to 

 38 bushels, of clover and rye grass each to one ton of hay, and of wheat 

 as before tb' 25 bushels. Then we have from the entire rotation in 

 pounds — 



• Common, among other counties, in that of Durham. There are cases, however, in 

 which this three years' course may not be indefensible, and it never could be compared with 

 some of the so-called improved rotations in East Lothian in the time of Lord Karnes ; as for 

 instance, fcMow, barley, clover, manure on the clover stubble, then tcbeat, barley, oats. — See 

 The Gentlenmn Farmer (1802), p. 147. 



