27 



per centage of vegetable mould (0. 5 to 3. per cent.) and is therefore to be removed cautiously. 

 Sulphuric acid is present to some extent iu all soils, abounding inaiost in ancient marls and gypseous 

 formations. The supplies of lime and alkalies are very much greater than any of the preceding 

 bodies ; the former attaining 10 and the latter 4 to 5 per cent, in rich alluvial lands. The extent to 

 ■which we may remove these in a rotation is as their probable amount in the soil, which may be 

 taken in general terms after the following rates per cent., in a perfect alluvial soil. Phosphoric acid 

 0. 20 — azotized matter 0. 25 — sulphuric acid 0. 10 — alkalies 2. 00 — lime and magnesia '5. 00. In 

 estimating the consumption we must know the amount and kind of bodies removed with each crop. 

 The difference of average crops in this respect is remarkably striking, and the subject has been fully 

 detailed in my lectiu-es in the University. It may be proper, here, to adduce by way of illustration, 

 a few cases. A crop of wheat of 25 bushels with straw removes 123 lbs. of inorganic matters, con- 

 sisting of about 12 lbs. of phosphoric acid, 90 lbs. of silica, 15 lbs. of alkaline salts. A crop of 

 lucern of two tons removes 425 lbs. of mineral bodies, of which about 250 lbs. are lime, 20 lbs. 

 sulphuric acid. Eight hundred bushels of beets remove about 360 lbs. of ashes, of which 316 lbs. 

 are alkaline salts. 



It would be tedious and out of place to read here the tables upon which these calculations are 

 made ; it may be enough to state that they have been made, and that they form one of the 

 necessary items of knowledge in constructing a perfect rotation. In addition to this, every expedient 

 used by practical men, as the introduction of cleaning crops, green fallows, depasturing fall crops, 

 the employment of roots, etc., are to be attended to in carrying out the design of the rotation — the 

 economy of the mineral and organic aliments of the soil. These expedients do not however, 

 constitute principles to be incorporated in the system, but are only practical adjuncts to be used or 

 otherwise according to local circumstances. 



Recurring to the foregoing explanation of the two precepts deduced from practical writers, we 

 find that they are sustained by the attraction of particular plants for certain aliments, and are there- 

 fore two principles for the government of rotations. To these we now add a further precept, that 

 in the employment of foliage or root crops, to economize phosphoric acid, such as succeed each other, 

 should differ in respect to their affinity for lime, alkalies and sulphur. Thus we have attained the 

 following principles : 



1. Seed crops exhaust the soil of phosphoric acid ; and are to be introduced at intervals from 

 each other as remote as may be expedient. 



2. Certain plants require a large proportion of azotized matter from the soil ; and are therefore 

 to follow the application of the manure or to open the rotation in rich soils. 



3. Certain crops recruit the soil, as respects azotized matter ; and are to be employed after its 

 partial exhaustion. 



