ROTATION OF CROPS. 249 



more nitrogen over a given area than the other crops may, 

 at any rate in part, be dependent on their being able, by 

 virtue of the range and character of their roots, to gather up 

 more nitrogen in the form supposed than the plants with 

 which they are alternated. Such a view does not, however, 

 exclude the supposition that some of their nitrogen is derived 

 in other ways, as above referred to. 



In connection with the foregoing results of direct experi- 

 mental investigation into the mineral composition of legum- 

 inous crops, it may be observed — that clover at any rate grows Applica- 

 more favourably on land that has recently been chalked or f^^nd 

 limed ; that chalking or liming of the mixed herbage of grass potash for 

 land also favours the development of the leguminous herbage ; legurmnous 

 and that the application of gypsum to clover has been found 

 very effective on some lands, especially in America, though 

 it has not proved to be at all generally useful when it has 

 been so applied in this country. Indeed, the direct application 

 of potash as manure is certainly more frequent, and is more 

 generally recognised as effective for leguminous crops, than 

 is that of lime, notwithstanding its obvious importance, and 

 its great influence on the luxuriance of growth of such crops. 

 This may perhaps be partly explained by the fact that, in 

 mauy, if not in most, soils, the immediately available supply 

 of potash within the root-range of the plant will probably be 

 sooner exhausted than will that of lime. 



Summary and General Conclusions. 



It remains, in conclusion, very briefly to summarise the 

 facts brought out in this extended inquiry on the subject of 

 rotation, and to endeavour to draw from them an explanation 

 of the benefits arising from the practice of it. 



At the commencement it was pointed out, that although Variations 

 many different rotations are adopted, they may for the most ** rota ' 

 part be considered as little more than local adaptations of the 

 system of alternating root-crops and leguminous crops with 

 the cereals. Thus, there are rotations of five, six, seven, or 

 more years. But these variations are, in most cases, only 

 adaptations of the principle to variations of soil, altitude, 

 aspect, climate, markets, and other local conditions ; and they 

 consist chiefly in the variation in the description of the root- 

 crop, and perhaps the introduction of potatoes ; in growing a 

 different cereal, or it may be more than one cereal consecu- 

 tively ; in the growth of some other leguminous crop than 

 clover ; or the intermixture with the clover of grass seeds ; 

 and perhaps the extension of the period allotted to this ele- 

 ment of the rotation to two or more years. 



