120 EXPERIMENTS UPON ROOT-CROPS 



on practically the same lines as have been maintained for the 

 mangel crop to-day, and are summarised in Table XLIV. 



These results show the great dependence of the turnip crop 

 upon the supply of a phosphatic manure like superphosphate, 

 whereas there seems but little need of any external supply of 

 alkaline salts when turnips are growing on a soil like that of 

 Rothamsted. The crop was increased by each addition of 

 nitrogen, but the increase was not large, and affected the leaf 

 more than the root. Until the soil had become depleted of its 

 available nitrogenous compounds by repeatedly growing the 

 crop without any aj^plication of nitrogen, superphosphate alone 

 without any nitrogenous manure gaA^e rise to a comparatively 

 good crop. The value of superjDhosphate as a manure for 

 Swedes and turnips of all kinds was found to lie in the 

 extended root-development it induced, especially when the 

 plant was young. It was from these early experiments that 

 agriculturists first learnt how essential were phosphatic 

 manures to the growth of turnips, and the fact that the 

 manure for turnips should consist of superphosphate in the 

 main with but little nitrogenous manure, soon passed into 

 the common stock of farming knowledge and is universally 

 acted upon to-day. At the same time the success of super- 

 phosphate manuring for Swede and turnip crops led to an 

 enormous development of the manufacture of superphosphate 

 from mineral phosphates, which was then beginning under the 

 patents taken out by Lawes. 



In the earlier years l3ut little return was obtained for potash 

 manures, but as the plots continued to be cropped with nitrogen 

 and phosphoric acid but without jDotash, the soil became 

 gradually depleted of available potash and the potash manures 

 began to show large eff'ects. 



As has already been mentioned, it was soon found necessary 

 to discontinue the attempt to grow Swedes year after year on 

 the same land. The soil at Rothamsted is not very well 

 suited to the crop, being heavy and awkward to work, and in 

 consequence of the use of saline manures and the restricted 



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