34 FERTILISERS CONTAINING NITROGEN [cHAP. 



crop throughout the year. A crop of wheat, still un- 

 manured, completes the rotation. In the table the 

 yield on the portions which have grown clover is com- 

 pared with that on the portions without crop ; it will 

 be seen that although a crop of nearly three tons of 

 clover hay has been taken away from the one portion, 

 the wheat which follows it is 23 per cent, better than on 

 the portion where no clover had been grown in the 

 previous year. Nor is the benefit due to the clover 

 exhausted by the wheat crop, for it is seen to persist in 

 the root crop following the wheat and in the barley 

 which comes a year later still. 



The only practical limitation to the gathering of 

 nitrogen by this method lies in the difficulty that is 

 found in growing leguminous crops frequently on the 

 same land. Although the Rothamsted experiments 

 have demonstrated that it is possible to grow wheat 

 year after year for more than half a century and 

 maintain the yield if the appropriate manures are 

 employed, on few soils can clover be grown with success 

 more frequently than once in four and even once in 

 seven years. As the farmer says, the land becomes 

 "clover sick," and though the clover seed germinates 

 and grows for a time, the constitution of the plant is so 

 weak that it almost inevitably succumbs during the 

 winter to an attack of fungoid or other disease. The 

 determining cause of this weakness of constitution 

 which lies at the back of "clover sickness" is still 

 unknown, but preventing as it does the more extended 

 use of these nitrogen-collecting crops it would be of 

 real economic importance to find the cause and a 

 remedy. 



More recently, however, other bacteria have been 

 discovered in the soil which are capable of fixing free 

 atmospheric nitrogen without association with any 



