34 



SOIL CONDITIONS AND PLANT GROWTH 



duce too much straw if the nitrate supply is excessive, and the straw 

 does not commonly stand up well, but is beaten down or " lodged " by 

 wind and rain. Swede and potato crops also produce more leaf, but not 

 proportionately more root or tuber, as the nitrogen supply increases ; 

 no doubt the increased root would follow, but the whole process is 

 sooner or later stopped by the advancing season the increased root 

 does in fact follow in the case of the late-growing mangold. Toma- 

 toes, again, produce too much leaf and too little fruit if they receive 

 excess of nitrate. On the other hand, crops grown solely for the sake 

 of their leaves are wholly improved by increased nitrate supply : 

 growers of cabbages have learned that they can not only improve the 

 size of their crops by judicious applications of nitrates, but they can 

 also impart the tenderness and bright green colour desired by pur- 

 chasers. Unfortunately the softness of the tissues prevents the cabbage 

 standing the rough handling of the market. 



Three cases are illustrated in Table XL : wheat shows increases in 

 straw greater than those in grain as the nitrogen supply is increased ; 

 white turnips show increases in leaf greater than those in root, but 

 mangolds show substantially the same increase both in leaf and root, 

 because their growing period is so much longer than that of the other 

 crops, continuing until the end of October. 



TABLE XI. EFFECT OF VARYING SUPPLY OF NITROGENOUS MANURE ON THE GROWTH 



OF CROPS. ROTHAMSTED. 



The actual increase of growth brought about by successive incre- 

 ments of nitrogeneous food depends on the amount of water and 

 other nutrients, on the temperature, and so on ; any of these may act 

 as limiting factors. Table XII. shows the crops obtained on some of 

 the Rothamsted mangold plots ; in one case the supply of potassium 

 is so small that it becomes the limiting factor, in the other sufficient 

 potassium is supplied. 



