106 EXPERIMENTS UPON ROOT-CROPS 



salts, the magnesium sulphate and sodium chloride added to 

 Plots 4 but not to Plots 6, though providing no direct plant 

 food, yet so assist to render soluble the reserves in the soil and 

 to economise the supply of potash, that the crop receives an 

 indirect benefit equivalent to an addition of the more indispen- 

 sable elements of nutrition. 



The great effect of potash, and to a less degree of the other 

 alkaline salts, upon the mangel crop is very striking, and is to 

 be correlated with the fact that the mangel is essentially a 

 sugar-producing plant, and that large supplies of potash seem 

 to be essential to the processes in the plant which result in the 

 formation of sugar and similar carbohydrates. 



Doubtless the long period over which the experiments have 

 been continued has intensified the effect, because the soil of 

 Plot 5, which has received no potash for at least forty- seven 

 years, must by this time have been very thoroughly exhausted 

 of available potash. The poor returns of Plots 5, receiving no 

 potash, have been progressive, getting worse each year as the 

 initial stock of potash in the soil has become more and more 

 exhausted. For this reason, the farmer taking his mangel 

 crop in rotation need not expect to find an addition of potash 

 produce such a very large proportionate increase as is here 

 manifest. 



The effect of potash and of the other saline manures is 

 plainly visible in the appearance of the plants themselves. 

 On the plots receiving potash the plant begins to ripen early, 

 the leaves turn yellow and become flaccid, so that in October 

 these plots may be seen outlined from the rest by their lighter 

 tint at any distance from which the field can be viewed. The 

 ripening effect and the lighter colour are even more apparent 

 where the complete mineral manure, containing also magnesium 

 sulphate and salt, has been applied, than where potash has 

 been used alone. On the contrary, the plots receiving no 

 potash show all the signs previously described as indicating an 

 excess of nitrogen — the premature death of the outer leaves, 

 and the dark green, curled, and unhealthy appearance of the 



