REASONS FOR ROTATION. 203 



111 the cultivation of these crops, the ground between 

 the rows should be turned over and stirred as often as 

 possible, six or eight times at least. The ploughing 

 will then come in this order : earliest, for carrots and 

 beets, next, for potatoes, next, for corn, next, for turnips. 

 The grass-field, for potatoes, may be turned over after hay- 

 harvest ; the ploughing for parsnips may be later and the 

 seed be sown in the autumn. All the hoed crops should 

 be cultivated with the cultivator, the horse hoe or the 

 horse plough, whenever time can be found till the crops 

 are too far advanced to admit of it. 



Good reasons can be given for the seven-years course 

 here recommended. Potatoes require large portions of 

 the alkaline salts and of lime. These are succeeded by 

 corn, which requires more of silica, together with alka 

 lies ; then come the roots, which require lime and the 

 alkalies, with a good deal of nitrogen ; after them rye, 

 which calls for silica. This is succeeded by clover, which 

 demands a great deal of lime, and this, by the grasses, 

 which again demand silica and the alkalies. 



715. All plants require, but in different proportions, 

 carbonic acid, phosphoric acid, sulphuric acid, the alka 

 lies, potash, soda and ammonia, and lime, magnesia and 

 iron. The acids combine with the other elements of fer 

 tility, and, while the corn is growing, they are preparing, 

 from the particles in the soil, carbonates, phosphates and 

 sulphates of potash, soda, ammonia, lime, magnesia and 

 iron, for the other crops, and new supplies of silica for the 

 grasses. 



The substances most frequently needed for the restora 

 tion of fertility are ammonia, phosphoric acid and potash, 

 and the most valuable manures, next to barn manure, 

 are accordingly bones and ashes, 

 is* 



