FEETILITY 167 



produce of a grass field contains a large amount of plant 

 food, and, if it were regularly cut and carried off, the 

 fertility of the soil would not be restored but reduced. 

 When the field is grazed the produce is all consumed, but 

 only a comparatively small proportion of the plant foods 

 is permanently retained by the animals. Generally, 1 in 

 round numbers, about three-fourths of the phosphoric acid, 

 nine-tenths of the nitrogen, and practically all the potash 

 are restored to the soil in the droppings. The quantity of 

 available plant food permanently abstracted from the soil 

 is thus greatly reduced, and, when the land is ploughed 

 up after several years lay, it will be found that they have 

 accumulated and that the productiveness has considerably 

 increased. If the grass is cut in alternate years, or more 

 or less frequently, the benefit to the land will be corre- 

 spondingly reduced. 



Restitution. It has been shown above that a cropped 

 fallow and laying down the land in grass are advantageous 

 as a means of increasing the supply of available plant food 

 in the soil only when the crops are consumed on the land, 

 and that the benefit consists essentially in restoring the 

 ingredients of the crops to the soil. It frequently happens, 

 however, that it is more convenient to remove the crops 

 and consume them in the stables and cowhouses of 

 the farm. If the droppings of the animals are collected 

 and carried back to the land again, the result is much the 

 same as before, except that a certain amount of loss which 

 may be minimised by careful management is incidental 

 to the process. All the ingredients of the crops, save the 

 small proportion retained by the animals and what is lost, 

 may be thus restored to the soil. Boots and hay are gene- 



1 The amounts retained depend on the kind, age, and condition of 

 the animals. Of the total nitrogen in the food consumed, fattening 

 sheep and oxen retain about 5 per cent., pigs about 15 per cent., and 

 milking cows about 25 per cent. 



