196 SOILS AND MANURES 



first in the rotation, i.e., so that they may obtain the full 

 benefit of the available plant foods restored to the land in 

 the farmyard manure. In that case it is not usually 

 necessary to give special nitrogenous or potash manures 

 to turnips and swedes but a dressing of superphosphate 

 generally proves a useful addition. The root crops are 

 more favourably situated than the cereals for obtaining 

 nitrogen directly from the soil, because their most 

 active growth takes place later in the year and there is 

 therefore a larger supply of available nitrogen produced 

 by nitrification. This does not, however, apply to man- 

 golds, for which nitrates are much more important than 

 phosphates. Mangolds are deeper-rooted plants and 

 better able to obtain phosphates directly from the soil 

 than turnips and swedes. They are grown earlier in the 

 year and the quantity of nitrogen contained in a crop 

 is very large. 



Potatoes. The manurial requirements of potatoes re- 

 semble those of turnips and swedes, but potatoes contain 

 relatively more potash and give a better return for potash 

 manures. Even a full dressing of farmyard manure will 

 generally give a better crop when supplemented with a 

 special potash manure. Though potatoes contain less nitro- 

 gen than turnips and swedes, they are more dependent on 

 manures for the supply of that element. Excessive 

 quantities of nitrogenous manures," however, are some- 

 times harmful, especially if the supply of potash be 

 insufficient. Nitrogenous manures are superfluous when 

 the customary quantities of farmyard manure are em- 

 ployed. 



All these crops can, if necessary, be grown without 

 farmyard manure if they are amply supplied with nitrogen, 

 phosphoric acid and potash in the form of artificial 

 manures. The farmyard manure must, however, be 

 applied to the land at some time for purposes of resti- 



