III.] NITROGEN REQUIRED— HABIT OF GROWTH 91 



although large amounts of nitrogen, 100 to 150 lb. 

 per acre, arc taken up from the soil. The turnip 

 is a shallow-rooted crop possessing a considerable 

 development of small fibrous roots, but which are 

 confined to a surface layer of restricted depth ; as a 

 rule, the crop is grown with a moderate dressing of 

 farmyard manure and 4 to 5 cwts. per acre of phosphates. 

 When farmyard manure is not used, ^ cwt. per acre 

 of sulphate of ammonia or its equivalent is found to 

 be enough nitrogenous manure ; and in the south and 

 east of England even that is sometimes omitted when 

 the land is in good heart. But the land receives a very 

 thorough preparation during the spring months before 

 the seed is sown, so that the fine seed-bed has already 

 been enriched by an accumulation of nitrates, the pro- 

 duction of which has been greatly stimulated by the 

 working and aeration of the soil. The seed is not 

 sown until the end of May or early June, by which 

 time temperatures are high and nitrification very 

 active, and the growth of the crop is accompanied by 

 continual hoeing and working of the land between 

 the rows. There thus continues to be produced in a 

 rich soil sufficient nitrates for the requirements of the 

 crop, and large external supplies in the manure are 

 unnecessary. Mangolds, on the contrary, are a much 

 deeper rooted plant, are sown earlier and generally on 

 stronger soils less adapted to rapid nitrification, and 

 are found by experience to require a far greater 

 supply of nitrogenous manure. 



One of the most important questions to be settled 

 in connection with nitrogenous manures is their relative 

 availability and rapidity of action. It has already been 

 stated that the nitrates are both soluble and can be 

 taken up without further change by the plant; the 

 ammonium salts as a rule require to be nitrified, but 



