ROOT AND ALLIED CROPS 305 



The following, taken from Warrington," shows the 

 amounts in pounds per acre of phosphoric acid, nitrogen, and 

 potash, removed by good crops from the soil : — 



Phosphoric Acid. Nitrogen. Potash. 



Wheat ... '21-1 48 28-8 



Potatoes '21-5 46 76-5 



Mangels 52*9 138 300-7 



Mangels are looked upon as exhaustive of available plant- 

 food, and that this is correct is to be seen from the above table. 

 They should therefore receive heavy applications of kraal 

 manure, or should follow well-decomposed green manures. On 

 most soils a dressing of 100 to 200 pounds of superphosphate is 

 required. Although the above analysis shows a high potash 

 requirement, and under European conditions the crop responds 

 well to heavy dressings of potassic fertilisers, in the Union 100 

 to 200 pounds per acre of sulphate of potash is usually all that 

 is necessary. 



When farmyard manure or green manures are used these 

 must be allowed to become thoroughly decomposed, as the 

 seed-bed should be compact and finely pulverised. 



Cultural Methods. — A good, fine, mellow seed-bed 

 should be prepared by ploughing, cross-ploughing, and harrow- 

 ing, and, if necessary, rolling to break down clods, to pulverise 

 the soil, and to compact it. Ploughing should be deep and 

 thorough, so as to extend the feeding zone of the roots. 



Mangels are grown on ridges or on the level. Under irriga- 

 tion the former method is employed, on drylands the latter. 

 On irrigated land the seed is drilled in on top of the ridges, 

 which are generally 80 inches apart. On drylands the width 

 of the rows is usually 36 inches. 



The seed is put in either by means of a root-crop drill or 

 a single row planter, e.g., Planet Junior, and should not be 

 planted deeper than one inch, except in sandy loams, where 

 the depth may be slightly more. Eidging is necessary for 

 irrigating, and assists in drainage during excessively wet 

 spells. 



In South Africa, unfortunately, all the seed used at present 

 is imported, and must be tested for vitality before planting, 

 as poor seed is the chief source of failure in this country. 

 Germination tests often show a viability of only 5 to 10 per 



