172 PLANT PRODUCTS 



eighty pounds of nitrogen, forty pounds phosphoric acid, 

 one hundred and fifty pounds potash, and two hundred 

 pounds common salt, a relatively somewhat expensive 

 mixture. The root crops in general, when grown with a 

 large amount of nitrates, especially nitrate of soda, decrease 

 in food value, the plants being of a rather watery, poor 

 feeding quality. Potash, which is so essential for the 

 mangold crop, can be economized to a partial extent by the 

 use of sodium salts. A particularly useful waste industrial 

 product is a mixture of calcium sulphate and sodium chloride, 

 obtained from some salt works. Where sodium chloride 

 is desirable for cultivation, as it is in the case of mangolds, 

 the sodium has the tendency to render the clay sticky, but 

 an admixture of calcium sulphate overcomes this difficulty, 

 as it prevents the formation of colloidal compounds. 



The Leguminous Crops obtain much of their nitrogen 

 from the atmosphere, and therefore do not require nitro- 

 genous manure, excepting very small quantities to get over 

 their early stages, when they are particularly subject to 

 the attack of all kinds of pests. Small quantities of nitro- 

 genous manure enable them to get out of the reach of many 

 of their enemies at a period when their capacity for obtaining 

 nitrogen from the air is very small indeed. They are 

 particularly dependent upon lime, potash, and phosphoric 

 acid. The importance of clover in the hay crop as part of 

 the rotation has been recognized from the earliest times, 

 the procedure being known to the Romans. This system 

 is also adopted in tropical countries, like India, where a 

 leguminous crop is grown either mixed with one of the millets, 

 or as a separate part of the rotation. The increase in the 

 nitrogen content of the soil, by the growth of clover, has 

 been already alluded to (see p. 8i). Among the different kinds 

 of clover, the wild white clover has been found particularly 

 suitable for development in the pastures. Where, however, 

 the conditions of the soil are of a rather moist character, 

 probably the wild red clover is equally suitable. There is 

 a great difference between growing mixed crops of herbage 

 and„ growing a single crop in the ordinary way. Where 



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