The Chemistry of Garden Crops 



soil deficient in potash, however well supplied it may be with soda. 

 The cheaper alkali in combination as salt (chloride of sodium) may, 

 however, be usually employed in aid of quick-growing green crops ; 

 and more or less with tap-roots and Brassicas. Salt, too, is very 

 useful in a dry season by reason of its power of attracting and re- 

 taining moisture. As regards Potatoes, it seems worthy of observa- 

 tion that they contain but a trace of silica, and yet they generally 

 thrive on sand, and in many instances crops grown on sand are free 

 from disease and of high quality, although the weight may not be 

 great. The mechanical texture of the soil has much to do with this ; 

 and when that is aided by a supply of potash and phosphates, 

 whether from farmyard manure or artificials, sandy soils become 

 highly productive of Potatoes of the very finest quality. On the 

 other hand, Potatoes also grow well on limestone and chalk, and yet 

 there is but little lime in them. Here, again, mechanical texture ex- 

 plains the case in part, and it is further explained by the sufficiency 

 of potash and phosphates, as also of magnesia, which enters in a 

 special manner into the mineral constitution of this root. 



Thus far we have not even mentioned nitrogen, or its common 

 form of salts of ammonia j nor have we mentioned carbon, or its 

 very familiar form of carbonic acid. These are important elements 

 of plant growth ; and they account for the efficacy of manures derived 

 directly from the animal kingdom, as, for example, the drop- 

 pings of animals, including guano, which consisted originally of the 

 droppings of sea-birds. Some of the nitrogen in these substances, 

 however, is of an evanescent character, and rapidly flies away in the 

 form of carbonate of ammonia ; hence, a heap of farmyard manure, 

 left for several years, loses much of its value as manure, and guano 

 should be kept in bulk as long as possible, and protected from the 

 atmosphere, or its ammonia will largely disappear. One difficulty 

 experienced by chemists and others in preparing artificial manures is 

 that of ' fixing ' the needful ammonia, so that it may be kept from 

 being dissipated in the atmosphere, and at the same time be always 

 in a state in which it can be appropriated by the plant. We cannot 

 supply plants with nitrogen directly, but in all good manures there is 

 a certain proportion of it in combination, and in many instances the 

 percentage of nitrogen is made the test of the value of a manure. 



The importance of humus the black earthy substance resulting 

 from the decay of vegetation in a soil is that it contains in an 

 assimilable form many of the ingredients essential to plant life. 

 Humus when it decomposes gives off carbonic acid, which breaks up 



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