242 A TEXTBOOK OF BOTANY [Ch. V, 6 



together. Most important are the mineral salts necessary in 

 the nutrition of plants, and therefore commonly, though not 

 quite correctly, called " plant foods " (page 28) . They consist 

 in compounds of nitrogen, sulphur, phosphorus, magnesium, 

 iron, potassium, and calcium, having the uses in the plant 

 already described (page 230). They come into the soil so- 

 lution chiefly through chemical disintegration of the rocks 

 which contain them, but to some extent through action of 

 living organisms, as will be further described a page or two 

 later. These natural sources of supply are sufficient in 

 case of wild plants, which, by decay, return their substance 

 to the ground ; but under cultivation, where great quantities 

 of mineral matters are annually removed with the crops, 

 some are apt to run short and must be replaced artificially, 

 which is accomplished through fertilizers. The mineral 

 salts which usually first become scarce are compounds con- 

 taining nitrogen, phosphorus, and potash; and since all 

 three are abundant in barnyard manures, we can see the 

 agricultural value thereof. Nitrates, phosphates, and potash 

 salts, obtained from other sources, are also used commonly 

 as fertilizers. Such, at least, is the older and, among 

 farmers, still prevalent belief as to the role of fertilizers in 

 the fertility of land. But of late some leading investigators 

 have advocated a different view, based on the claim that the 

 soil solution supplies all of the mineral salts which plants ordi- 

 narily need, even on much-cropped land, the fertilizers 

 finding their use chiefly in the neutralization of other un- 

 favorable conditions in the soil. 



The functional use of the different mineral salts to plants 

 is inferred from various lines of evidence, but chiefly from 

 the results of water culture (Fig. 171). Many herbaceous 

 plants can be grown from seed to maturity with the roots in 

 water, their well-developed aeration systems providing suffi- 

 cient oxygen to their roots. By using pure (distilled) water 

 as a basis, it is possible to supply to a plant all of the neces- 

 sary mineral salts except some given one, in which case the 



