88 PHYSIOLOGY OF NTTTRITION 



This is shown by Wagner's experiments' with oats (Fig. 58). Three culture 

 vessels received equal amounts of phosphoric acid (0.5 g.) as pulverized Thomas 

 slag; but different kinds of slag were used, showing different solubilities of their 

 phosphates in ammonium citrate solution. The fourth vessel received twice 

 as much phosphoric acid (i.o g.), in the form of pulverized phosphate rock 

 (phosphorite), and the fifth received no phosphorus fertihzer at all. The fol- 

 lowing table shows the effects of these fertilizers upon the growth of the plants. 



This experiment shows very clearly how fertilizers may differ in quality. 

 Although the fourth culture contained more phosphoric acid than any of the 

 others, its yield exceeded that of the unfertilized plants by only about 15 g.; 

 the plants could not assimilate this particular phosphorus compound. It 

 appears that the greater the amount of phosphorus compounds that can be 

 dissolved out of the fertOizer by ammonium citrate solution, the better can the 

 fertilizer be utilized by the plant and the greater is the yield. 



Not only the properties of the fertilizer, but also the peculiarities of the 

 plants under cultivation must receive attention. The same fertilizer, added to 

 a given soil, may be beneficial to one plant and entirely useless to another. In 

 Prianishnikov's "experimentSj^ for instance, various plants were cultivated in 

 sand supplied with the necessary nutrient salts. In one series of experiments 

 phosphorus was supplied as mono-sodium phosphate (NaH2P04), in the other 

 as phosphate rock (phosphorite), which contains calcium phosphate, calcium 

 carbonate, sand, loam, iron oxide, and aluminium. Millet grown in these two 

 media gave a yield of 29.07 g. with the soluble phosphate and one of only 0.57 g. 

 with phosphate rock (Fig. 59). Millet and other grains either cannot utilize 

 phosphorite in sand cultures at all, or else they can utilize it only to a very slight 

 degree. The Papilionacese (peas, beans, etc.), however, show an entirely dif- 

 ferent behavior toward phosphate fertilizers. Scarcely any difference can be 

 discovered between the pea cultures supplied with soluble phosphates and those 

 supplied with phosphorite (Fig. 59). 



The value of phosphate rock as a fertilizer depends not only upon the nature 



1 Wagner, Paul, Diingungsfragen unter Berticksichtigung neuer Foreschungsergebnisse. Heft III. 

 S6 p. Berlin, 1896. 



« Prianishnikov, D. N., 1st die Phosphorsaure der Mineralphosphate der Kulturpfianzen zuganglich? 

 1 Russian, with German abstract.) Ann. Inst. Agron. Moscou 5 (Partie non offioielle): go-iio. 1899. 



