268 Journal of Agricultural Research Voi. xviii. no. s 



purify the water sufficiently for wheat seedlings was inadequate for the 

 citrus seedling. It was necessary to use an extra large amount of carbon 

 black or to bring the water to the boiling point in the presence of carbon 

 black and filter in order to obtain water in which citrus seedlings would 

 grow. Individual variation in the resistance of the seedlings was fre- 

 quently observed. In a toxic solution that would kill nine of the seed- 

 lings, the tenth would sometimes grow vigorously. 



ORGANIC EXTRACTS 



In the early experiments organic extracts from acid upland peat (leaf 

 mold) and horse manure were employed. The organic matter was first 

 extracted to obtain the water-soluble portion and then extracted with 

 3 per cent ammonia. These extracts were filtered and evaporated sepa- 

 rately to dryness and the dried material used to make up standard organic 

 solutions. The peat extract proved to be fully as satisfactory for cul- 

 tures as that from manure and has been employed in most of the work.* 



The effect of organic matter upon the growth of the lemon seedling is 

 shown in Table I. The result in each instance represents the total root 

 growth of lo seedlings in six days. It will be seen that the water-soluble 

 extracts of peat in concentrations of lo parts per million or more pro- 

 duced root elongation double that produced with cultures in carbon- 

 treated water. 



Table I. — Effect of peat extract on root elongation of citrus seedlings 



Treatment. 



Root growth. 



Carbon-treated water, control 



Carbon-treated water, plus 5 p. p. m. water-soluble peat 



Carbon-treated water, plus 10 p. p. m. water-soluble peat 



Carbon-treated water, plus 50 p. p. m. water-soluble peat 



Carbon-treated water, plus 100 p. p. m. water-soluble peat. . . , 

 Carbon-treated water, plus 500 p. p. m. water-soluble peat. . . 

 Carbon- treated water, plus 1,000 p. p. m. water-soluble peat. . 



Mm. 



90 



"5 

 197 

 197 

 236 

 178 

 190 



A similar series of experiments with grapefruit seedings, carried on 

 at the same time, gave results almost as marked. The root growth in 

 tlie two cultures is illustrated in Plate 33, A, B. 



The stimulating effect of the soluble organic matter in very low con- 

 centrations was verified by repeated tests and was observed not only 

 for grapefruit and lemon seedlings but for Blood, St. Michael, Tahiti, 

 Valencia, and Tangerine oranges as well. The ammonia extract of the 

 peat, freed from ammonium hydrate (NH^OH) by evaporating to dry- 

 ness, was as effective as the water-soluble extract in stimulating the root 

 growth of the seedlings. 



1 This peat is the same as that employed by Coville in his investigation of blueberry culture. (CovrLLE, 

 F. V. EXPBRIMENTS IN BLUEBERRY CULTURE. U. S. Dept. Agr. Bur. Plant Indus. Bui. 193, 100 p., 31 

 figs., 18 pi. 1910. 



