74 



SOIL CONDITIONS AND PLANT GROWTH 



Schreiner and Shorey (250) have attempted a resolution of the 

 " humic acid " and of the " crenic acid " (the part not precipitated by 

 HC1) and have obtained the following substances from the alkaline 

 extract : 



Substances precipitated by Acids (the so-called 

 Humic and Ulmic Acids). 



Substances not precipitated by Acids (the so-called 

 Crenic and Apocrenic Acids). 



Resin acids. 

 Resin esters. 

 Glycerides.\ 

 Paraffinic acid, C^H^O^ m.pt. 45-48, 



probably identical with the acid formed on 



treating paraffin with fuming nitric acid. 

 Lignoceric acid, C^H^O^ m.pt. 8o-8i, 



isomeric with above. 

 Agroceric acid, C 2 iH 42 O 3 , m.pt. 72-73, a 



hydroxy fatty acid. 

 Agrosterol, C^H^O, m.pt. 237. 

 Phytosterol, C^H^O-HaO, m.pt 135. 



Both of the cholesterol group. 



Dihydroxystearic acid, CjgHggO^ m.pt. 



98-99, identical with the acid formed 



on oxidising elaidic acid. 

 a-Picoline y-carboxylic acid, C 7 H 7 O 2 N!, 



m.pt. 239, identical with the acid formed 



on heating uvitonic acid to 274. 

 Xanthine, C 8 H 4 O 2 N 4 . 

 Hypoxanthine, C B H 4 ON 4 . 

 Cytosine, C 4 H 5 ON 3 H 2 O. 

 Histidine, CgHgO-jN,,. 

 Arginine, C 6 H 14 O 2 N 4 . 

 A pentosan. 



Methods for estimating the amount of " humic acid " or " humates " 

 in the soil have been devised and numerous analyses have been made, 

 but no conclusions of any consequence have been drawn from the 

 results. On the Rothamsted plots about one-half of the total nitrogen 

 is contained in compounds soluble in alkalis, the proportion varying 

 but little with the scheme of manuring. It has been maintained by 

 Grandeau (in) and Hilgard (133) that these compounds are by far the 

 most useful for making plant food, but there is no evidence in favour 

 of this view. They are also stated to have a special stimulating effect 

 on soil bacteria, but in the only case systematically examined this 

 was ascribed to the iron invariably present (see p. 94). 



No examinations have been made of the part of the organic matter 

 insoluble in alkalis, but there is no reason for supposing that it is any 

 less important than the soluble part. 



Wax-like Constituents. 



Some of the soil organic matter is wax-like in properties, interfering 

 very much with the wetting of the soil and the movement of the water- 

 As it only decomposes slowly it tends to accumulate in rich soils 

 and to become rather troublesome. It can be extracted by organic 

 solvents, e.g. toluene, and obtained as a yellowish-brown mass contain- 

 ing appreciable quantities of nitrogen (a soil yielded '003 per cent, of 

 a substance containing 3 per cent, of nitrogen in one of the writer's 

 analyses). 



