144 SOIL CONDITIONS AND PLANT GROWTH 



useful material for making plant food. Methods for deter- 

 mining its amount in soils were elaborated and many estima- 

 tions were made. But nothing came of them : on the 

 Rothamsted plots about one-half of the total nitrogen is soluble 

 and one-half insoluble in alkalis whatever the manurial treat- 

 ment. The idea rested on no experimental basis and, in the 

 only recorded tests, Weir (300^) found that the removal of 

 this soluble nitrogen caused no diminution in the productive- 

 ness of the soil. If this turned out to be general it would 

 show that the really important nitrogen reserves are in the 

 insoluble part, as is not unlikely in view of the circumstance 

 that they probably consist of protein and similar bodies which 

 do not dissolve in alkalis. 



An indirectly beneficial effect on the plant is suggested by 

 Briggs, Jensen, and McLane's observations that the percentage 

 of mottling in citrus trees of S. California is inversely related 

 to the amount of soluble humus in the soil : they therefore 

 recommend mulching with straw so that the decomposition 

 products can be washed into the soil.^ 



Kaserer ^ suggests that an important function of humus in 

 the soil, so far as micro-organisms are concerned, is to act as a 

 carrier of the numerous inorganic constituents they require. 



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 decomposes only slowly it 

 tends to accumulate in rich soils and to become rather trouble- 

 some. It can be extracted by organic solvents, e.g. toluene, 

 and obtained as a yellowish-brown mass containing 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). 



^ yourn. Ag. Research, 1916, 6, 721 ; and 1917, 9, 253. 



^ H. Kaserer, Internat. Mitt.f. Bodenkunde, 1911, i, 367-75. 



