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THE ALUMNI JOURNAL. 



complex, form on decay, a mixture of 

 the different kinds of soil. But when 

 they are not complex the soil sometimes 

 requires the addition of the materials in 

 which it is wanting. A quartz soil re- 

 quires the addition of limestone and fels- 

 pars in order to make it fertile. Clays 

 require the addition of lime. Shales and 

 sandstones require lime or gypsum. 



We also hear much of the "virgin 

 soil " of the West and the " spent soil " 

 of New England and "old fields" of 

 Virginia. The latter is spent and old 

 because the constituents (principally the 

 compounds of nitrogen, potassium and 

 phosphorus) that have been removed 

 from the soil in the process of agriculture, 

 have never been replaced. This condi- 

 tion is met to-day by inducing a proper 

 rotation of crops and by the addition of 

 fertilizers, manures, bones, guano, com- 

 mercial phosphates, gypsum, cretaceous 

 greensands and marls. 



Soils are produced not only by the dis- 

 integration of rocks, but also by the de- 

 cay of animal and vegetable matter 

 which latter forms " humus " and mixes 

 with the minerals from the rocks. The 

 disintegration of rocks is produced by the 

 action of the atmosphere and in climates 

 exposed to varying amounts of humidity 

 and sudden changes of heat and cold, 

 there results an unequal cohesion and 

 breaking up of the rocks. This influence 

 is noticeably present in the splintering 

 and peeling off of pieces of the Obelisk 

 in Central Park in New York City. This 

 disintegration of the rocks is further in- 

 fluenced by the plants themselves, as may 

 be seen in our mountains at all times. 

 The roots of plants secrete from their 

 hairs an acid which corrodes the mineral. 

 In this manner is produced a soil consist- 

 ing of a fine scaffolding of stone particles 

 between which the humus is scattered. 

 It is the humus that assists the soil in 

 absorbing liquids and gases, and in ex- 



amination of humus will reveal on the 

 surface a film of water, called by Sachs 

 " hygroscopic water." 



The character of food required by the 

 plant has been ascertained by synthetic 

 experiments. Water cultures have 

 shown that a mixture of salts, amount- 

 ing to 0.17 per cent, of the water is suf- 

 ficient for the plant. The plant derives 

 its food from the air or the soil, or 

 both (sometimes from water). It 

 was formerly supposed that all of the 

 carbon used by plants containing chloro- 

 phyll comes from the atmospheres as C0 2 

 (that of fungi comes from organic mat- 

 ter), notwithstanding that the humus 

 contains 300 times as much as is found 

 in the air. The question of the source of 

 the nitrogen taken up by the plant, is 

 still unsettled. In cereals there is a 

 direct ratio between the soil and the 

 nitrogen thev contain ; hence it is sup- 

 posed that they derive their nitrogen 

 from the soil. In the Leguminosae there 

 is a marked difference observed between 

 the nitrogen in the soil and that which 

 they contain, there being an excess of 

 nitrogen in the plants over that in the 

 soil. Recent investigations have shown 

 that they are collectors of nitrogen from 

 the air. Their roots are found to con- 

 tain tubercles which have been variously 

 pronounced as excresceces and organs of 

 the plant. Frank found, in these 

 tubercles, bacteria which have been 

 since studied and are known as nitrogen 

 bacteria, and possess the ability to change 

 N and NH 3 into nitrites and nitrates. 



I. Carbon represents about 50 percent, 

 of the weight of the dried plant ; in plant 

 constituents it varies from 26 66 percent, 

 in oxalic acid, to 51 47 percent, in legu- 

 min and is as high as 92.31 per cent, in 

 styrol. The form in which it is absorbed 

 depends upon the plant. (1) Plants that 

 possess chlorophyll use the CO, of the 

 atmosphere when the sun shines. (2) 



