136 



THE GABDENEK'S ASSISTANT. 



and cling together so closely that few, if any, 

 open spaces are left between them for the ad- 

 mission of water. 



The figures in column 1 of the next table 

 represent the percentage amounts of water that 

 were imbibed by the soils which had been 

 screened to a tolerably uniform state of moderate 

 fineness, while in column 2 are given the per- 

 centage amounts of water imbibed by the same 

 soils after they had been finely pulverized. It 

 will be noticed that there is but little difference 

 between the second column and the first in the 

 case of soils which are naturally porous. Other 

 experiments have confirmed these results by 

 Zenger. Thus Wilhelm noticed, for example, 

 that a garden loam that naturally imbibed 114 

 per cent of water could absorb only 62 per cent 

 after it had been pulverized. 





Moderately 



Finely 





fine. 



pulverized 



Quartz sand, 



26 



54 



Marl, 



30 



55 



Brick-clay, 



66 



58 



Moor earth, 



105 



101 



Calcareous soil, ... 



108 



70 



Garden loam, 



123 





Soil from a moor meadow, 



178 



103 



Peaty soil, 



377 



269 



The chemical composition of a soil, though 

 far less important than its porosity, may, never- 

 theless, have some influence upon the rapidity 

 of the capillary movement. 



Naturally enough, the power of a soil to hold 

 water tends to retard evaporation from the soil. 

 From some experiments of Sachs it appears 

 that plants cannot exhaust the retentive soils 

 so completely of their water as they can the 

 soils which are non-retentive. Thus, in a loam 

 capable of holding 52 per cent of capillary 

 water, a tobacco plant wilted at night when the 

 soil contained 8 per cent of moisture. In a 

 mixture of humus and sand competent to absorb 

 46 per cent of moisture, another tobacco plant 

 wilted when the moisture had been reduced to 

 12 per cent; and in coarse sand which could 

 hold 21 per cent of moisture, a third plant 

 wilted when the proportion had fallen to H 

 per cent. Here the tobacco plant was able to 

 pump the soil almost absolutely dry. In these 

 experiments 44 per cent, 34 per cent, and 19 

 per cent of water respectively were more or i 

 less available for the plant. 



Different kinds of plants appear to resemble 

 one another more closely than might have been | 

 expected in respect to this power of exhausting 

 soils of their moisture, and the researches of 

 Hellriegel have shown that any soil can supply | 



plants with all the water they need, and as fast 

 as they need it, so long as the moisture within 

 the soil is not reduced below one-third of the 

 whole amount that it can hold. 



Humus or Vegetable Mould. — The importance 

 of humus as a source of nitrogen for crops is 

 most conspicuous when wild plants are con- 

 sidered. It is from the humus of the soil that 

 forest trees and most of the other natural plants, 

 including grasses and clovers, derive the greater 

 part of their nitrogenous food. Some nitrogen, 

 indeed, comes to the land with the rain, dew, 

 and other aqueous deposits that fall upon it. 

 But the amount of this atmospheric nitrogen 

 brought down by rain is comparatively small, 

 and is, by itself, no more than sufficient to 

 nourish a sparse vegetation, or vegetation of a 

 very low order. Some assimilable nitrogen, in 

 the form of nitrates, is found in the waters of 

 brooks also; and the plants which have access 

 to such waters profit by the nitrates that are 

 contained in them, as is shown by the results 

 obtained with irrigation. But only compara- 

 tively few plants are so situated that they can 

 be nourished by brook water; and, besides, a 

 good part of the nitrogen in such water is derived, 

 doubtless, from the oxidation of humus higher 

 up the stream. 



Humus, in the condition in which it is met 

 with in garden soils, is a mixture of remains of 

 plants in the most diverse stages of decomposi- 

 tion or decay. When roots, leaves, wood, straw, 

 or dung, lie upon or in the earth, they absorb 

 moisture and gradually become brown and soft, 

 then blackish-brown and crumbly, lastly black 

 and pulverulent, as may be very easily traced 

 in the soil covering the ground in forests. 



There are two kinds of leaf-mould in use 

 among the French gardeners, called la terre 

 de bruyere, and le terreau de feuilles. We learn 

 from Georges Truffaut that these two kinds 

 of mould are formed from decomposing plant 

 leaves and roots mixed with ordinary soil or 

 sand and ferric oxide (irony matter), and that 

 these moulds have a peculiar acid property from 

 the humic acid which they contain, which can 

 be made most beneficial for horticultural pur- 

 poses. 



The first description of leaf-mould, la terre 

 de bruyere (peat), is formed on sandy heaths by 

 the decay of the leaves and roots of Erica tetra- 

 lix, E. scojxiria, E. vagans, and frequently also 

 from E. cinerea, which flourish there, the soil 

 being held together in such a manner by the 

 roots of the growing plants as to allow of its 

 being cut into square blocks, and so despatched 



