i87 



supply (it carbon dioxid in the soil must he (lepended upon to disintegrate 

 and dissolve the fragments of rock ; hence great care must he used in the 

 choice of the soil covering. How much the mulching disturbs the circu- 

 lation of the air is shown by Amnion's experiments\ A\'ith 40 mm. of water 

 pressure in an hour there passed through a layer of earth 19.6 sq. cm. in 

 cross-section and 0.5 m. deep, the following amounts of air: — 



With a Grass Co\'ering. Straw Covering. Uncovered. 



1.60 1. 6.30 1. 7.32 1. 



In better aerated soils more carbon dioxid will also be produced and 

 this, in spite of its increased elimination into the air, will make itself felt in 

 an increased amount in the soil. The result of letting the soil lie fallow con- 

 sists directly in the greater production of carbon dioxid due to the action of 

 micro-organisms and to the greater decomposition of the rock debris. 



Another disadvantage of mulching is the lessened availability of the 

 precipitation for such covered soil. The amount of this disadvantage will 

 vary according to the kind of covering. It will increase with the increased 

 sponge-like substance of the covering. Riegler's- statement may serve as 

 an example of this diversity. He tested various forest litter and peat moss 

 (Sphagnum) as to permeability. Of the 500 g. of water, sprinkled daily in 

 a fine stream on the air-dry litter, the following amounts were absorbed or 

 ran throught : — 



Beech Litter Hemlock Litter Sphagnum Turf 



Ran through-absorbed. Ran throtigh-absorbed. Ran through-absorbed. 

 1st day.. .400.3 99.7 441.3 587 216.0 284.0 g. 



8th day.. .487.6 12.4 499.6 0.4 493.5 6.5 g. 



This sprinkling corresponded to 10 mm. of rain and accordingly possi- 

 bly 20 per cent, of the falling water was retained by beech litter, 12 per 

 cent, by fir and 57 per cent, by moss. The mulch was 8 cm. deep all over. 

 From Riegler's other tables it is found that, in the next 3 or 4 days, still 

 greater amounts were absorbed daily, gradually up to the 9th day the litter 

 became so saturated with moisture that almost all the water which fell upon 

 it ran off. Ten mm. of rain setting in after hot, continued dry weather, wet 

 the earth under the beech mulch only to a depth of 8 mm. ; under the fir 

 mulch, 8.8 mm. ; and under the moss, 4.3 mm. Besides this, the conditions 

 vary according to the strength with which the water falls on tlie mulch. If 

 the water, finely distributed, was sprayed on the moss cushion, 70 per cent, 

 of the given moisture was soaked up, while of the same amount of water, 

 supplied in the form of a fine running stream, only 14 per cent, was retained. 



Forests. 



The proximity of larger tracts of trees, viz., forests, must be considered 

 as a means of saving the moisture in the soil of culti\'ated land. According 



1 Biedermann's Centralbl. 1S.S0. p. 405. 



2 Forseh. auf. d. Geb. d. ■Agrikultuii)liy.sik, ISSO, 



