to Matthieu's^ observations, extending over ii years, the air in forests, 

 1.5 m. above the soil, is on an average colder than above bare ground, the 

 difference being the greatest in summer. The forests exert the same de- 

 pressing influence on the mean air temperature as they do on the temperature 

 extremes, which are less in forests. When the temperature differences 

 amount perhaps to only o.5°C., they are perceptible when a rain cloud passes 

 over the region. Air will become saturated above the forest sooner than 

 above uncovered land. Thereby the rain will begin sooner and be more 

 abundant than on the land which is not forested. In fact measurements of 

 Matthieu and Fautrat- prove greater amounts of rain above forests. Hygro- 

 metric determinations have shown that the weight of water vapor in one 

 cubic meter of air above a spruce forest amounted, on an average, to 8.66 g., 

 while above forests of deciduous trees it amounted to 8.46 g. ; above un- 

 covered soil at the same height (104 to 122 m. high), at a horizontal distance 

 of 100 m. from the conifer forest, to 7.39 g. ; at the same horizontal distance 

 from the deciduous trees, to 8.04 g. Thus the proximity of the forest in- 

 fluences the moisture vertically and may also exert the same influence 

 horizontally. 



Fallow Land. 



"Fallow Land" has less effect on the retention or increase of the water 

 supply in the soil than on the accumulation of nutritive substances. Accord- 

 ing to Wollny's^ statements, the peculiarities of fallow land may be sum- 

 marized as follows : — Soil lying fallow is warmer in summer and colder in 

 winter. Fluctuations of temperature are greater ever}'where in fallow land 

 than in soil overgrown with plants. During the time of growth the soil 

 covered by plants has always a lesser water content than when lying fallow. 

 This greater moisture content is retained in bare soil even when worked 

 more frequently. Bare soil also gains more from atmospheric precipitation 

 since, during the time of growth, considerably larger amounts of water per- 

 colate through soil lying fallow, than in fields provided with a growing 

 plant covering. From the standpoint of nutrition the carbon dioxid con- 

 tent of fallow land is most noteworthy. WoUny's researches show that the 

 air in fallow soil contains approximately 4 times as much carbon dioxid as 

 in grass land. Therefore, the means for the solution of mineral elements in the 

 soil are present much more abundantly ; which explains in part the greater 

 accumulation of nutritive substances in fallow land. This greater enrich- 

 ment also depends partially on the quicker decomposition of the organic 

 substances because of the greater temperature fluctuations, the increased 

 moisture and the more vigorous activity of the micro-organisms. It should, 

 however, be pointed out finally that soils with less power for holding water 

 and in greater depths (sandy soils) with their greater permeability lose 



1 Matthieu, Met^orologie comparec agricole ct forestigre. Paris 1878; cit. in 

 Forschungen auf d. Gob. d. Agrikulturphysik 1879, pp. 422-429. 



- Fautrat. Ueber den Einfluss der Walder, den sie beriihrenden Regenfall 

 und die Anziehung der Wasserdampfe durch die Fichten. Aus Compt. rend. 1879, 

 Vol. 89, No. 24; cit. Biedermann's Centralbl. f. Agrikulturchemie. 1880, p. 241. 



3 WoUny, Die Wirkung- der Brache. Allgem. Hopfenzeitung 1879, Nos. 55, 56. 



