other hand, this condition also retards working the soil and planting the fields, 

 thus becoming a cause of poor harvests. Especial consideration should be 

 given to the fact that, for all our cultivated plants, the usual planting time 

 has been determined by observing the behavior of the plants in our climate. 

 It can be shown at any time that variation in the periods of cultivation pro- 

 duces changes in the character of the plants (the change from winter to 

 summer grain). Such a delay of the seeding time often acts injuriously, 

 as, for example, in peas. The same seed that furnishes a fine crop of healthy 

 plants, when sown early in spring, very often produces low plants with 

 small pods, greatly injured by mildew, if sown in summer. Kohlrabi, planted 

 too late in spring, easily become woody, etc. 



Similar phenomena may be observed in fine sandy heath soils (loose 

 loam). Grabner^ characterizes this form of soil as consisting of sand grains 

 almost as fine as flour with only small clay admixtures. The whole mass 

 when wet looks like loam. In a dr}^ condition, however, it may be dis- 

 tinguished from loam proper by its porosity. Thus, as a result of its very 

 fine granular structure, it can become as hard as stone. In places which 

 are cultivated constantly and kept loose by means of animal manure, such 

 soil is often valuable but in forestry it is not, for, after the usual single 

 loosening, the fine sand is at once packed together by rain and too little 

 oxygen from the air can get to the roots of the trees. 



The Covering of Soil with Silt. 



In heavy rain storms and floods soils with a large content of very finely 

 broken particles are washed together and, after the evaporation of the water, 

 are left in the form of a thick, close crust. The moisture holding capacity 

 of a soil increases with the fineness of its pulverization, as has been men- 

 tioned above. Increased pulverization of the particles deepens the upper 

 surface and the power for retaining water depends on surface attraction. 

 By pulverizing a soil mass, consisting of coarse pieces of quartz from i to 

 27 mm. in size, which had an absolute saturation capacity of 7 per cent., the 

 capillary absorptive power was so increased that a fine sand produced from 

 the quartz, the size of its grains being 0.3 mm., held back more than 6 times 

 as much water. One sees that under certain circumstances the kind of 

 mineral may be unimportant and only the mechanical constitution of value ; 

 that, therefore, even quartz dust can assume the role of clay. Naturally 

 this dustlike sand has no coherance whatever, and can therefore never in 

 itself take over the role of a binding substance such as clay. Principally, 

 however, it is clay soils which sufifer from erosion in the form of silt and. 

 by making air tight layers, cause the decay of seeds and plant roots. At 

 times the roots form accessory organs in order to find the necessary air in 

 marshy soils. In this connection, attention should be called to the knee-like 

 outgrowths of roots which struggle to the upper surface of the soil, as those 



1 Grabner, Handbuch der Heidekultui', 1904, p. 200. 



