ing back of potatoes and pasturage, while those which had been covered 

 had sufifered no especial injury. 



This discovery indicates that we have to look for the chief period of 

 injury in spring, so far as frost phenomena in moor soils are concerned. In 

 cultivating trees this becomes clear, if we consider that the humus soils in 

 cold seasons usually contain an excess of moisture. The fine pored humus, 

 saturated with water, will cool more slowly in the fall than do soils less rich 

 in water, but will warm up much more slowly in the spring. However, the 

 longer the roots are in a warm location, the longer they remain active and 

 the more water will be forced up into the aerial axes. Trees growing poorly 

 on moor soil with its diluted nutrient solutions start the winter with a large 

 water content in their tissues. The more water the tissues contain and the 

 less cytoplasm, the more susceptible are they to frost, no matter whether the 

 effects of winter or spring frosts are concerned. Hence the frequent and 

 great injury from frost in moor pines, as is shown in the example from the 

 Liineburger moor. 



For short-lived field plants the most disasterous are the spring frosts 

 w^iich are produced in rays of cold. This may be easily recognized from the 

 fact that the phenomena of discoloration produced on the leaves and stems 

 by the cold are abruptly cut off, if such a part of the plant is partially cover- 

 ed by overlying leaves. 



It is now pertinent to ask when cold, due to radiation, will be greatest 

 and how much of it is due to evaporation. If both factors become effective 

 to a high degree, the air layers close above the surface of the soil will be 

 noticeably colder than the average temperature. Polis^ has proved such a 

 lowering of the temperature of the air layers above a covering of snow. This 

 will be the greater, the less the movement of the air. Hence May frosts in 

 still, clear nights. The moor soils and those bordering on moors with their 

 wealth of water wall evaporate strongly in the early spring when the soil and 

 subsoil have not been warmed through, even if, as cultivated land, they have 

 been mixed with sand and accordingly more cooled down. Evaporation will 

 also be still more increased by the dark color of the soil, as Wollny's- experi- 

 ments show. Covering with a layer of sand from 6 to lo cm. deep acts as a 

 preventive. Then but little water can reach the sand from the humus layer 

 and, correspondingly, only small amounts will be evaporated. For the same 

 reason the dead layer also acts as a protection against drought. One dis- 

 .^dvantage of the sand covering is found when fine, surface-rooting grasses, 

 are sown which are easily stunted in sand, poor in nutrition^. 



If the cultivation of fruit trees on moor soils is involved, the following 

 may be recommended for protection against frost: (i) The planting 

 of trees on the west and southwest side of the orchard, in order to modify 

 the temperature differences in spring. The bark cracks almost without ex- 



1 Meteorologische Zeitschr. 1896, Part I. 



^ Blatter fiir Zuckerriibenbau, 1899, No. 9. 



3 Mitteil. d. Ver. z. Ford. d. Moorkultur, 1895, Nos. 5 and 6. 



