148 E.B.OKSBJERG 



The water content in April after a winter rich in precipitation is taken 

 to represent field capacity. Very infrequently the state of field capacity was 

 established artificially. With the available facilities the estimation of 

 permanent wilting point was undertaken by balancing the soil samples 

 against sulphuric acid in closed jars, placed in a cool dark cellar. 



The water capacity or the accessible water is a vague idea comprising 

 among others a quantity which I have called the apparent water capacity, 

 i.e. the difference between field capacity and wilting capacity in the soil 

 space occupied by the root system. The investigation led to the advancing 

 of another concept, the physiological water capacity, which means the 

 difference between field and wilting capacity in the soil space from which 

 the roots can take up all accessible water during a critical period of drought, 

 i.e. the volumes of the soil where the density of roots is so high that complete 

 utihsation occurs after a longer period of drought. 



In some cases in the following account the water capacity of the raw 

 humus layer (or better the spruce mor) is indicated in brackets after the 

 figure of the total water capacity, for exainple in locahty nr. i,Rosenholm: 

 120 (45). This arises from the assumption that a part of the water content 

 in this superficial layer is exposed to atmospheric influence and conse- 

 quently open to direct evaporation, a water loss which does not benefit 

 much the physiological processes of the stand. 



Locahty i , Rosenholm. A spruce stand planted more than a hundred 

 years ago on drift sands formed as dikes alternating with strips bared to the 

 hard pan. A larger sand drift is examined and the course of soil moisture 

 is graphed in Fig. 4. hi Table i some data of the profile are collected. 



Excluding the horizons 5 and 6 which are nearly free of roots, the apparent 

 water capacity of the given example is about 115 (45 mm) of rain. In Fig. 4 

 it appears that even durhig a drought of two months the accessible water 

 is not exhausted in the three uppermost horizons. To avoid further indis- 

 tinctness in Fig. 4, the curves of the horizons 4, 5 and 6 for the summer 1955 

 are not drawn. In these horizons the water content was reduced slightly 

 but significantly, by 15-20% of the field capacity even though the root 

 density is very low and very different. Roots occur most frequently in the 

 horizon 4, near the boundaries to horizon 3 and 5. 



The uptake of water from the soil seems to desiccate it only very 

 moderately during a 'normal' summer as that of 1953, considering the 

 balance of the soil in Fig. 4. But even in a very dry period the accessible 

 soil water is not completely evacuated from the horizons penetrated by 

 roots. Judging from the conditions of 1955 the physiological water capa- 

 city of the profile in question is assessed to be about 60 mm. 



