290 MARGARET S.JARVIS 



THE RELATION OF GROWTH TO SOIL MOISTURE 



REGIME 



Method. In this series of experiments the plants were grown at three 

 controlled soil moisture regimes. In the first the soil v/as kept watered so 

 that measurable soil moisture tensions did not develop; in the second and 

 third the soil was allowed to dry out to pre-deter mined levels of soil 

 moisture tension (SMT) before watering took place. Results, from field 

 experiments particularly, on the relation of growth of plants to soil mois- 

 ture conditions, have often been confused by the difficulties involved in 

 obtaining an integrated value of the soil moisture tension in the soil 

 actually exploited by the root system, and also by differences in the rate of 

 extension of the root system into regions of moister soil. In order to control 

 these factors it was considered essential that the plants used in these experi- 

 ments should be grown in containers, so that there would be uniform 

 exploitation of the whole enclosed volume of soil by the root systems and 

 consequent even drying-out. The containers were circular polythene 

 bowls, 30 cm diameter, 13 cm deep. The same type of soil was used in all 

 these experiments, and in all other experiments described in this paper. 

 This was a limestone soil, from Haydale, Derbyshire, on which all the 

 species grew well, so that nutrient and aeration factors were unlikely to 

 be limiting. 



The soil moisture tension was measured daily, using Bouyoucos plaster- 

 of-Paris electrical resistance blocks. These had been placed in the centre of 

 the volume of soil in each container at the time of planting. The blocks 

 were calibrated in a modified form of a Richards' pressure membrane 

 apparatus (Richards, 1941). The structure and manufacture of the blocks 

 and the apparatus used for calibration have been described in detail by 

 Jarvis, M.S. (i960). 



The experiments were carried out in the summer of 1959 in an unheated, 

 well-ventilated glasshouse. The position of the containers was altered daily 

 to minimise the effect of gradients of temperature and light. Measurements 

 were made at the beginning of the experiment and at intervals, for ten to 

 twelve weeks. Total leaf area was used as the most suitable estimate of 

 growth which could be made on species of different growth forms without 

 destroying the plants. The length of each lenfofFilipendula vulgaris, and the 

 length and breadth of each leaf of Thelycrania sangninea and Pmmis padus 

 were measured and the leaf areas were calculated from relationships 

 established from samples of leaves taken at the end of the experiment. The 

 projected area of each plant o£ Saxifraga hypnoides was measured. The num- 



