The control of seed germination by moisture as a soil physical property

Abstract

The germination (i.e. the first emergence of the radicle) of Medicago denticulata Willd., M. confinis Koch, M. tribuloides Desr., and Juncus vaginatus R. Br. under controlled moisture conditions has been examined. Moisture regime was defined by the soil moisture suction (pF). The influences of temperature and light, which would otherwise obscure the importance of soil moisture, have been minimized by comparing the number of seeds germinated under each physical regime with the maximum number germinated for each species under identical physical conditions when suction was approximately zero (i.e. standard cotton wool germination count procedure). The differing water regimes were obtained by using methods common in soil physics – the tension plate and pressure membrane methods – the seeds being in contact with the plate or membrane kept at a constant water suction. Additionally, two soil media, were used with the Medicago spp. It was found that:

1. For all species an increase in suction produced a decrease in rate of germination until at 10 atm, germination practically ceased.
2. Medicago spp. showed a smaller decline in rate as suction increased than did J. vaginatus.
3. The moisture conditions to allow J. vaginatus to germinate at rates comparable with those for the Medicago spp. were quite restricted and corresponded to high water-tables or soils wetter than field capacity.
4. The moisture condition of the soils controlled the germination rate not only by means of the suction effect, but also in terms of hydraulic conductivity (or permeability). A drier soil having less ability to transmit water than a wetter soil reduced the rate at which water could reach the seed, with a consequent decline in the germination rate.
5. As the relationships between: (a) moisture content and suction, and (b) moisture content and hydraulic conductivity, are soil characteristics, it is necessary to define the soil moisture characteristic as well as moisture content in germination experiments.