Nematode activity in the soil depends on the presence of free water. We conducted pressure plate experiments to understand better how soil matric potential and structural degradation affect the population growth of three bacterial-feeding nematodes (Cephalobus, Pristionchus, Rhabditis). We took undisturbed cores from six soils (sand, silt loam and silty clay loam with four management regimes), and removed all fauna from them. Ten or 30 nematodes were added, and pressures corresponding to soil matric potentials of −10, −33, −50, −100 or −1500 kPa were applied for 35 days. The nematodes were then counted. Significant reproduction of all bacterial-feeding nematodes occurred when the diameters of water-filled pores were approximately 1 μm. This confirms observations using repacked soils and field manipulations. Only for Pristionchus did declining populations match the reduction in total soil porosity related to intensification of land use on the silty clay loam. We had not expected Cephalobus to have the fastest increase in population of the three nematodes in intact soil cores, and our evidence questions the relative importance given to the three nematode families in soil processes. The differing rates of population increase of the three nematodes in the various soils reflect both habitable pore space and trophic interactions. This suggests that the very diversity of nematode assemblages is crucial in the resilience of biological soil processes. That water-filled pores as small as 1 μm provide suitable spaces for sizeable populations of bacterial-feeding nematodes accords with the observed migration of infective juveniles of trichostrongylid nematodes and mermithids in water films on herbage. Our results imply that assessment of the role of nematodes in soil processes may be a key to the understanding of biological interactions in water films, and the selection pressures on nematode morphology.