Soil life is complex, and huge body-size changes of organisms like soil nematodes remain enigmatic along environmental gradients and across ecosystems. Such a knowledge gap is surprising, given the importance of these invertebrates for ecosystem functioning. Moreover, differences in the biological stoichiometry between terrestrial systems are still poorly understood. Within one of the most intensive ecological soil surveys worldwide ever, containing 29 552 individual records, we monitored the nematodes of 200 rural and natural areas in The Netherlands. In addition to the body length, width, and estimated mass of nematodes, this data set includes information on taxonomy, life stage, sex, feeding habit, trophic level, geographic location, sampling period, ecosystem type, soil type, and soil chemistry (pH, organic carbon, and total nitrogen and phosphorus contents). Physical, chemical, and biological information was organized over different categories regarding four soil types and three land-use types (resulting in seven combinations). Of the soil nematodes 70.8% were juveniles, 15.9% females, 5.5% males, and 7.8% Dauer larvae. Our empirical data set shows that the responses of the nematodes' body mass to a 50-fold change in the soil molar C:P ratio were as strong as the same trait responses to a four orders-of-magnitude change in the H+ concentration in the soil. Traits like body lengths are so dependent on the life stage (here, adult vs. juvenile) that they must be taken into careful account for modeling predator–prey relationships, since nematodes are well represented in all feeding levels of the soil food webs. Traits from this novel data set can be helpful in determining habitat–response relationships, predicting effects of biological stoichiometry, and understanding the dynamics of ecosystems.