Increasingly, researchers are expected to make their research data openly available. However, scientific fields differ in their research practices and norms for sharing research data publicly. We provide quantitative evidence of differences in data practices and the public sharing of research data at a granularity of field-specificity that is rarely reported in open data surveys. Based on a survey of 8,822 researchers at German Universities, we find considerable variation, within and across disciplines, of data practices and rates of open data sharing. For experimentally oriented subject areas we further observe a relationship between data self-sufficiency and public data sharing which likely reflects a link between data sharing and the epistemic specificity of data. Our findings underline that in order to quantitively assess and evaluate rates of public data sharing, a better understanding of the embedding of public data sharing into field-specific research practices is needed.