Plant-soil feedbacks (PSFs) contain two steps: (a) different plant species must create different soils and (b) these different soils must alter subsequent plant growth. Two-phase experiments are used to demonstrate PSF. In Phase I, soils are cultivated by known plants. In Phase II, plants are used as phytometers to measure soil differences created in Phase I. Ideally, a third phase, Phase III, is performed in which soil analyses and inoculation experiments are used to identify mechanisms of PSFs. A review of the literature reveals 25 studies that performed phases I and II. Of these 25 studies, 22 used grassland species, 21 were performed in a greenhouse, 19 were published between 2004 and 2006, and 12 used non-native species. Across studies, 271 feedback values were reported. Negative feedbacks represented 147 of the 271 values and produced, on average, a 37% reduction in plant biomass. Positive feedbacks represented 41 tests and produced, on average, a 43% increase in biomass.