Vegetational development in areas covered by volcanic ash from the Paricutin Volcano shows striking differences among types of plant cover at the time of the eruption. Forests affected by deposition of ash recovered soon after the eruption (1943–1952), but areas lacking vegetation at the time of the eruption today lack a closed canopy. We tested the effect of adding organic matter, straw, and a combination of organic matter and straw on recruitment of seedlings in a sand deposit formed in agricultural fields covered by ash from the Paricutin Volcano in Michoacán, Mexico. Number of seedlings varied significantly among treatments. At the end of the rainy season, plots with organic matter showed the highest number of seedlings (154 ± 20 seedlings/m²) followed by plots with organic matter and straw (82 ± 7 seedlings/m²), plots with straw (68 ± 14 seedlings/m²), and control plots (21 ± 13 seedlings/m²). In contrast, plots with organic matter and straw accumulated significantly more biomass (61.7 ± 10.8 g/m²) than organic-matter plots (14.5 ± 3.1 g/m²) or mulched plots (9.9 ± 3.1 g/m²). Clustering of experimental plots indicated that species composition of seedlings differed between plots with organic matter and straw, and plots with straw or organic matter. Our results suggest that the nature of organic matter that accumulates in soil can have a significant effect on composition of the seedling community, and support Eggler's hypothesis that lack of organic matter prevents development of vegetation in sandy substrates of volcanic origin.