Phosphorus uptake rate, plant top weight, and grain yield were measured for corn (Zea maysL.) planted in field plots with or without fumigation, with different levels of P fertilization, and under conventional till and no till management. Plant growth and grain yield in P‐deficient, unfumigated conventional till and no till plots were significantly higher than those in the corresponding fumigated plots. The heightened responses were attributed to enhancement of P uptake by biological activity. Therefore, the effect of differential microblal activity on P uptake must be considered in estimating the lowest level of P fertilization needed for adequate growth in low P soils.