The poultry industry is very important in Iowa and manure is being sold for its fertilizer value but there are doubts about manure P availability for crops. Eighteen field trials with corn included manure and fertilizer treatments arranged in a complete block, split-plot design. Main-plot treatments were a control and two manure rates. Subplot treatments were 0, 25, and 50 kg fertilizer P ha^-1. Measurements were early plant growth and P uptake (V5-V6 stage), grain yield, grain P removal, and post-harvest soil-test P (STP). High N and K rates were applied across all plots. An incubation study with two soils, fertilizer, and manure from egg layers or turkeys evaluated P effects on STP under controlled conditions. Manure or fertilizer P increased early growth at 13 sites and P uptake at 15 sites, and responses were unrelated to STP or the P source. Grain yield was increased at 10 sites, where STP was < 21 mg kg^-1 Mehlich-3 P. Fertilizer P in addition to manure did not increase early growth or grain yield. Bray-P1, Mehlich-3, and Olsen tests assessed manure and fertilizer P effects on soil P similarly, but fertilizer increased water-extractable P more than manure. The studies provided inconclusive evidence for differences in P availability between fertilizer and poultry manure P sources.