Biotechnology by-products and animal manures have received considerable attention in recent years because of the problems associated with their disposal and because of their potential use as soil amendments and as sources of nutrients for crops. A laboratory experiment was conducted to assess the potential N mineralization in five biotechnology by-products and three animal manures added to an agricultural soil with pH values of 5, 6, and 7 and incubated at 20°C or 30°C. Two greenhouse experiments were conducted by using the same soils to evaluate those materials as sources of N for plants; corn (Zea mays L.) and ryegrass (Lolium multiflorum L.). The biotechnology by-products used were sterilized microbial biomass (SMB), slops (SS), hydrolyzable soybean oil meal (HSOM), Ajinomoto proto ferm (APF), and Heartland lysine ferm (HLF). The animal manures included dairy, chicken, and swine manures. The experimental design was a 9 x 5 x 3 factorial, with six rates (0, 100, 200, 300, 400, and 500 mg of N kg-1 of soil) of nine N sources, including urea-ammonium nitrate (UAN). All macro- and micro-nutrients were added in sufficient amounts, with N as a variable. After 40 days, corn tops were harvested, whereas ryegrass was harvested every 30 days for a total of three cuttings. The plant samples were dried at 65°C for three days, weighed, ground, and analyzed for N, P, and K. The results showed that, in general, N mineralization increased in the following order pH 7 ≥ pH 6 > pH 5. The pH effect, however, was not pronounced. The Q 10 values of N mineralization were mostly between 1.1 and 1.3. The percentages of N mineralized, potential N mineralization ( No ), and the rate constant (k) varied, depending on the organic material tested, incubation temperature, and the soil pH used. Almost at all rates of N and at an pH values, APF, HLF, and SS produced greater yield of N in corn tops than those produced by SMB and HSOM. The APF and HLF materials produced dry matter yield of ryegrass that was equal or greater than those produced with UAN. Organic N uptake of corn tops and three cuttings of ryegrass was significantly correlated with the cumulative N mineralized during 6 or 20 weeks of incubation at 20°C or 30°C, and with No and the k values. The results of some of the materials evaluated deviated from those relationships.