Exam on Soil Fertility Management | SOCR 350, Exams of Soil Mechanics and Foundations

Download Exam on Soil Fertility Management | SOCR 350 and more Exams Soil Mechanics and Foundations in PDF only on Docsity! Name_____________________________ Soil Fertility Management SOCR 350 / Exam 2 November 5, 2007 Circle the underlined correct answer(s) on multiple choice or write the answer in the blank where appropriate. The number in parenthesis indicates the points for each question. Show work on problems. GIVEN: 1% O.M. supplied 30 lb. N/A Legume contributions: Manure contains 10 lb. N/T Alfalfa - 50 lb. N/A 2.7 x ppm = lb/A Ft. H2O Beans - 30 lb. N/A Alfalfa Grass - 30 lb. N/A ppm x 3.6 = lb. NO3-N/foot/acre ____________________________________________________________________________________ 1. (6) There are six forms of inorganic N that are found in the soil. a. Name four of them using their correct chemical formula. b. Which of the six forms are taken up by the plant? 2. (15) The following processes control N turnover in the N cycle. Define the following processes in words and write the letters that correspond to the reactions on the next page that chemically describe the process in the correct order in the blanks to the right. (There may be extra blanks.) Chemical reaction order a. Mineralization: _____ _____ _____ b. Dentrification: _____ _____ _____ c. Ammonification: _____ _____ _____ d. Urea hydrolysis: _____ _____ _____ 3. (5) Why are the concentrations of organic P higher in the surface soil then in the subsoil? 4. (8) In classifying forms of P for environmental evaluations, we classify various P forms as “environmentally” active. a. Biologically available P is measured by extracting the soil with 0.1N NaOH (500:1/NaOH:Soil). What environmentally active form of P in the soil does this measure? 8. (12) You have the following soil chemical analysis of a field you want to grow “giant pumpkins” next year. Your yield goal is 5T/A. Giant pumpkins require 75 lb N/T to grow the big ones. The rooting depth is 3 feet. What will be your N recommendation under the following management. Apply 10 T manure/A (50% available next year) Soil 0.M=1.5% ET=28 inches Irrigation water contains 8ppm NO3-N (assume credit for 85%) Soil pH=7.4 P=18ppm K=375ppm Previous crop=Alfalfa Zn=2ppm Residual Soil NO3-N 0-1 ft = 12 ppm 1-2 ft = 8 ppm 2-3 ft = 6 pm 3-4 ft = 10 ppm 9. (8) There are two soil extractants used to determine the P soil test level of soils, the Bray and NaHCO3 extractants. a. Why is it necessary to have two different soil extractants? b. Describe the general chemical reaction of NaHCO3 extractant. c. Describe the general chemical reactions of the Bray extractant. 10. (15) Using the solubility diagram and the given chemical reactions (below), answer the following questions. (Some of the sentences below may have two underlined words, numbers, or chemical equations. Circle the one you would select as the correct choice in the sentence.) Given: 1. AlPO4 ● 2H2O + H2O <-----> Al (OH)3 + H + + H2PO4 - 2. HPO4 2- + H + <-----> H2PO4 - 3. CaHPO4 ● 2H2O + CO2(g) + H2O <-----> CaCO3 + HPO4 2- + 2H + + H2O 4. CaHPO4 ● 2H2O + H + <-----> Ca 2+ + H2PO4 - + H2O 5. None of the above a. DCPD is the dominant precipitated P form in soils with a pH greater than _____________. b. Strangite would provide the most P to a plant in which soil? pH 5.0 or pH 6.0? If I lime this soil from pH 5 to 6, reaction _________________________ is occurring and it is going to the left, right? c. As the soil pH changes from 7.2 to 6.2, the H2PO4 - + HPO4 - concentration increases, decreases ___________________________________ times. This change is described by reaction ________________________(see above reactions) which would be going to the right, left. d. Reaction ____________________ controls P solubility of pH range 6.2 to 7.2. If I add S 0 to the soil, the reaction would be going to the left, right. e. What would be the concentration of H2PO4 - + HPO4 - in soil solution in equilibria with DCPD at pH 7.4? ________________________________. f. In soils that have a pH>7.8, the attached solubility diagram shows that P solubility increases as the soil becomes more alkaline. Reaction _____________________________ describes this process. However, the P availability to the plant actually decreases in soils with pH>7.8. Why?