SOIL SCIENCE 109
Homework 3 Questions
Spring 1998



1.   Assume you have a 10 hectare field and 35% of the cation exchange capacity of the 25 cm thick A horizon is occupied by sodium (Na+). You wish to remove enough Na, using CaSO₄ so that only 2% of the exchange sites are occupied by Na. Assume the bulk density of the A horizon is 1.2 Mg/m³ and the cation exchange capacity is 375 mmol_c/ kg of soil. What is the minimum amount of CaSO₄ needed to remove the Na. Some helpful numbers: One hectare is 100 meters by 100 meters, the molecular weight of CaSO₄ is 136. Assume the CaSO₄ is 100% pure.







2.   You have been called upon to recommend how much crushed limestone (CaCO₃) to add to the A horizon of a soil in a 20 ha field to reduce the exchangeable acidity by 10%. The 15 cm thick A horizon has an exchange capacity of 300 mmol_c/kg of soil. The exchangeable acidity is 25% of the exchange capacity. The bulk density of the A horizon is 1.3 Mg/m³. What is the minimum amount of limestone you recommend to add? What assumptions have you made about the limestone?







3.   The results of a CEC analysis are 300 ppm NH4+/L using 200 mL of extractant and 20 g of soil. Additional analysis of the extrantant reveals 20 ppm Na. Calculate the Exchangeable sodium percentage of this soil. What can you tell me about these soils, such as soil characteristics and kinds of crops to grow?







4.   . Calculate the available N (mineralized or immobilized) from the following data. A field has 4000 kg residue with 45% C and 0.7% N. Previous studies on this soil indicate that the soil biomass has a C to N ratio of 6 to 1 and a substrate use efficiency of 30%. The residue is plowed into a vineyard soil in the spring. What is the N availability to the grape vine.