Soil Nutrient Management
SOIL 4234

Soil Nutrient Management

Basic Chemistry Lab

              Laboratory Exercise 2           
           Review of Basic Chemistry Related to Soil Nutrient Management

Overview:
Soil nutrient management is strongly related to the behavior, in the soil, of chemical elements that are plant nutrients.  Because plants absorb nutrient elements in their ionic form, except for boron, understanding the simple inorganic chemistry of them is very useful to nutrient management.  Students will work in groups of four or five.

Objectives:
In this exercise we will examine:
·         How nutrient elements differ in their solubility in chemical compounds with other ions.
·         How pH of solutions is influenced by the relative strengths of the acid and base from which the compound’s cation and anion came.
·         The pH buffer characteristics of some solutions and soils.

Materials:
1.      Chemical compounds listed in Table 1 (about 0.1 mole of each in labeled containers).
2.      Five 100-ml graduated cylinders and about fifty 120-ml clear plastic cups.
3.      Standard acid and standard base, 100 ml each of 1.0 N (1.0 milliequivalent /ml) strength, which can be dispensed in 1.0 ml volumes (disposable pipettes).
4.      About 250 g each of a sandy acid, clayey acid, sandy alkaline, and clayey alkaline soils.
5.      Analytical balance(s) for weighing to 0.01 g and a standardized pH meter(s).
6.      Stirring rod, weighing paper, spatula, and distilled or de-ionized water.

 Procedure:

         The instructor will identify five groups of students.  Each group will be responsible for performing the activities listed below and completing the exercise assignments.  Results from each group will be shared by recording information in a posted version of Table 1.

 Exactly 0.01 mole of each of the assigned compounds and 15 g of the soils, listed in Table 1, will be available in labeled plastic cups.

 1.      Add 10-ml of distilled water to each cup containing a chemical compound, intermittently stir or swirl the contents until it dissolves or 5 minutes have passed.  Note, in the appropriate column of Table 1, if the compound dissolves (S = dissolves; I = did not all dissolve). 

2.      Add an additional 90-ml of distilled water to each of the cups containing chemical compounds. For compounds that did not initially dissolve, continue to stir or swirl again until it dissolves or 5 minutes have passed.  Again note whether or not the compound dissolved. 

3.      Measure the solution pH for each of the compounds and record the result in Table 1.

4.      Add 15 ml of distilled water to each of the soils, stir intermittently, wait about 5 minutes, measure and record the pH in Table 1.

5.      Add 1 ml of standard acid to solutions (chemical compounds and soils) that were basic in step 3 and 4, and 1 ml of standard base if the solution or soil was acidic.  Stir, wait a minute or two, measure and record the pH.  Identify the buffer capacity of the mixture as low (L = pH change 5 or more units), medium (M = 2 to 5 units) or high (H = less than 2 units).

6.      Write the cation and anion (include charges) of each compound in the last two columns of Table 1.

 Table 1. Chemical characteristics of compounds common to soil nutrient management.

*pH after all water has been added.
**pH measured 5 minutes after addition of acid or base.

 Results and Discussion:

1.      The most insoluble compound in water is _____________.  The most soluble compound is ______________.

2.      Is the solubility of the compounds in Table 1 related to the charge of ions that make up the compound? _____________

·   Identify and contrast two or more compounds from Table 1 that support your answer.

3.      The most acidic solution was found for the compound _______________, which could be formed from reaction of the acid _________________ with the base _________________.

4.      The most basic solution was found for the compound _________________, which could be formed from reaction of the acid _________________ with the base ________________.

5.      A compound (other than water) that is weakly buffered against pH change is ____________, and one that is strongly buffered is _______________.

6.      The soil that is most strongly buffered against pH change is the ________________ soil.  The acid soil that is most strongly buffered against pH change is the _______________ soil. 

Relative information

Nutrient uptake
·         Mass flow: driven by evatraspiration
·         Diffusion: due to ion movement from area of high to area of low concentration
·         Root interception: root surface contact with nutrient ions.

Soil nutrient availability to plants is governed by
·         solubility
·         ionic charge
·         ionic absorption
·         biological immobilization

Solubility
·         a chemical property referring to the ability for a substance (the solute), to dissolve in a solvent.
·         measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium.
·         Solubility of compounds and availability of soil nutrient to plants can be discussed as the following reaction: Aⁿ⁺ + B^m- =A_mB_n.
pH: a measure of acidity/alkalinity of a solution.

Unlike weakly buffered solutions, the strongly buffered solutions are resistant to small additions of a strong acid/base.

Acid/base reaction: acid (donates a proton) and a base (accept a proton) react to form a salt and water. During the reaction, a proton is donated by the acid to the base to yield water. The remaining ions form a salt.