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Soil Macronutrients

Soil Macronutrients Kit – NPK/pH tests (LaMotte Model AM 31)

Overview: Soil may be defined as the naturally deposited unconsolidated material that covers the earth’s surface, whose chemical, physical, and biological properties are capable of supporting plant growth. This kit tests for the presence of nitrogen, phosphorus, potassium and pH levels in soil - all of which are major essential nutrient elements that are required in varying proportions by plants - as well as the easy-to-use  soil sampling tube, soil sample bags,  and  the Soil Texture Unit.

Requirements: Representative soil samples can be taken in soil texture of any type. Samples must be returned to the classroom and air-dried in preparation for further testing.

Directions: The following directions describe how to take a soil sample and measure its texture, adapted from LaMotte instructions. For directions on the individual macronutrient and pH kits, please refer to the manuals enclosed.

Taking the Sample:

1.       Holding the tube in a vertical position, force it into the soil approximately six to nine inches. Leave about one inch of the cutaway portion of the of the tube showing above the surface of the soil.

2.       Twist the tube back and forth to sever the core.

3.       Continuing to twist the tube back and forth with a rapid motion, lean the tube toward you with the cutaway portion facing up and slowly withdraw the tube from the soil.

4.       Remove the soil sample through the cutaway portion. For samples needed at a specific depth, simply measure downward from the top of the soil core.

5.       Use a clean utensil to dislodge soil remaining in the tip of the sampler.

Preparing the Sample:

1.       Spread wet or damp samples on a paper plate or sheet of paper that has been marked according to the number of the sample. Soil lumps should be broken with a spatula and the soil left to dry at room temperature.

2.       When the soil is dry enough to easily sift through the fingers, it should be screened through a wire screen mesh, to remove pebbles and foreign matter.

Measuring Soil Texture (Sand, Silt, and Clay)

1.       Label the Separation tubes “A,” “B,” and “C,” and place in the rack. Add the soil sample to tube “A” until it is even with line 15, making sure to tap the bottom of the tube to eliminate air spaces.

2.       Use the pipet to add 1 mL of Texture Dispersing Reagent to the sample. Dilute to line 45 with tap water. Cap and gently shake for two minutes, making sure all the soil sample is thoroughly mixed. Be sure to shake gently shake the separation tube up to the time of the first separation.

3.       Place tube A back in the rack. Allow to stand undisturbed for exactly 30 seconds.

4.       Carefully pour off all the solution into Soil Separation Tube “B”. Return tube “A” to th rack. Allow Tube B to stand undisturbed for 30 minutes. Then, pour off the solution from “B” into “C.” (Read manual if further experiments necessary on clay suspension in C)

5.       Sand: Read tube “A” volume at top of soil level. Divide reading by 15, multiply by 100, and record as % sand.

6.       Silt: Read tube “B” at top of soil level. Divide reading by 15, multiply by 100, record as % silt.

7.       Clay: Subtract “A” and “B” totals from initial volume (15), multiply by 100, record as % clay.