Tech Challenge


Tech Challenge

Measuring the pH of a soil sample

Whether for commercial or recreational purposes, gardeners express a growing interest in taking accurate pH measurement of soil samples. The pH of soil indicates more than its alkalinity or acidity strength, it affects the relative availability of nutrients, the soil life, and the type of plants that will thrive. What is the best method to measure the pH of a soil sample? Most pH electrodes are designed to measure a liquid sample which presents a challenge.

    A: You can use almost any pH electrode to measure soil. Just insert the probe into moist soil and wait a few minutes for a measurement. It just takes longer for the readings to stabilize.
    B: Only conductivity measurements should be used in soil testing.
    C: You can use almost any pH electrode to measure soil. You will need to mix the soil with deionized (DI) water before measuring.
    D: Only pH paper (litmus paper) can be used for measuring the pH of soil.

Answers:

    A: You can use almost any pH electrode to measure soil. Just insert the probe into moist soil and wait a few minutes for a measurement. It just takes longer for the readings to stabilize.
    Incorrect. This would not give you an accurate reading. While most probes can be used for measuring soil, pH electrodes work best for aqueous samples and don’t give great readings in solids. Try again.
    B: Only conductivity measurements should be used in soil testing.
    Incorrect. Conductivity is another measurement which is helpful for soil testing, but pH is definitely measurable. Try again.
    C: You can use almost any pH electrode to measure soil. You will need to mix the soil with deionized (DI) water before measuring.
    Correct. To make a correct soil measurement you need to use the following procedure.

    a. Weigh 20 g of soil sample into a 100 mL beaker.

    b. Add 20 mL of deionized (DI) water and place on a stirrer to mix for 30 minutes.

    c. Cover and let stand for an hour.

    d. For the most accurate measurements, allow the buffers and the soil sample both to come to room temperature. (A difference in temperature will add error to your measurement.)

    e. We recommend a 2-point calibration with a pH 7 and a pH 10 buffer solution. The electrode slope should be between 92 and 102%.

    f. Rinse electrode and ATC (automatic temperature compensation) probe with DI water and blot dry. Place probes in soil sample and measure pH and record measurement.

D: Only pH paper (litmus paper) can be used for measuring the pH of soil.

Incorrect. Litmus paper also can only measure aqueous samples. Standard pH electrodes will work for measuring soil and give a much more precise and accurate reading to litmus paper. Try again.

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Using a peristaltic pump for irrigation

Using a venturi chlorinator injector pump is common for an irrigation feed application, but Ronnie went with a peristaltic (or tubing) pump instead. Which of the following are the advantages with using a peristaltic pump such as Masterflex®? There is more than one correct answer.
    A: Reduced pulsation
    B: Accurate flow metering
    C: High viscosity
    D: Easy cleanup
    E: Low maintenance

Answers:

    A: Reduced pulsation.
    Incorrect. This is one of the few obstacles when using a peristaltic pump. A peristaltic pump works by spinning a rotor with multiple rollers over a tube. Each roller causes a pocket of fluid to move through the tubing and each pocket creates a pulse of flow. For many applications including this one, pulsation is not a factor. That being said, there are still ways to dampen or remove the pulsation with a peristaltic pump. See full article at coleparmer.com/techinfo for details.
    B: Accurate flow metering.
    Correct. The peristaltic pump used allows for calibration and accurate control of the flow rate. It also makes it easy to vary the flow of the chemical you are injecting with the press of a few buttons or the turn of a dial. Venturi pumps require use of a needle or ball valve to reduce or choke off the flow. This isn’t as easy to control accurately and can waste a great deal of the chemical you are injecting. See if you can find the other two correct answers.
    C: High viscosity.
    Correct. Peristaltic pumps have a great suction lift-ability as well as the ability to pump high viscosity fluids. There is a drop off in flow rate which usually means you need to move to a larger tubing size or a larger pump. Venturi pumps are typically only used for fluids with water-like viscosity. See if you can find the other correct answers.
    D: Easy cleanup.
    Correct. The only material that comes into contact with the fluid moving through a peristaltic pump is the tubing. This makes cleaning out the pump extremely easy—simply replace the tubing. Ronnie is also using the Easy-Load® style of pump head, which makes changing out the tubing even faster. See if you can find the other correct answers.
    E: Low maintenance.
    Incorrect. A venturi pump requires almost no maintenance except for the valves required to use it. A Masterflex® peristaltic pump only requires that the tubing be replaced periodically.