Soil is an effective environmental filter and helps purify water as a three-way filter: physical, chemical and biological. As a physical filter, sand grains have rough edges which help capture wastes and large suspended in water are too big to pass through the small pores in soil.
Updated: 8 November 2024
- Physical filter – sand grains have rough edges which help capture wastes and large suspended in water are too big to pass through the small pores in soil.
- Chemical filter – soil almost always has a negative charge due to the type of clay minerals it contains. This allows soil to adsorb cations (particles with positive charges) and remove them from the soil solution (water and everything dissolved in it).
- Biological filter – billions of microorganisms live in the soil -and they are all hungry! The primary role of microorganisms in the soil it to decompose organic compounds – materials that come from things that used to be alive. Wastewater is filled with organic compounds, and the organisms eat (decompose) the compounds as they pass through the soil.
This simple experiment can be used to demonstrate the physical and chemical filtering properties of soil when grape Koolaid and “floaties” (large particles) are poured through different soils.
I always use simple equipment for those who do not have a fancy lab with all the bells and whistles and for those who want to try it at home. The write-up suggests using 3-oz and 5-oz paper cups and toothpicks.
This is a great activity to teach the scientific method: asking a question, making a hypothesis, gathering data and making observations, determining if the hypothesis was correct, and discussing the science behind the outcome.
I first posted this activity more than twenty years ago on Dr. Dirt’s K-12 Teaching and Learning Activities. Through my work with the Soil Science Society of America K-12 Outreach Committee, it has been refined and demonstrated at the National Science Teacher’s Association conference and in teacher workshops as ready-to-go lesson plans. These documents will help you get started and be prepared to answer questions you or your students may have.
Soil is Charged! is a great companion activity to apply deductive reasoning to determine the charge on clays and then the charge on the red and blue dyes.
Grape Koolaid is purple because it is an organic salt – a red anion and a blue cation bonded to each other like sodium chloride in table salt. When the Koolaid is added to the water, it dissolves and the red and blue dyes in the water cause it to appear purple. When passed through soil, the most common result is for the blue to remain in the soil and the red to pass through, because opposite charges (soil and blue dye) attract and like charges (soil and red dye) repel on another. But, as you can see below, sometimes the result is different.
Some types of clay minerals can switch charges in acid soils so that they have a positive charge instead of a negative one. That is why the last cup in the right photo has blue water instead of red as you see in the photo on the left.
Purple water comes through sands mostly unchanged since sands are essentially inert, chemically nonreactive.
À une époque où la confiance numérique est primordiale, lucky 31 se positionne comme un espace de jeu transparent et fiable. Un programme de fidélité bien structuré permet aux joueurs réguliers de bénéficier d’avantages exclusifs. La plateforme propose régulièrement des défis et missions qui ajoutent du piment aux sessions de jeu. La bibliothèque de jeux dépasse souvent les mille titres, offrant un choix quasi illimité aux joueurs. L’avenir s’annonce prometteur pour cette plateforme qui ne cesse d’innover.
Brownish colors are not uncommon in high organic matter soils or compost because the organic acids dissolve and leach – much like the color of coffee or tea.
Almost clear water can result from an organic-rich loam soil.
The photos below show two reasons water may have color: solution and suspension.
Solution: Solutions are composed of free ions – anions (negatively charged) and cations (positively charged) – in the water. They are not particles, so they do not fall. Water in the soil is never pure, it is always a solution. The most common solution is salt water; put salt crystals (sodium chloride, NaCl) in water and stir, and they dissolve into water, sodium cations (Na+) and chloride anions (Cl–). In the beakers below, a purple solution (grape Koolaid) was poured through different soils, resulting in the colored solutions.
Suspension : The Red River in Texas is red due to the red soil particles “floating” (suspended) in the water. Sands (0.05 to 2.00 mm) fall out of suspension within 40 seconds, but I have witnessed clays still remaining in suspension after more than a year. Mud is essentially a very thick suspension of soil particles in water. A slurry has enough water that it will flow.
Updated: 8 November 2024