Plants require at least 14 essential elements, or essential mineral nutrients, that they usually absorb via their roots from the soil solution. The six macronutrients, needed in fairly large amounts, are nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. The eight micronutrients, needed in much smaller amounts, are iron, boron, manganese, zinc, copper, molybdenum, chlorine and nickel. No natural deficiencies of chlorine and nickel have been seen because those two elements are relatively abundant in soils relative to plant needs. Potted plants are often irrigated with a fertilizer solution. Plant scientists often grow plants without soil in a Hoagland solution, or other mineral nutrient solution, to assure their plants have all the essential mineral nutrients. Here is an excellent website on plant mineral nutrition:
http://www.soils.wisc.edu/~barak/soilscience326/essentl.htm
The following USDA website indicates that cider vinegar contains some essential mineral nutrients, including significant amounts of potassium (730 mg/liter), phosphorus (80 mg/liter), calcium (70 mg/liter) and magnesium (50 mg/liter). Compare these numbers to those for a Hoagland Solution at the bottom. Note that the USDA tables use units of mg per 100 grams. Multiply by 10 to get standard units of mg/liter.
http://www.nal.usda.gov/fnic/foodcomp/search/
The main ingredient in vinegar is acetic acid, which has an acid pH and would make water less available to the plant roots. Aloe vera is a succulent. Succulents can withstand low soil water availability better than most nonsucculents. Thus, perhaps it can tolerate the acetic acid better than many plants. Soil pH has a major effect on the availability of several mineral nutrients to plant roots. Thus, vinegar changing the soil pH might affect the amount of mineral nutrients available to the roots. See the soil pH nutrient availability graph in the following webpage:
http://www.extension.umn.edu/distribution/horticulture/components/1731-03.html
Milk has too high a salt (sodium chloride) concentation for irrigation of most plants. The higher the concentration of salts and other dissolved substances in the soil solution, the more difficult it is for plant roots to absorb water. In addition, the proteins, sugars and fats in milk provide an energy source for microbe growth. Microbes can produce waste products that harm plant roots or compete with plant roots for mineral nutrients. Check the soil of the milk-irrigated plant for bad odors, a sign of toxic microbial waste products.
Based on the label, Hershey (2001) calculated that skim milk contained 520 mg/liter sodium, a very high level for plant irrigation water. If you can get an electrical conductivity (EC) meter, you can easily compare the EC of your three solutions to get an idea of how good they are for plant irrigation (Hershey and Sand 1993). The higher the EC, the worse the water is for plant irrigation (Hershey 1993). Hershey and Sand (1993) reported that 1% milk had an EC of 5.2 dS/m. Excellent irrigation water has an EC below 0.25 dS/m.
Milk does contain very high concentrations of many of the essential nutrients for plants, especially calcium, magnesium, phosphorus, zinc and potassium. Check the USDA website mentioned earlier and compare the levels in milk to the Hoagland solution.
Important questions in this experiment are the following:
1. The chemical and physical properties of the soil. Was it field soil or potting soil? Did it contain any starter fertilizer? Field soils in pots often give very poor plant growth because of their poor physical properties (Hershey 1990a).
2. The chemical composition of the tap water used as the control. Did the tap water provide significant amounts of any essential mineral nutrients, such as calcium, potassium and magnesium? Your water company can provide a chemical analysis of the tap water.
3. Was any fertilizer added? If so, what type and amount?
4. What effect did the vinegar have on the soil pH compared to the control?
5. Which is cheaper per liter, a fertilizer solution designed for irrigating potted plants, such as Miracle-Gro, or milk or vinegar (Hershey 1990b)?
Irrigating plants with human beverages, such as soda, milk, tea, coffee, fruit juice, etc., is a common student project but contradicts basic biological facts that photosynthetic plants get their energy mainly from light and require only mineral nutrients and water from the soil. A better way to present such projects would be to consider what the effect on plants would be if tanker truck of milk or vinegar spilled or whether outdated milk or vinegar could be safely disposed of or recycled by using it to irrigate plants.
References
Hershey, D.R. 2001. Re: Why did the liquids kill the plants? Why did the tea do well?
http://madsci.org/posts/archives/2001-02/981520898.Bt.r.html
Hershey, D.R. 1993. Evaluation of irrigation water quality. American Biology Teacher 55:228-232.
Hershey, D.R. and Sand, S. 1993. Electrical conductivity. Science Activities. 30(1):32-35.
Hershey, D.R. 1990a. Container-soil physics and plant growth. BioScience 40:685-686.
Hershey, D.R. 1990b. Sleuthing the nutrients that make your houseplant grow. Science Activities 27(4):17-20.
http://www.angelfire.com/ab6/hershey/safertilizer.pdf
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