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) re****ted
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.
Im****tant 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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