Vinegar & Eggshells; Effective Source of Calcium for Plants?
I have read that many of the claims for eggshells in the garden, e.g. shells repel slugs, are incorrect. And have also observed that eggshells will dissolve in vinegar. There are lots of claims that eggshells dissolved in vinegar makes a calcium-rich foliar spray for plants. On the other hand, there are claims that vinegar spray can be used as a herbicide. 1. Is the eggshell-vinegar claim verified? 2. Is the herbicide claim verified? 3. If both are true, is the difference that the eggshells mitigate the herbicidal effect of the vinegar? Or that the fertilizer involves household vinegar while the herbicide involves industrial-strength vinegar?
District of Columbia County District of Columbia
A soil test should first be performed on the area of the garden in question to see if there is a calcium deficiency. If any residues from a spray are washed into the soil, calcium may alter soil pH over time. Excess calcium can interfere with the absorption of other needed nutrients such as potassium, magnesium, phosphorus, iron, manganese, boron, and zinc. Additionally, excessive amounts of any nutrient can cause plant health problems and lead to environmental pollution if the nutrient is water-soluble. Leaves are not designed for the uptake of nutrients; roots are, which is why testing the soil is important. (Information on soil testing can be found here, though you may have to check with the labs to see if they're currently open: https://extension.umd.edu/hgic/topics/soil-testing)
Sites discussing eggshell use in the garden state that they should be finely ground first. https://extension.illinois.edu/blogs/good-growing/2018-03-28-using-eggshells-garden-and-compost. The calcium carbonate in eggshells is insoluble and unavailable to plants unless ground finely. While calcium is freed by the chemical reaction with household vinegar (the other byproducts being carbon dioxide and water), you will end up with a solution of unknown quantity of calcium and potentially leftover acetic acid. Calcium in fertilizer form (that is, manufactured or processed for sale as fertilizer) will have a nutrient analysis that enables you to know exactly how much calcium is present per amount (weight) of product. This article addresses ways to avoid blossom-end rot on tomatoes, a use commonly associated with attempts at foliar calcium treatments: http://extension.missouri.edu/phelps/documents/Horticulture_News/BlossomEndRot.pdf.
Yes, acetic acid (vinegar) can be used as an herbicide, but in such cases the strength of the concentration of acid is well beyond what one would find in the grocery store. It is a registered herbicide and must be handled with caution as it is caustic and can damage human tissues; it can be more dangerous to human health than other more "conventional" herbicides and is labelled as such. It does not kill plant roots, so perennial weeds would return after rebounding and sprouting again. Repeated treatments may exhaust the weed's reserves and kill it, but care needs to be taken as with any non-selective herbicide to not contact any desirable plants. See the mention here under "organic lawn herbicides": https://extension.umd.edu/hgic/topics/lawn-herbicides.