Question
Thanks for your detailed reply, Doc. I have a followup.

If sugar dehydrates things in the soil, I get the impression that sugar would be a negative thing.  If you want to control weeds with salt, and you don't want to grow anything else there, then you can put salt down; if you want to put sugar instead, same results?

Maybe the sugar effect gets over quickly?

Also, it seems like this would be bad for microbes that we actually want.  Instead of "feeding" microbes this would actully damage them or maybe even kill them.  Isn't that the opposite of what we're trying to do?  Don't microbes somehow work in favor of the plants?

Last: If you put this on grass in your lawn, will it do just as much damage to the insects (beneficial) and things living in the grass as it does to the weeds?


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The text above is a follow-up to ...

-----Question-----
What is sweet soil and is this bad for plants or good? I am asking because of the recommendation I received earlier to put sugar about for killing weeds. I am very confused now. If you could clear it up for me I would appreicate that.
-----Answer-----
Hi Jason,

The term "sweet soil" is an old farmer's term to indicate an alkaline soil, i.e. one with a higher pH. This was a term to contrast the "sour soil" which had a lower pH.  As the grandson of an old-time farmer, I can vividly remember my grandfather tasting the soil to determine whether it was too acid to grow certain crops.  I never tried it, so I don't know whether there is any true "sweet" taste in the soil.

Most garden plants thrive in pHs between 6.0 and 7.0, and though that is near a neutral pH, it is sometimes called a "sweet soil" relatively speaking.  Certain plants such as blueberries, azaleas, etc. require a more "sour" (acidic) soil at a pH of approx. 4.0.

As you can see, these imprecise terms lead to much, much confusion, so the very best way to determine the pH of a soil is to have it tested, or to use an electronic probe.

As far as the application of sugar for weed control, that is another complex story. The application of salt or sugar in high concentrations can dehydrate tissues by drawing water out of the cells of microbes. (Salt and sugar curing are examples of this.) Most plants are not affected by these applications, but these methods do control bacterial growths.  

Weed control is really a secondary consequence of the microbial community in the soil.  In applications to soil, sugar serves as a carbon source for the soil microbes, so with the change in population size of the soil microbes, certain plant nutrients are less available (nitrates, for example) as they are sequestered temporarily in the microbes.  This can affect the germination of certain annual, weedy species.  Since weedy species have very high metabolic rates and nutrient requirements, they fail to grow because of the unavailability of the nutrients.

I must caution you that if you are attempting to grow plants that also have high nutrient requirements, they will also be affected by the nutrient deficiencies.  Many grasses and native woodland species are capable of thriving in nutrient-poor soil, so they are less affected by the sugar applications.

Sorry for the long, involved answer, but I hope this has helped.  

Good Luck and Good Gardening.  

Answer
Hi Jason,

Salt would have a greater effect on the plants themselves.  If you live in an area that uses salt for snow and ice control, you probably see brown, dead plants along roadsides.  This is because the salt solutions break down into sodium ions and chloride ions which can enter the plants through the root systems.  Sugar molecules cannot move through the root systems into the vasculature of the plant.

Sugar in high concentrations can affect the number and diversity of soil microbes. Applications of sugar will certainly become more dilute as they dissolve and enter the soil. Your point is dead on target, though; applications of sugar will initially affect the microbial diversity in the upper parts of the soil.  As the sugar becomes more and more dilute, the non-affected microbes will enter the areas and use the carbon (sugar) for metabolism.  

I do not know what the sugar concentrations are that have been researched.  It is my suspicion that the sugar solution is mild enough to minimally disturb the microbe community.

Good Luck and Good Gardening.