The Q&A Archives: Soil Amendments

Question: The soil in my area is mainly on the clay side and has many rocks. I have removed the large rocks from the soil and tilled the garden. I am wondering what the best amendment is that I could add to the soil to make it the best
possible soil for a vegetable garden. I am thinking of adding sand or composted horse manure. I was also wondering if adding too much composted horse manure to my garden soil could actually harm the plants.
I was thinking on the order of 6 cubic yards of composted manure for an area 20' X 40'. Is this too much? Or would
I be better off with sand. Will I burn new seedlings if they are planted in the manure mixture? Also, could you tell me the last frost date for my area.

Answer: You are on the right track with adding organic matter. That is the best addition you can make to build good soil. (If you add sand to clay you will end up with brick when it dries out.) Most vegetables do best on a rich, moist but well drained soil (meaning not soggy) with plenty of organic matter.

If the composted manure is truly well aged it should not burn the plants especially if you work it into the soil. In most cases it also has a substantial amount of straw or shavings mixed with it which also reduces the nitrogen. (The difficulty is in determining the actual analysis of the material because it depends on the animal's diet, the age of the product and how it was composted and whether or not it was covered....)

If your soil is truly poor you might add even more organic matter such as chopped leaves or other less nitrogen-rich material along with it. (Ultimately you will need to run some basic soil tests to judge the results.) For the record, a cubic yard will cover about 160 square feet about two inches thick, so that amount should be fine for your 800 square feet. In fact, you could add up to ten inches of organic matter (other than manure) and it wouldn't be too much! Using an organic mulch during the growing season can also help build your soil as it decays over time, too.

I can't tell you the frost date for your local area, but your County Extension (461-1000) should be able to. They should also be able to help you with the soil tests and interpreting the results, as well.

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