Adjusting Wet Acidic Soil in the Garden
Answered by: Richters Staff
Question from: Christine Houghton
Posted: Before April 1998

We are preparing a new garden and have had the soil analysed to show a pH of around 3.5 in a black clay base which becomes very sticky and muddy when wet.

What should we use to prepare the soil for gardens around our new home. One plant nursery says gypsum as well as dolomite. Do these calcium-based substances have different soil-conditioning properties? What else should we do?

We are having an earthmoving machine come in 2-3 days to level the block and believe it would be easier to add any soil conditioners to the soil at this time.

Your pH is too low for most herbs which prefer a pH in 5.5 to 7 range. A pH of 7 is neutral; below 7 your soil is acidic and above 7 your soil is alkaline. Your soil is at least 100 times more acidic than the ideal pH for herbs. Poor drainage caused by the heavy clay presents an added problem because most of the popular herbs require good soil drainage to prevent root rot.

Adding dolomite limestone will increase the pH (reduce acidity) but you must watch for the soil’s natural capacity to buffer pH changes. Clay soils have a high buffering capacity and may require several applications of limestone to stablilize the pH in the desired range. Even so, the pH can creep downward over several years and repeat applications of limestone may be necessary.

Gypsum is recommended to break up heavy clay. It does not always work well, but it is worth trying. We recommend adding organic material also which will help "break up" the clay. A well-rotted compost or manure or pH-adjusted peatmoss will help improve the soil.

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