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Can dolomite lime be used as mulch?

25 Apr 2008

I would appreciate your comments on the idea of adding dolomite to the surface of the ground where I am thinking of planting a field of lavender. I want to reduce acidity in the soil, provide a reflective groundcover to raise summer temperatures around the lavender plants, improve drainage (south-facing slope) and deter weeds from growing.

GardenWise Horticulturist Carolyn Jones,

In Spain I saw lavender growing in rock screes, so this approach would mimic the native terrain of lavender. As you probably know, dolomite lime is ground from limestone with a high magnesium content; coastal B.C. soils are often deficient in magnesium, so that's another good reason to use dolomite lime. When using finely ground dolomite lime, as it is sold in garden centres, the recommended rate is about 2.2 kg per 9 square m (5 lb per 100 square ft.). It takes one to three months for the pH to become one degree higher (less acidic). I would test the pH of your soil before you start, just to have a base line measurement. I assume you are using a larger crush, like gravel, as a mulch. Even though this adds more weight, the rate of dissolution of the stone is slow because each particle has a smaller surface area to volume ratio (compared to a finer crush). I don't have figures for the rate of change of pH with gravel versus fine crush, but it is not likely to be dramatically different. The light colour of the dolomite will reflect heat. This means the soil will warm more slowly in spring and stay cooler in summer. The mulch will greatly reduce the rate of soil evaporation, as well, keeping the plants' roots cool. This style of gravel gardening is a great water-conservation strategy and is increasing in popularity. The only downside that I can see is that it's not as easy to tidy up fallen leaves, trimmings (when the lavender is harvested) and so on, but you could use a blower rather than a rake.

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