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Monday, March 19, 2012

Putting Rock Mulch on Top of Composted Soil/Wood Mulch for Fruit Trees


Q. In my backyard in 2010 I planted a semi-dwarf Early Elberta Peach tree and a Katy Apricot.  I hand-watered them and left their tree wells uncovered until I completed the irrigation system around April 2011.  Now each tree has drip emitters. I put a thick layer of Dr. Q's Pay Dirt potting soil on top of each tree well and then covered the potting soil over with a layer of the ruby red stone that I have throughout the floor of my backyard.  Both trees are doing very well and I want to be sure to do everything I can for their long-term health.

            Per your View article, should I move the red stone back off the top of the tree wells to a radius of 6 feet away from each tree, and then cover each tree well with a layer of wood mulch instead?

 A. One soil problem we have when growing fruit trees in our desert is the small amount of organic matter in our soils. This miniscule amount of organic matter is not enough for nearly all plants including so-called desert plants.

            Fruit trees, and nearly all other plants, perform much better in our soils if organic matter (compost preferably) is added to the soil surrounding the roots. I hope that you added a lot of amendments to the soil at the time of planting, not just the surface.

            I have compared two application methods when compost is applied to fruit trees: mixing it only to the soil at the time of planting and adding it only to the soil surface after planting. Adding it to the soil, not just the soil surface, dramatically improves plant growth in our desert soils.

            The best soil amendments to use to use when increasing organic matter in our soils are homemade composts. Commercially made composts, available in bags or bulk and extremely variable in quality, would be next. The good ones (there are good ones out there now thanks to the explosion in organic gardening) are expensive. The cost of enough good quality compost, added to the hole at planting time, in some cases might rival the cost of the plant itself.

            If trees are planted in amended soils and then the soil surface covered with rock, over time, the organic matter in the soil is “used up” so to speak by soil microorganisms. When the majority has been used up, we say the soil has become “mineralized”. Without addition of organic matter to the soil surface every two to three years, the soil slowly reverts back to its previous desert condition.

            From my observations of fruit trees and nondesert landscape plants growing in our desert soils amended only at the time of planting and mulched with rock only, the soil is typically “mineralized” by the fourth or fifth years. Cacti and desert plants are much more tolerant of mineralized soils but still grow better in amended soils.

            By placing wood mulch on the soil surface where the soil is wet, it slowly decomposes. Through its decomposition it adds organic matter to the soil. Through their decomposition, mulches add a lot of organic activity such as beneficial microorganisms and earthworms leading to improved plant health. Rock mulch cannot add organic matter and so in a few years none of the benefits of organic mulches will be present.

            I am guessing your trees will be fine for several years with rock mulch covering the soil surface. But what may happen in the fourth through the sixth years, as the soil becomes mineralized, is that they may begin to decline in health.  The trees may become more and more yellow, leading to leaf scorch, followed by branch dieback and insect attacks such as borers.

            Borers come into play due to a decline in tree health. Decreased plant health causes a thinning of the tree canopy leading to an increased amount of sunburn damage to the limbs. Sunburn damaged areas are the ideal locations for borers damage to occur.

            My concern would be how you might add organic matter to the wet soil surfaces as it decomposes. If you can do this then it will probably not be a problem.  It is best for the trees if you can put wood mulch in the wetted area under the trees. In the drier areas under the tree the wood mulch will not decompose.

1 comment:

  1. My problem with rock mulch in the desert is two fold. It is hot in the desert. Soil temps already can exceed ground temp for 1-3" of depth...not healthy for roots. Adding rock on the surface only magnifies this thermal transfer and limits soil life in that upper inch or two.

    Secondly that rock reflects a heck of a lot of sun and heat (IR) up off the ground and onto the trunk and UNDER the canopy. Leaves are made to take the sun above not from below. You essentially cook the leaves undersides in the summer.

    Now rock in the winter can get trees to break dormancy sooner (if desirable) and promote earlier root growth/rejuvenation in Jan.-Feb. by warming the ground sooner.

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