Orchard with wood mulch in the Mojave Desert Q. I noticed that even though my drippers water deeply, the mulch around the surface of the plants and trees form a crust. Does that prevent oxygen from getting to the plants? The summer heat can only make worse. What do you suggest? A. I have never noticed this with the organic wood mulch that we use at the Orchard. It is very important that air gets to the roots. If this is a problem, then break up the crust with a rake.              Very fine mulches can compact or lay flatly on the surface of the soil over time. This can happen with sawdust, finely shredded newspapers and even very fine rock mulch such as 1/4 inch minus. Salts that are pulled up from the soil from surface evaporation can accumulate and cause crusts to form.             Coarse mulches seldom if ever have this problem because they cannot lay flat. Air moves through the coarse mulch where it supplies oxygen to microorganisms that help break down the mulch at the soil surface.             This constant breaking down of wood mulch adds organic matter to the soil which further aids in getting air into the soil. With time, the breakdown of wood mulch loosens the soil even further and causes the soil to allow even more air to the roots, particularly at the soil surface.             Hopefully you added compost to the soil at the time of planting. The wood mulch even adds more nutrients to the soil as it breaks down. It is important to have a wide diversity of different types of wood in the wood mulch; the more the better.             If you want a measurement of the activity, pull the wood mulch back where the soil is irrigated. After just a few months the soil will become much darker indicating its enrichment and biological activity which continuously improves the soil. Wood mulch also helps to keep the soil cooler aiding in decomposition as well.