Benefits of organic mulch are numerous

: I read your column explaining fertilizing plants. I recall you mentioned using organic mulch. Can you please explain the technique for applying the fertilizer? We have rocks surrounding our plants, no lawn; must we remove the rocks to apply the fertilizer. Also, we have three crape myrtles that have become very dry and they look like they are dying although they receive water.

A: Organic mulch is a product derived from plants and applied to the surface of the soil usually at a depth of 3 to 4 inches. Organic mulches can be wood products, paper products, plant residues such as clean straw and others. I think you get the picture.

Organic mulches prevent weeds from growing, slow evaporation of water from the soil surface, improve soils as they decompose and typically increase the number of beneficial microorganisms and larger critters such as earthworms. These are all good things.

Organic mulches break down or decompose over time when in contact with wet soil and in the presence of microorganisms.

To break down organic mulches, microorganisms need lots of nitrogen. Organic mulches, although full of nutritional value for plants, have very low amounts of nitrogen. If the microorganisms are going to break down the mulches, they are going to take nitrogen wherever they can find it.

Microorganisms that are decomposing organic mulches will outcompete plants for any available nitrogen. When there is not enough nitrogen present for both, the plants become nitrogen starved, displayed by stunted growth and yellow leaves.

To prevent this, make sure that the plants surrounded by organic mulch such as wood chips are receiving adequate amounts of fertilizer. Fertilizer, which contains nitrogen, moves to plant roots in moist soils. It cannot move to plant roots in dry soils.

If you are using drip irrigation, apply the fertilizer between the drip emitter and the plant. Do not get fertilizers too close to the plants because concentrated fertilizers can damage or kill plants due to their high salinity.

I would highly suggest that you use slow-release fertilizers whenever fertilizing plants. These can be granular or in the form of stakes. Several readers have told me they have fertilizer injectors coupled with their irrigation systems; that is certainly a high-tech solution to applying fertilizers in small amounts. Just remember, if you have a fertilizer injector, make sure that you also have adequate backflow prevention so that you do not contaminate your neighbor's drinking water.

You also can spray fertilizer solutions directly onto plants' leaves. This works quite well, but is short lived and needs to be done frequently during the growing season.

I do not know why your crape myrtles are turning brown; it is probably water related -- either too much or too little. They may not have enough drip emitters. Make sure that they have at least four emitters if they are smaller plants. As they get larger, you will need to add more emitters.

Crape myrtles do not do particularly well in rock mulch, so make sure it is organic. The soil under the canopy of these plants should be wet to a depth of 18 inches to 24 inches after each irrigation. This could mean that the emitters may have to run for an hour or more at each irrigation.

If you are not sure if they are getting enough water, especially during the heat of the summer, take a hose and thoroughly wet the soil around the plants. However, with an adequate irrigation system, you should not need to do that.

They also need iron. Apply iron and a soil-applied fertilizer in January each year. The iron should be a chelated form, containing the chelate EDDHA in the ingredients. Don't use any other chelate for soil applications.

Follow this up with a good quality liquid fertilizer applied to the leaves in about March. This could be Miracle-Gro, Peters or something similar.

Bob Morris is an associate professor with the University of Nevada Cooperative Extension. Direct gardening questions to the master gardener hot line at 257-5555 or contact Morris by e-mail at extremehort@aol.com.