I have a 5-by-5-foot planter box in my backyard. When pulling out my tomato plants, I found that a neighborhood cat has been using a corner of the box as a litter box.

Since I like to grow vegetables and herbs in this box, do I need to completely remove all the soil that is currently in the box and replace it with clean soil before using that raised bed again for edibles?

The location of the box doesn't permit any winter sunlight, so I didn't plant it this winter. But I would like to discourage this behavior from any cats in the future.

– Jane Collins, Sacramento

Replacing the soil in your raised bed is not necessary; just remove the cat feces, says UC master gardener Bill Pierce.

Dispose of the feces; do not add it to the compost pile or use it for fertilizer.

There are several devices on the market for discouraging cats and dogs from visiting garden soil. One of the most effective is a motion-sensor sprinkler; these are available at several local nurseries. The sprinkler will squirt the intruder with water when it enters the raised bed.

The easiest and least expensive cat barrier is wide-mesh wire poultry netting. Just lay the netting over the soil in your bed.

Most seedlings – other than tomatoes, for which you will need to clip a few of the wires – can be planted through the holes in the netting. What you plant from directly sown seeds will accommodate themselves to the wires.

Also, since you don't plant a winter vegetable garden, consider covering your raised bed with black plastic when it's not in use. It will discourage the cat, and you'll have fewer weeds to deal with in the spring when you uncover it.

I live in northeast Galt on land that used to be dairy and farmland.

Our backyard is partially shaded and damp on one end. The grass on that end has bare spots that are covered with a black fungus-looking stuff. I'm not sure how to kill this black stuff and maybe let the grass have a better chance in the area. Can you help?

– Roberta Raw, Galt

According to UC master gardener Bill Pierce, this black material is probably algae that has grown because of the excess moisture.

To keep this from happening, you need to improve the drainage in the area by incorporating organic material into the soil and/or reducing the frequency of irrigation.

In clay soils, the particles of clay are very fine and they pack together. This inhibits the movement of water through the soil.

Working 4 inches of compost into this area should overcome the problem. Various types of compost are available at landscape supply yards. One option is blended steer manure – manure plus ground forest products such as redwood bark.

Some feed stores sell large bags of rice hulls; it takes hulls several years to decompose in the soil and they would do an excellent job for you.

If you use the rice hulls, you will need to add additional nitrogen fertilizer to the area before you replant the lawn because the hulls need nitrogen in order to decompose.

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