cat defecation problem

You have to make the area as unattractive as possible to the animal. Contaminating the soil with chemicals will probably not have the intended effect. Medium-sized rock, a pan of water, covering that nice scratchable dirt with heavy plastic or something sticky will probably work as long as the occluding material is left on the dirt.

You could try putting twigs or bark chips on the dirt patch - cats prefer plain earth for littering. Other ideas are mothballs or orange peel (also, our cats absolutely loathe the citronella candle we have, so citronella oil might work too)

on topic: the SF SPCA suggests:
Push disposable wooden chopsticks or 10-inch plant stakes into
flower beds every eight inches. Prevents digging or
scratching.

Cats dislike the smell of citrus: try putting orange and
lemon peels out; spray the area with citrus-scented sprays;
or sprinkle with orange-scented pet bedding like Citrafresh.
Spray cat repellent (available at pet supply stores) around the
perimeter of your yard and along the top of the fences.

Scatter coffee grounds or pipe tobacco in the area.

Try growing the herb rue�cats dislike the smell. You may also be
able to buy it dried in health food stores.

Soak strips of old towels or rags with old perfume and hang them
near target plants.

that is not true of this one:

http://www.conceptresearch.co.uk/cats_1.htm