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I'm Gwen Outen with the VOA Special English Agriculture Report.
Last week, we discussed how farmers can control harmful insects by mixing different kinds of plants with the main crop. But there are also other ways to use plants to protect crops without chemicals. Some plants provide food and protection for insects that help control harmful insects.
Organic Gardening magazine, published by the Rodale Institute, once described some examples, such as ladybugs.
Ladybugs are beetles that like crimson clover and hairy vetch. They find food, water and a resting place in the clover and vetch. Ladybug larvae eat harmful aphids. Aphids are tiny insects that feed on many different kinds of crop plants.
Plants also help each other through their root systems.
For example, scientists say the roots of the marigold flower reduce harmful nematode populations in the soil. Nematodes are tiny worms. There are more than 10,000 different kinds of nematodes. And some of them feed on corn.
Wild mustard is another plant that releases a poison through its roots. This poison kills nematodes. It also kills some kinds of fungi.
A researcher at the University of California, Santa Cruz, said the wild mustard should be cut close to the ground after the first 15 days. After that, it should be cut once a month. If left to grow freely, wild mustard will compete with the corn for nutrients in the soil.
Canadian researchers discovered that the dandelion weed can protect tomato plants from fusarium disease. Fusarium attacks the plant roots. It reduces the number of tomatoes that the plant produces.
Dandelion roots produce cichoric acid. This acid prevents the disease from getting iron from the soil. Fusarium needs iron to survive.
There are, however, plants that should never be grown together. The roots of the black walnut tree, for example, produce a poison that kills potatoes, peas, tomatoes and peppers.
Dying parts of the brassica family of plants produce a poison that prevents the seeds of some plants from growing. Brassica plants include broccoli, cabbage and cauliflower.
Plants with small seeds, such as lettuce, are especially affected by the brassica poison. A professor at the University of Connecticut said brassica plants should be removed from the soil after they have produced their crop.
This VOA Special English Agriculture Report was written by Bob Bowen. Internet users can find the first part of our report at voaspecialenglish.com. I'm Gwen Outen. | 
