MEXICO CITY - Ten-year-old Adriana Salgado spends her days in a field in northwestern Mexico, picking the spinach, cabbage and other vegetables that fill American salad bowls. Adriana doesn't know how to read. She attends school for only one hour a day.

Her 15-year-old sister, who works with her, can't read, either. Adriana had an 8-year-old brother, too, until he was crushed to death by a tractor while working in a tomato field last year.

About 300,000 youngsters such as Adriana work illegally in Mexico's fields, the U.N. Children's Fund says, making child labor a major link in the chain that increasingly supplies American dinner tables.

In his annual May Day speech last week, Mexican Labor Secretary Javier Lozano pledged to eradicate child labor and impose "the quick application of workplace law." But with thousands of poor families dependent on the money that their children bring in, experts say it is an uphill battle.

"It is the worst form of exploitation," said Nayeli Ram�rez, director of Ririki Social Intervention, a Mexican group that campaigns for children's rights.

Mexican law prohibits children 13 and younger from working, and those 14 to 16 can work only in jobs that do not "jeopardize their development." That clause is not defined.

Nevertheless, children under 15 make up 20 percent of Mexico's migrant farmworkers, the Mexican Labor Secretariat says. Less than 10 percent of these children attend school, and 42 percent suffer from some form of malnutrition, government studies show.

They persist in the fields despite harsh criticism from international groups, rules imposed by U.S. distributors and increasingly strident warnings from the Mexican government.

Child deaths

In December, nine children were killed when a truck carrying coffee pickers flipped over in Mexico's central Puebla state. In January, the government threatened to fine another coffee farm for using child laborers.

"Enough of this, this making a minor less than 14 years old work without good school conditions or medical care," Lozano told agribusiness executives during a hearing in the Mexican Senate last year. "We cannot approve of what is going on."

Adriana is a migrant worker from the southern state of Guerrero. Interviewed by telephone from a migrant camp in Mexico's Sinaloa state, she rattled off the names of other children who work with her on a farm run by a wholesaler of Asian vegetables that exports to the United States and Canada.

She recited the produce they pick: baby lettuce, Chinese cabbage and spinach. She earns about $7 a day, said her father, Cruz Salgado.

From 6 p.m. to 7 p.m., she attends a school run by the company and staffed by student teachers provided by the Mexican government.

Asked if she enjoys working, she said, "Not much."

"I like school," she said.

NAFTA byproduct

The 1994 North American Free Trade Agreement is partly to blame for the child-labor phenomenon, Ram�rez said.

The trade pact has opened up the U.S. market to Mexican farmers, encouraging large-scale packing and exporting operations in Mexico's northern and central states. From 2003 to 2007, U.S. imports of Mexican fruit and vegetables rose 35 percent, to $4.4 billion from $3.2 billion.

Small-scale farmers, mostly Indians from southern Mexico, have found it hard to compete. Instead, they began migrating north each harvest season to work the big fields, Ram�rez said.

The children of these families went from working on the family farm to working for hire, she said. To most families, it just made sense: Most farms did not offer day care or schools, so the children might as well stay busy.

In most cases, the children do not appear on the farms' payrolls, said Teresa Rojas, a professor at Mexico's National Teachers' University who has studied the phenomenon.

But because adult workers earn bonuses for picking more than their daily quota, parents with "helpers" bring home more money. Farms save money because they do not have to pay social security for the youngsters.

"You never find the child on the payroll; it's always the head of the household," Rojas said. "That's part of the problem. These children are invisible (to the government)."

In many farms, children as young as 5 scoot on their hands and knees along rows of vegetables, cutting weeds, she said. In the state of Puebla, children work as "burros," carrying buckets of coffee beans down from the mountains.

In states that grow corn and wheat, which are easily harvested by machine, small children are seen less frequently in the fields. But adolescents are common.

In the central Mexican state of Guanajuato, Maria del Carmen L�pez, 16, said she had been working in the fields since she was 14.

On a recent day, she walked down rows of young corn plants carrying a small hoe. Every few minutes, she stopped to hack away at a weed or brush dirt off a corn sprout.

"I wanted to keep studying after middle school," she said, her voice muffled by a bandanna she wore to keep out the dust. "But I couldn't afford it."

In Mexico, many children are expected to start working after middle school. The cost of books, supplies, school clothes and school-maintenance fees are barriers to many families.

In a nearby field, 15-year-old Ivan Garcia cut broccoli for another exporter of vegetables. He said he had started working in the fields at age 13.

Cracking down

Child farmworkers came under greater scrutiny after the Jan. 6, 2007, death of Adriana's 8-year-old brother, David, at the Los Pinos tomato farm in Sinaloa.

"He was going to empty his bucket, and the tractor driver didn't see him and went right over him," said Cruz Salgado, his voice choking up.

The case made headlines across Mexico and prompted calls by UNICEF to ban child labor.

The farm's owner, Agr�cola Paredes, paid $3,308 in funeral expenses, according to a report by the Mexican Senate. But it refused to pay any further compensation, claiming that the boy was on a public road and that he was not a formal employee, the Senate report said.

Agr�cola Paredes officials declined to talk to The Republic.

The company sells tomatoes, bell peppers and eggplant in the United States under the SPV, Chelita and Paris brands.

One of its main importers, H.M. Distributors of Rio Rico, Ariz., said it had not heard about the boy's death and was unaware of any children working on its suppliers' farms. But Vice President Rene Monteverde said it was difficult for importers to make sure farms are obeying the rules.

Changes in policy

"We're not there every day, so it's tough," Monteverde said.

Before David's death, many companies in Sinaloa paid child workers just like other workers, with envelopes of cash, Salgado said. Now, most refuse to pay children directly, and some bar children from the fields entirely, he said.

U.S. distributors are increasingly putting rules against child labor into their supplier contracts, said Eduardo Garcia, operations manager of the Agr�cola Nieto farms in Guanajuato state.

Manager Let Tran of Agr�cola Buen A�o, the Salgado family's current employer, said he has no children on the payroll but added that many workers insist on keeping their children with them during the day.

"I do a school, and nobody wants to put their kids in school. I do a day care, and nobody puts their kid in day care," Tran said.

"It's really hard because we're trying to improve their lives."

Enforcement is difficult, Lozano has said, because of jurisdictional conflicts between state and federal officials. The federal Labor Secretariat only has 318 inspectors nationwide, while farms number in the thousands.

The Labor Secretariat said it did 3,047 inspections to check on the labor conditions of minors in workplaces from January to July 2007, but it could not immediately provide statistics on how many were farms or how many violations had been found.

Day care, school

The Mexican government has started building day-care centers nationwide and is boosting the number of grants given to families so that children can continue studying. But poverty and a long family tradition of farmwork remain a significant hurdle, Rojas said.

After losing his son, Salgado said he did not want his other children in the fields. But he said there was no way he could make ends meet on the $20 a day he and his wife would earn alone.

"I wish they didn't have to work," he said. "But we need the money."

Reach the reporter at chris.hawley@arizonarepublic.com