Cambodians are walking and driving oxcarts to the Thai border in dramatically growing numbers to pick up relief rice and farming tools, aid agencies here report.

Dwindling rice stocks, fear that the border may be closed by fighting and the freeing-up of farmers and oxen teams from plowing are believed to be the cause of the expanded traffic.

Yesterday, 12,000 carts crowded into muddy corrals erected by relief agencies outside the Thai village of Nong Chan for a twice-monthly distribution. Most returned to Cambodia after receiving 220-pound rations of rice.

Only 1,600 of the wooden vehicles were on hand when food and tools were passed out a month ago.

Separate distributions to people who arrive on foot have shown equally rapid growth: 5,000 people were issued 66-pound portions of rice on Aug. 28, 21,000 on Sept. 11, 33,000 on Sept. 22.

This unofficial "land bridge" into western Cambodia, opened late in 1979, has operated parallel to much larger programs that deliver aid directly to the Vietnamese-installed Heng Samrin government in Phnom Penh.

In June, Vietnamese troops occupied Nong Chan briefly and closed the land bridge. One purpose, diplomats believe, was to underscore Phnom Penh's opposition to any aid bypassing its official channels.

Relief agencies resumed distribution in July, and hoped to keep the scale smaller to avoid creating new political problems in Phnom Penh and drawing people away from their farm work.

Current tonnage passing through Nong Chan is still far below pre-June levels. But aid officials have been surprised at the speed with which arrival numbers are growing.

Aid from the West and Soviet Bloc countries has successfully averted mass starvation in Cambodia. But many rural families continue to live day-to-day. With almost no rice in their homes, they supplement their diets by catching small game, snails, crabs, fish and other food in the forests and canals.

Rice stocks can be expected to reach their lowest point just before the main rice harvest late this year, aid officials believe. Though Heng Samrin's distribution network is said to have improved markedly in recent months, many villages in western Cambodia will continue to look to the border to make up any shortfall.

This need for rice and fear that the Vietnamese Army might launch a major sweep this fall and close the border could quicken the traffic to Nong Chan in coming months, many aid workers believe. Nong Chan's thriving black markets, offering a wide range of consumer goods, could also attract people.

Moreover, with planting and plowing for the upcoming harvest largely completed, thousands of men, women and oxen have been freed up from the fields for the lengthy (two weeks is common) journey to the border along roads and trails turned to mud by monsoon rains.

"The food emergency is over" said one relief official. "But we want to keep the land bridge open until we're sure a good harvest is in for next year." k

Later this month a special team from the U.N. Food and Agriculture Organization will arrive in Phnom Penh to begin surveying the crop. The team's report will go before a meeting of representatives of the Cambodian program's donor countries, convening in New York on Nov. 19.

Heng Samrin's Ministry of Argriculture has said that its official planting target, variously reported at 2.5 million to 3.75 million acres, has been 75 percent met. Visitors to Cambodia have reported seeing miles of bright green paddy fields along highways.

A crop this size would put the country well on the road to self-sufficiency in food in 1981. But most relief officials expect that some foreign aid will still be needed next year.