International experts are preparing for a U.S. sponsored conference here next month on a problem that threatens to make a third of the world's arable land barren by the end of the century.

The Aug. 29-Sept. 9 conference on the spread of deserts is to be along the lines of the United Nation's 1972 Stockholm meeting on the environment and its 1974 Rome conference on world food supplies. A hudred countries and 60 international organizations are expected to send delegates, reflecting growing concern about the threat in Africa and elsewhere.

At a preliminary meeting here in April, Hugh Lamprey, coordinator of the U.N. arid-lands project, warned that unless emergency measures are taken, the effects of the spreading desert will soon be noticeable even in Nairobi, a well-landscaped garden city.

The problem of African deserts was dramatically brought before the world during the devastating drought in the Sahel in the early 1970s. Caused by overpopulation and related overgrazing, soil erosion and the wholesale destruction of forests, the encroachement of the desert has become the continent's major ecological porblem.

In Sudan, the desert advanced 62 miles during a recent 17-year period and continues to push forward like a pillaging army at the rate of almost four miles per year.

In Kordofan Province, it now takes five times the acreage to produce 75,000 tons of peanuts that it did in the early 1960s. Sesame seed producers have lost 95 per cent of their production capacity.

Declining food resources are compounded by an increasing population and when exacerbated by droughts, as occured in the Sahel, mass starvation is likely to result.

Researchers have estimated that about 250,000 square miles of arable or grazing land have been lost to the southern Sahara during the past 50 years.

Though massive in scope, the problem of the desert in Africa has low priority. In Kenya, for instance, where two-thirds of the land is arid, an official said. "We are a poor country and only 10 per cent of our population lives there, so our limited development resources must be focused on the third of the land where 90 per cent of all Kenyans live."

"People here depend on their camels, goats and cattle," said one U.N. reseacher in Kenya. "When there is a good rainfall, plant productivity increases proportionately and this is followed by large increases in livestock herds.

"The people know that droughts are cyclical and they figure that if half of their cattle are going to die anyway, it's better to start with 100 than with 50.

"But the large herds over-exploit the vegetation and after a prolonged drought the plant life doesn't recover and when the rains come, flash floods wash away the soil, leaving bare rock or sand."

Hydrologists tried to ease the problem with wells, but the excessive herds drawn to the water wiped out all traces of vegetation within a 20-mile radius.

Reforestation is often mentioned as a possible solution but large stands of trees are being cut for firewood in the Sahel many times faster than trees are being planted.

A panel of African experts who met in Nairobi last month called for establishment of a "green belt" to combat the spread of the Sahara.

The proposal incorporates not merely tree planting, but livestock management, crop cultivation, sand-dune fixation, soil conservation, land reclamation and irrigation.

According to one U.N. ecologist, "There is a tendency to be far too optimistic for the immediate future. We are working in the long range.

"It's very trendy to find deserts romantic, but a different view isthat they are a terrible degraduation of the face of the earth.

"We will be extremely lucky if attempts to revegetate work," he added.