This kingdom known for deserts and oil is plowing export earnings into a fertile new field, farming, with the goal of creating vast new acreage to supply the country with grain and dairy products. It hopes to achieve self-sufficiency in wheat by 1986.

Tycoons with extra millions on their hands are regarded as highly patriotic to take up agriculture, and government subsidies help produce handsome returns.

The latest boom -- after oil, construction and industry -- is more precisely agribusiness. Like everything in this land of vast wealth and vision, it is being done on a grand scale.

Poultry and dairy farms are sprouting like date palms around Riyadh and here on the plains, 140 miles to the east, the desert is blooming.

Spider-like wheeled, pivoted irrigation systems circle the sun-baked earth. Huge tracts of green-tipped winter wheat and forage grasses sprout. Well-digging machinery and stakes marking out new farms are also part of the new landscape.

According to Abdul Rahman Kanhal, the manager of the Harad Agricultural and Animal Production Co., the upsurge began two years ago.

"We were the first here in this region to introduce irrigation and mechanized agriculture," said Kanhal, 32, who in his long, creaseless white robe and red-checkered headdress is not exactly what one imagines of a Third World farmer. "The others saw how we did it and then started up."

Kanhal is rather typical, though, of the new Saudi farmer. The son of a vegetable and date-palm grower, he went off for a master's degree in agriculture from the University of Colorado and worked on a Colorado farm for a while.

When he came back, the only job for him was as a bureaucrat in the Agriculture Ministry. After four years he came here. Now, Saudis with agricultural degrees are a hot commodity on the tight labor market.

Kanhal manages a 10,600-acre farm, soon to be expanded to 14,400 acres, that grows wheat, peanuts, potatoes and forage grasses. It supports 500 dairy cattle imported from Texas and 2,000 local sheep.

The way Kanhal explains the origins of the Saudi farming boom, it combined generous government help and rich Saudi businessmen looking for good investments. "The idea is to get the sector going and then the government will withdraw," he said.

Government incentives include free land, interest-free loans up to about $5.9 million, farm equipment subsidized from 35 to 50 percent, fertilizer sold at half the cost, and prices guaranteed high above world market levels.

Locally grown wheat brings $1,000 a ton, compared to $300 on the world market. Milk sells at wholesale for $5.50 a gallon, four times the price in the United States.

The government Grain Silos and Flour Mill Corp. stores, mills and markets the wheat.

"There is no problem of marketing," says Kanhal. The state-run silos "just give me a check for whatever I bring in."

Crown Prince Fahd, the kingdom's day-to-day ruler, has just approved a $74 million investment in additional silos around Riyadh and in the Qasim region, 500 miles to the north of the capital, to accommodate the sharp increase in wheat production.

Wheat is a priority item. Last year, Saudi farmers met 30 to 40 percent of the country's needs, and Kanhal estimates they will double production this year. The way Saudis are opening new land, they may reach the goal of producing all they need by 1985.

While outsiders think of Saudi Arabia as typified by the endless desert of the Rub al Khali, or Empty Quarter, there are many regions suitable for agriculture, particularly in the mountainous southern Asir Province, on the plains around Kharj and Harad, in Qasim Province and in the Al Hasa oasis, the world's largest, 100 miles north of here.

Only 1.3 million of the kingdom's 640 million acres are now devoted to farmland, with less than one quarter of that irrigated. But the government estimates the total cultivable land at three to four times this, and has marked out territory in each of the kingdom's 13 provinces for new farms.

It has also set aside $1.47 billion for loans and $735 million in subsidies to private farms during the five-year development plan that began last year. Government statistics show more than 100,000 loans made during the five years of the preceding plan.

The farm that is now the Harad Agricultural and Animal Production Co. began 15 years ago as a government-sponsored Bedouin settlement scheme. But as Kanhal explains it, "They all left and went to the city or into their own business."

So three years ago, the government gave more than 75 percent of the project to three private owners. When the farm grossed $10 million on wheat alone the first year, pressure grew on the government to turn it into a publicly owned company.

In June, a National Agricultural Development Co. was created, with 2.8 million shares. The public buys shares from banks, as there is no stock market. The government still holds a 20 percent interest.

Kanhal said the company is the kingdom's largest, with the main working farm here and new ones coming next year in Wadi Dawassir in the south and Hail in the north. Others are being planned.

One problem for farming, like every other activity in the kingdom, is labor. Of the 130 to 140 employes on this farm, 70 are Saudi nationals. The others include Filipinos and Sudanese. Saudi Arabia, which is one fourth as large as the United States, has a population of only 8.5 million.

"We have to be highly mechanized and don't count on casual laborers," said Kanhal.

Water is not a problem here. The farm, which sits on one of the kingdom's untapped reservoirs of oil, also has a huge underground lake. For 15 years, it has provided all the water needed for irrigation, recharged by rain.

The farm's two 15-megawatt generators are fueled by flared gas from the oil field. It is provided free by the national oil company, Aramco.