Radio listeners were elated April 1 by the unexpected announcement that a large resvoir of oil had been discovered in southern Mali.

Hours later, however, they were angered by an apologetic radio announcer's broadcast, which had been preceded by a severe tongue-lashing from irate Malian officials, that the announcement had been an April Fool's Day prank. m

"That was a very bad joke," remarked an angry minister of industrial development, Robert N'Daw. The bulletin momentarily raised false hope for many Malians, he said, adding, "We don't have time for such nonsense."

The world's sixth-poorest country, according to the World Bank, land-locked, semi-arid Mali would be economically hard pressed under the best of circumstances, but it has gone through 20 years of deterioration and underuse of its meager resources. Spurred by economic pressures, Malian officials have begun a gradual program of reform of the country's socialist system and a partial move toward a more liberalized capitalist economy, in an effort to end an ever-worsening cycle of financial decline.

"We need a general reform of our economic system," N'Daw said. Mali's socialist model, he said, was created from "a position of opposition to the system of neocolonialist" economic links to the West, through which strong nations retain their influence over the weak.

"We now recognize a need for change," he declared.

Like all of West Africa's Sahelian countries, Mali was ravaged by the 1968-1974 drought and has continued to suffer from less than average rainfall.

Even so, other countries with identical problems, particularly neighboring Niger, have managed to do more to overcome difficulties endemic to the Sahara Desert region than Mali has.

With the same rainfall deficiencies as Mali's and a substantially smaller cultivable amount of farmland -- 12 percent of Niger compared to 40 percent of Mali -- Niger has been able to reach self-sufficiency in cereal-grain production while Mali suffers large annual food deficits. The major difference in the two countries' farm programs, foreign economists said, is the higher price the Nigerian government offers its peasant farmers for their produce.

In addition, Mali has some of the Sahel's best farmland and two major river systems, the Senegal and the Niger -- the latter with a 250-mile inland delta that in precolonial times was a hand-irrigated regional granary. Mali's peasants, 94 percent of the country's 6.5 million population, farm only 4 percent of the 40 percent of the land that is arable, in silent protest against the government's efforts to monopolize grain purchases at lower than market value.

This problem will continue for the immediate future, N'Daw said. Mali's urban wage earners are too poorly paid for them to pay market value prices for their staples of millet and sorghum, N'Daw said.

"The workers accept to live in low conditions to help their country so we must try and help them," N'Daw said in explanation of the government's policy of keeping grain prices down."We will eventually begin to increase prices paid to farmers, but it will take place gradually, over five years."

Socialist visionary Modibo Keita brought Mali to independence from France in 1960. Keita rejected the paternalistic economic ties offered by France, turned to the Soviet Union for development assistance and created a state-run industrial system of 26 companies. From the Soviets, Mali has received 450 military advisers, technicians, teachers and an arms bill, as yet unpaid, estimated at $200 million.

In 1968, after having jerked this conservative Moslem country from one ideological extreme to another, Keita inauguarted a cultural revolution in imitation of China's. Shortly afterward, the Army toppled Keita's government in a bloodless coup and installed a 32-year old Army lieutenant, Moussa Traore, as head of state.

On assuming power, Traore announced that his government would reform the state enterprise system, as it was an unprofitable drain on the Malian economy. But Traore's military government, which has ruled behind a facade of civilians since 1979, has left the system virturally intact.

For the past 13 years, Mali's traditionally powerful families, clans and chiefs, who saw early the economic utility of ganing control of their own state company as a source of income, have effectively thwarted any reform measures proposed by the Army. Dismantling the system, it is estimated, would also put 15,000 people out of work. Each of those workers supports to some degree up to 15 other persons.

But as the companies' financial losses mounted, the International Monetary Fund put increasing pressure on Traore's government to dismantle the system as a condition for backing Mali's effort to get more aid. Salaries of the companies' employes and other civil servants eat up more than 70 percent of Mali's $160 million yearly budget.

One World Bank official said government employes have grown increasingly less productive because their salaries are three to four months late. For the first time, earlier this year, the government was even two to three weeks behind in paying its soldiers.

One assassination attempt and two coup plots against Trarore in December and January have been linked to the downward spiraling economy.

The 26 state companies (only three are operating at a profit) have build up $26 million in debts and owe the government $4 million in back taxes. Mali's external debt was $539 million in 1978 and revenue earnings a minuscule $116 million the same year, according to World Bank figures.

Recently, Mali has begun to invite in Western investors. Esso and the French oil company ELF have signed expoloration contracts and the Japanese are searching for uranium. The government also would like to sell the unprofitable companies to outside investors.

"We plan to keep those that are strategic and viable and sell the rest," N'Daw said. "But this cannot be done immediately. It has to be done slowly."