On a flat, wide plain outside this bustling rural crossroads lies a monument to Africa's possibilities: 600,000 jute bags filled with husked corn. They are stacked neatly in a half dozen huge mounds, each taller than four men and longer and wider than a football field.

On a continent plagued by drought, starvation and malnutrition, these 200-pound bags and hundreds of thousands of others stacked at rural depots throughout the northern half of Zimbabwe are symbols of hope and a promise redeemed.

Minister of Agriculture Denis Norman calls them a "miracle." That is not much of an exaggeration. This corn was not supposed to exist. It was grown on small plots of marginal land by peasant farmers. They defied the worst drought in a generation and official predictions of failure to produce their biggest crop since Zimbabwe's independence four years ago.

The miracle began with some last-minute luck. An inch of rain fell unexpectedly late in April. It gave the nearly ruined corn crop a second chance at life.

But the rain would have fallen to no effect if Zimbabweans had not been ready for it. This nation was waiting with specialized seeds, fertilizer, pesticides, transportation, expertise and, most importantly, money. There was credit for peasant farmers to buy agricultural necessities, and crop prices were kept high enough to motivate them to take a risk.

Faced with a third consecutive year of drought, Zimbabwe did not fold and join the long and growing list of agriculturally anemic African countries. Instead, its farmers diversified. They planted less corn, the country's main staple food, and more drought-resistant crops such as soybeans, cotton, tobacco and sorghum. First Trade Surplus Since Independence

In the end, Zimbabwe this year earned more than $500 million dollars from agricultural exports, enough to produce its first trade surplus since independence. The country was forced to import nearly 25 percent of its corn needs. But it had expected to import more than twice that amount.

The country's peasant farmers, who five years ago were not producing enough corn to feed themselves, this year sold 325,000 metric tons -- one-third of Zimbabwe's entire crop. At the same time, white commercial farmers continued to produce remarkably high yields, the world's second-highest behind the United States.

Yet all is not golden on the Zimbabwean farm. Prime Minister Robert Mugabe's government is criticized by capitalists for socialist policies that eat into farmers' profits. Leftist critics contend that the government has abandoned its commitment to land reform and that it ignores the needs of landless peasants.

A fourth year of drought could force the country to buy more foreign grain. The movement into nonedible cash crops, while producing a financial windfall this year, could backfire if world commodity prices fall or grain is short. A staggering population growth rate of nearly 4 percent means that Zimbabwean farmers have 300,000 new mouths to feed every year.

Nonetheless, compared with its neighbors and to most of Africa, Zimbabwe is a case study in agricultural success. It is one of the few countries where the African peasant is producing more food per capita than he did a decade ago.

Zimbabwe's lesson, according to those who have studied it closely, is that there is no easy formula for growing more food. The country defies much of the West's conventional wisdom that high producer prices and heavy doses of free enterprise are the key. Prices here are not among Africa's highest and there is a lot of state intervention in the marketing process.

Yet Zimbabwe seems to have come up with the right agricultural prescription for this water-starved continent. It includes a relatively efficient marketing system, reasonable prices and Africa's best research and extension services. While Kenya imports all its fertilizer, Zimbabwe produces 15 domestic varieties. While grain spoilage averages nearly 15 percent in the rest of sub-Saharan Africa, in Zimbabwe it is less than 1 percent. Black-Ruled Government Responsive to Whites

Perhaps most important, Zimbabwe has a black-ruled government that from its first day in power has been committed to maintaining food self-sufficiency even when that goal has conflicted with its socialist principles.

"Obviously we all have our complaints, but on the whole the government has been extremely responsive to our needs," said John Laurie, president of the Commercial Farmers' Union, which represents the country's 4,400 white commercial farmers. "The attention they have paid to agriculture has been first class."

In Zimbabwe, two entirely different economies and ways of life have been bound together by history and necessity. It is an uneasy marriage, but so far it seems to work.

It dates back to the 1890s, when British pioneers financed by Cecil Rhodes, the entrepreneur and evangelist of imperialism, ventured north from South Africa to settle the land he named Rhodesia. The settlers laid claim to a fertile crescent around a new capital they called Salisbury. Over several decades, they constructed a web of laws and customs that gave 6,000 white farmers ownership and control of half of the colony's arable land. Several million Africans were confined to the other half. Whites claimed most of the good soil, while holding a virtual monopoly on credit, research and extension services.

The two separate and unequal systems, however, nourished each other. The difficulty of eking out a subsistence living on the African sandveld forced peasants to seek employment on white farms. This transformed the black "tribal trust lands" into a reservoir of cheap and plentiful labor for the large white farms.

"The two economies were closely linked," said Zimbabwean economist Roger Riddell. "The prosperity of the one depended largely upon the poverty of the other."

By the late 1970s, white commercial farms were supplying more than 90 percent of Rhodesia's marketable food and employed a quarter million black farm workers -- 90 percent of whom earned less than $30 a month. White farms received 99 percent of the available government-backed loans. By contrast, the tribal trust lands were stagnant, deteriorating and ravaged by a seven-year guerrilla war.

For African peasants, the war was a struggle for land. When it ended and black guerrilla leader Robert Mugabe took power, he faced both the heightened expectations of his landless followers and the heightened fears of his white former enemies.

He also had to find a way to feed a rapidly growing population at a time when most of Africa was slipping from food self-sufficiency into chronic reliance on imported western grain. In sub-Saharan Africa that usually meant black dependence on white-ruled South Africa, the region's agricultural powerhouse. Mugabe was determined that Zimbabwe not go begging to Johannesburg.

He adopted a dual strategy. Despite his personal commitment to Marxism, Mugabe encouraged whites to retain their holdings and continue capitalist-style production. To demonstrate his sincerity, during the first year of independence he approved a 40 percent increase in corn prices for farmers. To soothe the nerves of white farmers, he appointed Denis Norman, a white tobacco and corn farmer and the only white in the nation's Cabinet, as minister of agriculture.

At the same time, Mugabe attempted to improve the holdings of peasant farmers in the former tribal trust areas. He gave them access to credit, extension services and research. A series of new grain depots and distribution centers for seeds, fertilizers, pesticides and tools was built in remote peasant areas. It was financed in large part by grants and loans from Britain and the U.S. Agency for International Development. Farmers an Influential Lobby

The government also has encouraged farmers to organize themselves into village groups, and they have emerged as an effective channel for peasants to express grievances and exercise political leverage. Their voice, combined with that of the commercial farmers, is an influential lobby in Harare, as the nation's capital now is known. In the capitals of African countries where food production has dwindled in the past two decades, the farmer's voice is usually inaudible.

In part because of that voice, the increase in farm credit in Zimbabwe has been dramatic. In 1978, 3,000 peasant farmers received a total of $1.5 million in government-backed loans. This year, by contrast, the government is pledged to lend $56 million to 95,000 smallholders.

Farmers, both black and white, responded during the first year of independence with a record corn harvest of 2.6 million tons -- enough to feed the nation, refill storage bins and still export 1 million tons to a hungry continent.

But since that first year, drought and recession have dampened production and curtailed Zimbabwe's supply of foreign exchange. Spare parts, tires, fuel and other imported necessities now seem as scarce as rain. Seeking to placate urban consumers and avoid another huge surplus, the government waited two years to raise corn prices again. Farmers contend that the 29 percent increase announced in July is not enough to keep pace with increases in the costs of fertilizer and other essentials.

Mugabe's ambitious land-resettlement program also has withered. Under the scheme, the government bought unused land in the white commercial sector, subdivided it into 10-acre parcels and gave it to landless peasants, along with the tools, water and other ingredients needed to grow food.

The program, however, has fallen far short of projections due to lack of funds and, critics contend, lack of commitment on the part of the bureaucrats responsible for it. The goal of resettling 160,000 families by 1985 has long been abandoned -- at best 30,000 families will have been aided. The budget for land acquisition, slashed from $25 million to $6 million last year, has been reduced further, to only $2 million in the 1985 budget. Nearly all of the resettled plots have gone to individual families rather than to the communal or cooperative schemes the government originally had advocated.

A recent study by Edgar Lockwood, representative of the American Friends Service Committee here, concludes that government efforts in agriculture, while well-meaning, naturally favor those who are better off and those who have land and property, not the poor and landless classes. Citing what he sees as the government's reluctance to push for collectivization and land resettlement, Lockwood argues that Zimbabwe is on the road to a kind of state-led capitalism with socialist experimentation on the fringes.

Zimbabwe also remains vulnerable to the vagaries of the international marketplace. For three years it has been trying to meet stringent qualifications to export beef to the European Community, only to be stymied by outbreaks of hoof-and-mouth disease. The government has spent nearly $10 million dollars to upgrade its slaughterhouses and erect nearly 2,000 miles of fencing to separate cattle from the wild buffaloes that carry the disease. It also has sanctioned the slaughter of hundreds of buffaloes, all in what so far has been a vain attempt to obtain the quota, which would be worth as much as $50 million per year.

The huge corn surplus of 1981 revealed another harsh fact of Zimbabwe's relationship to the West -- its lack of competitiveness. Even though Africa is starved for grain, Zimbabwe cannot afford to grow too much. Given its relatively high production costs, its landlocked position, and the high costs and risks of African transport, Zimbabwe's grain often is not competitive. Agriculture Minister Norman estimates that his country sold much of its 1981 surplus at a loss of $10 or more per ton, a loss it cannot afford to repeat.

For all the country's shortcomings, Zimbabwe's black farmers appear to agree on one point. For them, life is better than it was before independence. "We never used to get the depots or the credits or the extension program -- now we do," said Robinson Gapare, president of the National Farmers' Association of Zimbabwe, which claims 200,000 peasant farmers as members. "Even not praising the government . . . it's hard to see how they could have done more than they have done."