President Anwar Sadat is making another tour to the remote villages and oases of southern Egypt this week to promote what he calls Egypt's "third revolution."

The first revolution was the coup d'etat in which he helped Gamal Abdel Nasser overthrow the Egyptian monarchy in 1952. The second, which Sadat calls the "corrective revolution," was the power struggle in which he overcame rival claimants to leadership after Nasser's death. The current one is the "green revolution," an attempt to fulfill Egypt's age old dream of making the desert bloom.

Last week Sadat visited the oases of the "new valley" of central Egypt, handing out titles to thousands of acres of empty land and urging farmers to put to use huge tracts of theoretically arable soil.

Front page photographs in the Cairo press showed Sadat planting palm trees to mark the start of land reclamation projects.

His itinerary this week includes the upper reaches of the Egyptian part of the Nile around Abu Simbel, the Aswan Dam and Lake Nasser behind it, and other provinces of Upper Egypt and along the coast of the Red Sea. The official purpose of his tour is to "inspect the projects for conquering the desert, food sufficiency projects and the programs laid down for implementation of the green revolution."

Sadat's tour coincides with one of the periodic shortages of food staples that afflict Cairo and has obvious political overtones as he inaugurates canals, distributes fishing boats and inspects water projects to promote Egypt's quest for "food security." Sadat, who describes himself as "a simple farmer," enjoys these forays into the countryside.

Yet, in the opinion of economic and agricultural experts, the projects Sadat is visiting have more than short-term political significance. Egypt has a booming population and an acute shortage of arable land - a combination that jeopardizes the futures stability of the country and makes agricultural advancement every bit as urgent as Middle East peace.

Egypt has a land area of almost 387,000 square miles, bigger than France and Italy combined. All but 4 percent is desert, however, with the result that most of the population is crammed into the Nile Valley and the Nile Delta. Egypt's population of about 40 million, according to one authoritative study, is "5.5 persons per acre of arable land, representing one of the highest man-land ratios in the world."

With much of the arable land planted in cotton, Egypt is increasingly unable to feed itself and is obliged to commit ever-larger amounts of scarce foreign currency to food imports. The bill for those imports is reportedly running more than $1.5 billion a year.

That is why land reclamation, Sadat's cherished conquest of the desert," is a necessity, but land reclamation projects have been part of national policy for many years, with little to show for it.

About a million acres were reclaimed from the desert between 1952 and the outbreak of the October war in 1973, mostly in upper, or southern, Egypt with waters from the Aswan Dam. Yet, nearly an equal amount is said to have been lost - to the expansion of cities, to saline encroachment from the Mediterranean, and to advancing sands from the desert that are eating away at existing farmland.