Trucks packed with cotton rumble on the roads outside Ashkabad, carrying the tufty "white gold" that is the key to the fortunes of Soviet Turkmenistan and its political leaders.

In recent months, a dramatic turnover has taken place in the top ranks of the Communist Party in Turkmenistan and other Central Asian republics. In most cases, troubles with the cotton crop -- including salinization of irrigated soil and padding of production figures -- have been a factor.

"In Turkmenistan, everything depends on cotton. If the harvest is bad, people lose their jobs. If it is good, they stay put. The difference this time was the consequences were felt higher up," said a resident of Mari, Turkmenistan's main cotton-producing province, where the first party secretary was dismissed a year ago.

As they head out of Ashkabad, the cotton trucks cross the Kara Kum Canal, a 680-mile waterway named after the formidable desert that makes up four-fifths of Turkmenistan, a republic with an area the size of Spain.

The canal, begun in 1954, will reach the Caspian Sea by 1990 and is viewed here as one of the triumphs of the Soviet era.

Taking 660 cubic yards of water a second from the Amu Darya River, the canal has brought water to the Kara Kum desert and fed the republic's appetite for cotton. The irrigated land in Turkmenistan has doubled in 17 years, and the republic is the nation's second-largest cotton producer, after Uzbekistan.

Total Soviet cotton production -- 8.75 million tons in 1985 -- ranks after China, in competition with the United States as the world's second-largest cotton grower.

But while irrigation has made cotton grow in Central Asia, it has also created its own problem: "white poison," which is what people here call the salt that seeps up from the underlying groundwater and poisons the irrigated soil.

A visitor flying into Ashkabad can see the problem from the air. Patches of the fields below are covered with what looks like snow -- but is in fact salt.

The problem of "secondary salinization" was a criticism of Turkmen agriculture by the Soviet Communist Party newspaper Pravda last August. Inadequate drainage -- "the result of man's stupid, irresponsible economic activity" -- was blamed for the annual cotton harvest shortfall of 350,000 to 400,000 tons, a third of the republic's raw cotton production last year.

According to Pravda, nearly 40 percent of the republic's irrigated land is "salinated" because construction of collector-drainage networks fell 60 percent short of goal. This failing was exceeded in other areas of agriculture, Pravda said, and despite year-round sunshine and ample labor, Turkmenistan cannot feed its population of 3 million.

"The republic supplies the country with cotton," local officials were quoted as saying. "But it has no strength left for anything else."

Pravda placed responsibility for the failure to meet production goals in cotton directly at the door of the republic's top party officials, naming Mukhamednazar Gapurov, who had headed the republic's Communist Party for 17 years.

"Despite promises and adopted party decisions, less cotton was picked" than planned, Pravda said pointedly, but "none of the republic's leaders, either in the press or in their reports, made any special effort to spread this news."

Published as Soviet leader Mikhail Gorbachev was sweeping out older, inefficient and corrupt party leaders around the country, the August article was viewed as Gapurov's political death knell. Despite his attempt in November to argue again that Turkmenistan's cotton crop had reached record levels, the 63-year-old party leader was ousted on Dec. 21.

He was the third Central Asian party chief to get the boot during the winter -- a remarkable turnover in a region that had seen no changes at the top from 1969 to 1983.

The change in Turkmenistan seemed largely the result of displeasure with the republic's economic performance, whereas in Kirghizia and Tadzhikstan, where party first secretaries also were retired this winter, the problems appeared to have been corruption and nepotism.

Uzbekistan, largest of the four Moslem republics along the Soviet Union's desert rim, underwent its major change in 1983 when party secretary Sharaf Rashidov died after 24 years in office.

But in the past year, more evidence of widespread corruption in Uzbekistan has surfaced, leading to a virtual purge of the leadership. Of the 65 regional party chiefs, 40 have been replaced, as have 260 local committee secretaries.

At the Uzbek party congress in January, the republic's leaders were accused of fraud -- padding cotton production figures "by hundreds of thousands of tons."