The Army Materiel Command's chief of staff is expected to send a letter to field offices today explaining what the current salary shortfall could mean for its 90,000 civilians. It was drafted in response to an item in yesterday's column. The column said AMC is reviewing plans that could mean a 21-day furlough starting in October. It also said the savings from such a furlough might eliminate the need to lay off 5,000 workers.

In the draft letter, Maj. Gen. William McGrath said times are tough, and that AMC hopes to avoid furloughs but that reductions-in-force are more likely. It says in part:

"Many of you saw The Washington Post column . . . in which Mike Causey wrote that AMC is reviewing plans that could lead lead to a 21-day furlough starting in October. We in AMC do face serious funding shortfalls and it is true that plans leading to a possible furlough are being reviewed.

"Such a furlough, however, would have to be executed no later than Aug. 30, 1990, not after the beginning of the new fiscal year in October." (AMC needs $330 million to meet this fiscal year's payroll, but so far has gotten only $134 million from the Army.)

McGrath's letter says that unless the Army provides the money, "we face a furlough of up to 21 days."

"A substantial reduction-in-force (RIF) for fiscal year 1991 {which begins Oct. 1} is another distinct possibility if funding is not increased. Unlike what the Causey column implied, a furlough in fiscal year 1990 would have no bearing on the need for a RIF in fiscal year 1991," McGrath added. He said he remains "optimistic Army will still come through" with the salary money.

McGrath's letter pledges that AMC's leaders will do everything possible to avoid furloughs and RIFs. "But the future does not look bright, and we must take appropriate steps to plan for the future," he said.

If AMC is forced into a RIF situation, notices could go out as early as August to 5,400 workers. If the layoffs are ordered, they would be effective in October.

AMC has asked the Office of Personnel Management for authority to offer senior workers early retirement. Officials expect it will be approved soon. Financial Help

The Federal Employee Education and Assistance Fund said it has provided $225,000 in emergency assistance to federal workers with serious financial and medical problems. But fund officials said that too many deserving workers are turned down in their requests for loans, grants or scholarships because of its cash flow problems. Most of the people calling the nonprofit fund are asking for money, rather than offering financial help. The fund is supported by worker contributions via the Combined Federal Campaign.

Jim Peirce, of the National Federation of Federal Employees, said that the fund has provided $190,000 in scholarships to children of federal workers. But because of the money squeeze, Peirce said, only one in every 10 applicants is helped. He serves as vice chairman of the fund.

The fund's address is P.O. Box 2811, Washington, D.C. 20013-2811.