Agriculture Secretary Bob Bergland is pressing for White House permission to propose two major changes in the food stamp program - one to liberalize it, the other to tighten it somewhat.

According to Bergland advisers in and outside the department, the two proposals would not increase the program's cost but would redistribute its benefits.

The first proposal, if the advisers have correctly estimated its effects, would increase the number of poor and families in the program. The second proposal would reduce benefits to families at higher income levels.

Bergland met yesterday with Health Education and Welfare Secretary Joseph A. Califano Jr. about the proposals. Califano's department is drafting a welfare reform plan for submission to President Carter on May 1. and food stamps, while they come from the Agriculture Department, are one of the government's largest welfare programs.

Bergland will seek concurrence from others in the administration next week. The Agriculture Secretary needs a decision from the White House by March 23, when he is scheduled to testify before the Senate Agriculture Committee on extension of all the government's basic farm and food programs, including food stamps, that expire this year.

The food stamp program has expanded mightily in the last 10 years and in the process has become controversial. Its cost 10 years ago was about $250 million; the cost this fiscal year will be about $5.6 billion. About 17 million people receive stamps each month, but there is considerable turnover in this group, and House Agriculture Committee staff members estimate that at one point or another last year 34 million people received food stamps. That is 1 out of every 7 Americans, or about as many as receive Social Security benefits.

The liberalizing step that Bergland favors is to eliminate the so-called purchase requirement in the program. This would not increase the benefits of households already on the rolls, but Bergland and his advisers think it would remove an obstacle that keeps some needy families off the rolls.

The way the program now works, all participating families of a certain size - all four-member families, for example - get the same amount of food stamps each month. Except for those at the very lowest income levels, they have to buy the stamps, and the amount each has to pay depends on its net income.

A family with a low net income has to pay only a small fraction of what the stamps are worth, while one with a higher income has to pay closer to the value of the stamps, and therefore gains less from the program as its so-called "bonus" is smaller.

Liberals say the trouble with this system is that some needy families cannot come up with the cash necessary to purchase food stamps each month and thus are barred from the program. Bergland's idea is to drop the purchase requirement and give eligible families the bonus stamps to which their incomes entitle them.

Bergland adviser and part-time House Agriculture Committee staff member John R. Kramer estimates this proposal will add about 1.2 million people to the monthly total of recipients at an increased annual cost of $400 million to $500 million.

On the other hand, there are many families - perhaps 10 per cent of all that take part in the program - that have low or no income in certain months, during which they qualify for stamps, but whose income the rest of the year is high - at least twice the federal poverty line.

Bergland would make these families pay back the bonus value of their stamps at income-tax time; they would thus get the bonus only temporarily, as a kind of loan. Kramer says this would recoup the money lost by dropping the purchase rule.

Bergland also is considering simplification of the complicated rules for calculating net income in the food stamp program, perhaps by turning to a standard deduction. But aides caution that the whole plan remains subject to change as it is reviewed.