President Carter met yesterday with representatives of the nation's major farm organizations but he failed to convince the most militant of the farm groups that the administration is sympathetic to their economic plight.

Following the 40-minute meeting at the White House, Laurence L. Bitner of the American Agriculture Movement said that Carter offered the farmers "just a defense of the current farm bill. I don't think the president really listened to what we presented."

"It looks like we're going to have to go home and do it ourselves, said Gerald McCathern of the same organization, referring to threats by farmers to stop or reduce their planting in order to force farm prices up.

Carter has already publicly rejected farmers' demands of 100 percent parity. Parity is a measure of farmers' purchasing power, with 100 percent parity meaning that farm prices would be high enough to give farmers the same purchasing power they enjoyed in the 1910-14 base period used for the measurement.

At the start of the meeting, with reporters present, the president told the more than 20 farm representatives that the 1977 farm bill was "a good step in the right direction." He said farm prices have slowly risen since he took office and that the administration has "a good program evolving."

After the meeting Bitner said, "The administration is still sandbagging the issue, but it will have to fave the issue . . . The target prices in the farm bill are below the cost of production. We want stable prices, too, but they must be above the cost of production."

Bitner predicted shap drops in plantings this spring, both because of the expected farm prices and because he said some farmers are so financially hard-pressed they cannot obtain sufficient credit to continue large planting operations.