Crocker National Bank said yesterday that it will report a loss of $215 million for the fourth quarter of 1984 because of continuing worsening in the quality of its agricultural and foreign loans.

But the bank's British parent will add more than enough new funds to Crocker to cover the loss. For the entire year, Crocker said it expects to lose about $324 million.

Crocker Chairman Frank Cahouet said that for the quarter just ended, the giant San Francisco bank expects to write off as uncollectable about $253 million in loans. He said the bank will add $326 million to the bank's reserves to cover loan losses.

Under accounting procedures for banks, additions to loan loss reserves are deducted from earnings, while loan losses themselves are written off from the special reserves.

Despite the massive loan losses, however, Crocker will have a loan loss reserve of about $300 million as of Dec. 31, up from $243 million on Dec. 31, 1983.

Cahouet told reporters at a San Francisco press conference that the loan losses involved a large number of credits, all of which were on the bank's books before 1984. He said the bank took a "very, very hard-headed view" of its loans.

Crocker, the nation's 12th-biggest bank with nearly $23 billion in assets, was able to take that hard-headed view because of the deep pockets of London's massive Midland Bank, which owns 57 percent of the California institution and hopes to buy the rest this year.

Midland will invest an additional $375 million -- purchasing $250 million of new Crocker preferred stock and loaning the bank $125 million for a year.

Crocker's problem domestic loans declined to $643 million on Dec. 31 from $680 million at the end of 1983, but its problem foreign loans jumped to $522 million from $98 million the year before. About $300 million of the problem foreign credits are loans to Argentina.

Argentina, as part of a complicated, nearly $20 billion financing pact with both the International Monetary Fund and its bank lenders, last week paid about $850 million of its $1.2 billion in past-due interest and pledged to bring all its loans current within six months.