The Presidential Task Force on the Arts and the Humanities, concerned that recent changes in federal tax laws might deter contributions to charities, will recommend that tax credits be given to individuals and corporations in an effort to encourage more giving to cultural activities.

In the task force's report, which will be sent to President Reagan probably at the end of this month, the group will recommend:

* a tax credit of 25 percent, in addition to the normal deduction for charitable contributions, for the second $1,000 of philanthropic contributions by individuals.

* the same tax credit of 25 percent on the first $100,000 of such giving by corporations.

Both credits would become effective in the 1982 tax year and would decline by 5 percent annually until phased out by 1987.

The task force does not know how much this tax credit plan would cost the government. "We're talking to the Treasury Department now," said David Morse, staff director of the task force. "We hope to get some revenue figures soon."

The tax credit plan was the idea of task force co-chairman Daniel Terra, ambassador-at-large for cultural affairs. The plan was one of several recommendations -- some final, some tentative -- drawn up two weeks ago by the 36-member task force at a meeting in Los Angeles. The group, which is also chaired by Charlton Heston and University of Chicago president Hanna Gray, will hold its final meeting here Sept. 16 to complete its report to the president. After that report is sent, the task force will disband.

Another task force recommendation would allow artists donating their works to museums or libraries to deduct for tax purposes the fair-market value of the works. Currently, individuals donating works of art can deduct fair-market value, but artists can only deduct the cost of the materials. The task force will recommend that fair-market value "be governed by the most recent arms-length sale, by the creator, of a comparable work," according to a public task force memorandum listing various recommendations.

Other proposals that will be discussed by the task force on the 16th for possible adoption include:

* The Federal Council on the Arts and the Humanities, in conjunction with the Treasury Department, could establish a panel to monitor current data on "the sources and magnitude" of giving to charities.

* The 50 percent limit on personal income that can be deducted for charitable giving and the deduction limit on contributions of stock and appreciated property should be increased.

* The distinction between gifts of artwork -- which can only be deducted if given to a museum or other organization with a related use for them -- and gifts of stock or other holdings should be eliminated.

* The 10 percent deductibility limit for charitable giving on corporate taxable income should be eliminated. (Many observers consider such a limit superfluous, since studies show that many corporations that donate to the arts give way below that limit anyway.)

* A tax checkoff to increase personal giving to the arts and the humanities should be established.

* Further tax incentives for sales and lease-backs of real property by individuals and corporations to charitable institutions should be established.