IN THE FARM BILL it passed last month, Congress refused to take full responsibility for all the changes it was approving in national policy. Some steps it legislated, but others it left to the judgment and nerve of the secretary of agriculture, on the theory that he and the president should be made to take part of the blame. Now outgoing Secretary John Block has taken nearly full advantage of these areas of discretion. A series of orders he issued this week will powerfully rearrange the agricultural landscape.

The most important orders have to do with price. Price supports for wheat and corn, the main supported products, will be about a fourth lower in the next crop year than they were in the last. In recent years support prices have been steadily higher than market prices, and farmers have sold to the government rather than to private buyers. The high prices here have cost U.S. farmers foreign sales. The idea is to reverse this. The support price for wheat was $3.30 a bushel last year. Congress took this to $2.70. Mr. Block then exercised the further power granted him to take it to $2.40. So also with corn, which last year was supported at $2.55 a bushel. Congress took it to $2.16, Mr. Block to $1.92.

The way the bill was written, the flip side of lower price supports will be larger income subsidies. Farmers are guaranteed minimum prices for their products, then also guaranteed the difference between what their products bring and so-called target prices. The farm bill, while lowering price supports, freezes target prices for two years. The target for wheat will remain $4.38 a bushel, for corn, $3.03. The gaps between market and target levels will increase, payments per farmer will be higher -- and more farmers will be drawn into the program. Last year the participation rates were about 75 percent for wheat and 71 percent for corn; this year they may be 85 to 90 percent.

That is why the bill and the secretary also took the second major step of restricting production. Farmers must idle land to qualify for supports. About a third of the nation's wheat land will be out of production in this crop cycle, and a fifth of the corn land. These figures are not as high as during the payment-in-kind program of 1983-84, when almost half the harvest was suspended and farmers were given grain from government bins to sell instead. But a huge amount of productive capacity is being put in reserve. Until target prices can also be reduced or market prices rise, that is the government's only recourse.