Agriculture Secretary John R. Block, saying that soil erosion has reached crisis proportions, yesterday outlined the administration's plan for dealing with it--an austere, no-new-money approach that raised quick doubts on Capitol Hill.

Block said that the plan, which the president will send to Congress early next year after considering public comment, will target areas of excessive farmland erosion and emphasize reducing flood damage in small, upstream waterways.

Without immediate action, the secretary warned, some of the country's most productive agricultural land will soon be ruined because of inadequate conservation measures and wasteful farming techniques.

He said that erosion is reducing productivity on one of every four acres now being farmed. Unless that rate is stemmed, between 50 and 75 million metric tons of annual grain production--half of 1980 exports--would be forfeited over the next 50 years.

Block also said that erosion and other deterioration on 60 percent of the nation's private rangeland is sharply reducing productivity, cutting forage to half its potential and reducing meat-production potential by up to 800 million pounds annually.

But, the secretary said, budget restraints mean that no new funds will be available for the administration's plan to combat this. Money will be shifted from existing programs and state and local governments will be expected to become more active in erosion control through the use of new block grants.

Block described the new program in appearances before the Senate and House Agriculture committees, where he was applauded generally for good intentions but questioned closely about the administration's frugality and direction.

Sen. Walter (Dee) Huddleston (D-Ky.) said he was concerned that the program would strive to achieve only "immediate benefits," while ignoring longer-range concerns of financing and technique.

Sen. Richard G. Lugar (R-Ind.), suggesting that Block and the administration were dodging a political hot potato, worried that federal farm policies were encouraging production in areas where erosion is intense or groundwater supplies are diminishing.

Block said that current federal conservation programs must be reshaped to emphasize priorities and to give local governments more authority to deal with problems unique to their areas. USDA, he said, will press farmers to adopt tillage techniques that reduce erosion potential.

Block said the administration is considering spending no more than $100 million over the next five years in the block-grant portion of its conservation program. Most experts regard that as inadequate.

Iowa's Republican Gov. Robert D. Ray, chairman of the National Governors' Association task force on soil erosion, told the House committee that more federal money is imperative if the control program is to work and that states must be given more flexibility in spending it.

Ray said the NGA supports the administration's new emphasis on block grants and targeting major erosion problem areas, but he added:

"I must reiterate my extreme concern that soil conservation funding may be reduced under the current Agricultural Conservation Program from $190 million to $132 million. Soil erosion continues at an alarming rate and unless efforts are increased, productivity will have severely declined by the end of the century."