Farming interests worry that Reagan budget cuts may pose a threat to America's topsoil.

The worriers are hurrying to defend programs and needs they see as vital. One of these is to expand programs to control soil erosion. Nationwide, the equivalent of the top inch of the State of Iowa disappears via erosion every year. It costs an estimated $18 billion a year to manufacture and apply fertilizers to replace the vital minerals lost through that erosion.

Agriculture Secretary John Block says "high priority" is being given to programs aimed at increasing farm productivity.

But farmers are concerned that decisions will be made in Washington by men who are skilled in economics and politics but unfamiliar with the very basis of Ameri can prosperity -- the nation's soil.m