AGRICULTURE Secretary John R. Block has put his family on a food stamp diet for a week. Like other politicians of the left and right who have tried out the Agriculture Department's "Thrifty Food Plan" to prove a point, the secretary will, no doubt, come through with no physical damage and a good deal of moral satisfaction. But what will the demonstration have proved?

It is no surprise that the average well-fed American can get by--and perhaps even benefit from--a few days of leaner living. But the secretary and his family might find their diet of bread, cheese, cereals, dried milk and an occasional hamburger a good deal harder to accept if they had to put up with it for 52 weeks a year with no time out for parties or even a trip to McDonald's. Even setting that aside, the secretary is treating himself too generously for a valid comparison.

Mr. Block has allotted himself the full weekly food stamp ration for a family of four. That's about $58 a week. But a poor family can't get that many food stamps unless it has almost no cash income-- in which case it is not likely to have a roof over its head, let alone a stove to cook on. Most recipients have their allotments reduced by 30 percent of the amount by which their weekly income exceeds $20.

As a result the average food stamp recipient gets stamps worth only about 47 cents per person per meal, or about $39 a week for a family of four--almost $20 less than Mr. Block's family is using. Of course, the family is quite free to supplement that amount with its free cash. But all food stamp recipients have income below the subsistence level defined by the official poverty line. And you can well imagine that a poor family trying to cover all its other costs--shelter, clothing, utilities, school expenses as well as the non-food grocery items such as soap that food stamps don't cover--won't find it easy to earmark additional money for food. That's why the end of the month now finds so many poor people frequenting soup kitchens and cheese lines.

If Mr. Block wants to find out what it's like to be a typical food stamp recipient he should move his family out of its comfortable house and try living on the $82 a week that Maryland welfare would provide them along with a food stamp allotment suitably reduced to account for their $62 in "excess" income. Then he might want to remember that last year he proposed increasing the food stamp offset to 35 percent of income. And he might understand why Congress saw fit to reject this idea.