THE PROCESSION of tractors chugging over the freeways in the farmers' demonstration here last weekend leaves a curious image in the mind's eye. It was high-technology agriculture trying to carry its grievances to its customers and dependents in the post-industrial city - if only it could find them. The parade ended at the White House more out of respect for convention, perhaps, than in any real conviction that the tenant can do much for them. When Congress was passing a rather expensive farm bill last summer, President Carter had repeatedly urged the committees to hold the cost down.

The demonstrators' emotions were genuine, but their tactics are inevitably going to prove ineffectual. For one thing, their support, even among farmers, is spotty; none of the major farm organizations is actively backing their threatened strike. For another, the present state of the market is not the only cause of their distress. Much of this protest appears to have originated in parts of the country where last summer's drought was severe.

Although the demonstrators talked about this autumn's falling prices, it's not a matter of a single harvest. The real trouble is the radical instability of farm prices over the past five years. Through a combination of unexpected circumstances and pure mismanagement, the Nixon administration sold off most of the nation's grain reserves in 1972. Under the pressure of unprecedented foreign demand, American prices soared. The benefits fell very unevenly. It was a tremendous bonanza, for example, for the people who raise wheat - but a disaster for the diarymen and beef producers who had to buy feed on a steeply rising market.

Farmers invested heavily in new equipment and expanded production to meet the new demand from abroad. But prices kept bouncing wildly up and down in reaction to every fluctuation in worldwide weather. For a couple of years, the crops have been very good in the United States and most other countries. As a result, prices have drifted downward. Meanwhile, inflation has been steadily raising the farmers- costs of production. Merely raising farm prices would constitute only a very temporary kind of relief, since it would promptly speed up the inflation from which these farmers are suffering.

The solution here is a system of grain reserves to act as shock absorbers against the impact of bad weather, crop failures and sudden market fluctuations. In political terms, reserves constitute a pact between producers and consumers. The Carter administration is now embarking, in a gingerly way, on the process of rebuilding the national reserves. But there is a lot of opposition to the idea - mainly from farmers like those who brought their tractors to Washington a few days ago. They believe that reserves tend to restrain rising prices.

That's quite right. But farmers can't have it both ways. If they want an unlimited free market when prices are rising, they can't expect much sympathy from their customers in the cities when prices start to move the other way.Perhaps it's possible to let the price cycles run, with farmers picketing the White House in the years of decline and the consumers picketing supermarkets in the years of upswing. But it's an expensive way for the country to live and ultimately destructive to rational farm production. There's a better way to do it. A restored system of grain reserves won't put prices where the farmers would like to see them. But it could greatly diminish the anxiety for the future, and the fear of collapsing prices, that brought the tractors to Washington.