Washington — When I write about agriculture I write with personal experience: I used to stand on the top of Grandpa Lang's hay wagon in South Lee, N.H., drawn by the ox-team and trample down the stuff with my bare feet as my uncle and the hired man pronged it up to me and my brother over the side rail. (Hay has a wonderful smell though it is scratchy.) That brings me to Lester R. Brown and his new study for Worldwatch Institute, a research outfit located here, entitled ''US and Soviet Agriculture: The Shifting Balance of Power.'' If ever there was a failure of centralized planning it is Russia's; if ever there was a victory of free enterprise it is America's. Each day two 20,000-ton freighters with grain leave America to help feed the Soviet Union. As I think of it it is perhaps the outstanding irony of international trade: the two superpowers arming for possible destruction and yet, how they need each other! Russia needs our grain (and corn and soybeans) and we need Russia's purchasing power to take our crop off the market. The semi-dependence of the Soviets on us would seem to be more humiliating, and yet we are tied in a way to Russia, too (though we don't like to admit it), which is why President Reagan so quickly ended the Carter grain embargo of Russia when he took office.
Lester Brown has the latest figures (Worldwatch Paper No. 51, October 1982), and some of the problems the Soviets have made for themselves with their centralized planning are almost funny. At a press conference he said that the Soviets have already planned their 1985 fertilizer budget and allocated it to regions. Neither the Soviet farmers nor the insect pests they fight quite understand the system. Having access to the right pesticide at the right time may be crucial to controlling weeds and insects. But the Soviet chemical industry in 1981 produced only 60 of the 144 necessary plant protection compounds. It was all centrally planned, you see. A curious thing is his discussion of the evolution of insect resistance to various controls; the communist insect ''shows little respect for the time lags of Five-Year Plans.'' In other words, ''without an agricultural chemical industry continuously altering its product to meet changing needs, Soviet farmers are frequently helpless in fighting an insect infestation . . .'' That's a thought that would never have occurred to Grandpa Lang, I guess, as he faced the seasons from his green and white New England farm house so many years ago.
On the other hand, the Carter grain embargo against Russia didn't work out the way it was expected, Mr. Brown explains. We cut sales to Russia and sent a lot of the left-over to Japan; simultaneously Japan reduced imports of grain from the Argentine and Argentina sent off more grain to Russia. Mr. Brown called it a kind of ''shuffling of the deck.''
And today the Soviet agriculture continues ''in deep trouble.'' The new study explains that ''this year the Soviet Union will attempt to import 46 million tons of grain, more than any other country in history. As a result one-fourth of all the grain for Soviet human and livestock consumption will come from abroad.''
Russia equips armies, lofts Sputniks, and makes nuclear bombs. Where it fails is in making the central planned socialist economy (to protect which the whole contest is all about) work efficiently.
''The Soviet economy is a planned economy,'' Mr. Brown explains, ''but their imports were not planned. Soviet food production shortfalls are rooted in the economic system itself. There is an inherent conflict between a centrally planned, controlled agriculture and a modern, highly productive agriculture.''
What happens now? Why, continues Mr. Brown, ''without fundamental economic reforms, perhaps as great as any since the Communist Party came to power in 1917 , even larger Soviet food deficits may be inevitable.''
There was a time when threatened food surpluses led the US government to pay farmers for not producing crops. Just the opposite problem plagues the Russians. But world trade connects us all. America's opposition to the Soviet natural gas pipeline, for example, might mean less Soviet foreign exchange to buy US grain. The superpowers cooperate in this curious and important exchange - they snarl at each other but grain-laden ships link US farms with Soviet dinner tables. Maybe some day it will spread to political cooperation, too.