HARVARD, ILL -- . The low, fast-moving clouds whipping over a muddy field in McHenry County promise more rain, which already has stalled spring planting for Mike Book and thousands of other farmers in the Midwest.

The Corn Belt farmers, who two years ago faced one of the century's worst droughts, now are losing time, crop yield and eventually money to the unusually wet conditions. Book, with any luck, would be back planting corn soon on the 2,800 acres he farms on the Wisconsin-Illinois border just south of Harvard.

But the rule of thumb is that every day after May 10 the crop doesn't get in the ground costs a bushel an acre in yield.

The soft-spoken Book had time this day -- in what should have been the busiest time of the year -- to talk about his wife, Jill, and four young children, about his partnership with his brother Ken, who manages a cattle feed lot, and about their father, Jim, who oversees both the family grain and cattle operations.

Standing by his frustratingly silent tractor and corn planter at the end of the 70-acre field, still littered with the remains of last year's soybean plants, the 34-year-old Book stood no chance of planting this day.

"Dad told me this morning that it would take only one good day of sunshine to dry out the fields, but I don't think we'll get a break," Book said, pointing to a few rows that had been planted before the rain hit.

The pace of corn planting in Illinois is far below normal, slowed by the unusually heavy rains, and nine-tenths of the soybean crop remains to be planted. Kenneth Kunkel, with the Illinois Water Survey, said most areas in the state received four to six inches of rain in the first half of May, which is two to three times the normal.

The wet fields are impassable to planting equipment. Book has about two weeks at most to complete the planting, given the shorter growing season this far north in the Corn Belt.

It's a costly delay. With some 600 acres still to plant, and corn at a cash price of about $2.70 a bushel, Book has a lot of money at stake. If the rule of thumb held, he would be losing about $1,600 a day.

In Illinois, corn planting was 65 percent completed when last week began, behind the 89 percent figure normal for this time of year. Indiana farmers had planted 60 percent vs. an average 72 percent. For Iowa, the top producing state, rain wasn't as much of a factor and planting was 80 percent completed, only slightly behind the average pace of 85 percent.

"Memorial Day is about the deadline on corn planting in this area if you expect to get a decent crop," Book noted.

If the rain delay makes the risk so great that a normal-maturity variety will suffer frost damage before harvest, Book could switch to a shorter-maturity, though lower-yielding, hybrid.

Still, he hopes to plant about 1,850 acres of corn, then immediately turn to planting 950 acres of soybeans, a crop that doesn't need as long a growing season. As with corn, the wet weather has meant that Illinois farmers had planted only 11 percent of their usual soybean crop by last week, far behind the average of 44 percent for this time of year.

In his crop rotation scheme, Book plants corn two years in a row in the same field, then switches to soybeans. Thus, every year about two-thirds of the cropland is planted to corn and one-third to soybeans. He would like to work oats into the rotation if the federal price-support programs allowed more flexibility.

"Flexibility" has been a Washington buzzword this year as Congress has struggled to rewrite farm legislation, an exercise undertaken about every five years. Several proposals have been offered for changes to the 1985 legislation, but for Midwest farmers the main idea is to allow some land allocated to corn to be planted with other crops without penalty.

Current law ties Midwest farm income to corn production, even if other crops might prove more profitable. To receive subsidies farmers, among other things, establish "base" corn acreage, which is the average planted over the preceding five years.

Thus, a farmer who wants to cut back on corn in any single year must consider that the action will reduce government payments for the next five years.

A "flexibility" proposal adopted Wednesday by the House Agriculture Committee seeks to encourage production of soybeans and other oilseeds. It would allow farmers to switch 25 percent of the acreage now protected by subsidies to some other crop without losing the potential to put it back into a subsidized commodity later.

Whatever the policy makers decide about "flexibility," Book wants the decisions made early enough so he can plan for 1991 crops. He participates in the farm program only because of a better net return and believes "most farmers would, if they could economically, get out of the programs and make their own decisions on what crops and how much to grow."

Book said rising prices preceding the planting season may have tempted some farmers to drop out of the program so they could plant more corn.

Most of his decisions for this spring's planting were made not long after last fall's harvest. For example, by January most of his seed, fertilizer and herbicides had been ordered. Soil tests, made every three years, showed three fields needed an application of lime to lower acidity, but cold weather allowed application on only two. Cold weather also limited nitrogen fertilizer application to only 450 acres.

Field preparation was completed on April 26 and Book began planting April 28. Almost a full week of planting was halted by showers on May 4. Then a broken hitch stopped the planting. It was fixed a day later, but the rain had started again. "I haven't been able to get back into the field since then."

The break imposed by wet fields has one compensation: Book gets to see his children.

During spring planting, long days in the field usually means he leaves home before the four youngsters, ranging in age from 7 months to 7 years, are awake, and he returns after they are asleep.

"I love being with the kids," he said, "but in the spring a farmer's first priority is to make sure the crops are planted."