Why did so many people plant soybeans during the window surrounding April 20, before rains the following weekend? Temperatures were average at best. It was a full three weeks ahead of when old-timers used to say soybeans should be planted for maximum results. Some people still hold to planting in early to mid-May. And who can say they are wrong. They weren't wrong planting June 5 last year- but that was one year!
Three factors come to mind. Two of the three explain why my neighbor asked me to spend a few hours on the drill when I had a chance. Meanwhile, he was planting corn and doing his own spraying, keeping up with what was planed and trying to stay ahead with burndown where crops would be no-tilled.
First, memories of last year linger. That shouldn't be a deciding factor, but in this case, it was a strong memory- the past two seasons were nearly carbon copies for planting across most of Indiana. Wet soils and plenty of rain at the wrong time made planting a challenge, and typically delayed it, in each of the past two years.
Second and perhaps most importantly, soil conditions were right. In fact, by April 20 to 22, soil conditions where I planted were by far drier and more workable than at any time last season. That includes planting in early June! When the calendar says it's the back half of April, there is data and anecdotal evidence that soybeans can yield well planted early, and soils are in excellent condition, it's difficult to stay out of the field.
The third reason for some was the amount of land they farm. If they farm large acreages and could run both a cornplanter and soybean planter or drill at the same time, many started the soybean planter or drill at the same time the cornplanter went to the field. The theory Howard Doster, a retired Purdue University ag economist, uses is that every acre planted now is an acre that won't be planted in June should weather delays push back planting. Long-term data indicates much better odds for higher yields for soybeans planted April 20 vs. June 5. Last year was the exception- the out-liar, if you well, on the graph of soybean yields vs. planting dates.
How do you know when soil conditions are right? If you're getting good, uniform coverage, even in no-till, then it's a good sign. You also want to dig and check to see how much sidewall compaction you might be creating as the seed opener discs pass through the soil. The less sidewall compaction, the better. Generally, sidewall compaction is less of a problem when soils are dryer.