2. You want to have your soybean crop transpire a greater fraction of the seasonally available water, simply because there is a linear relationship between the amount of total water transpired by the crop and final crop yield.

The seasonally available water includes off-season rainfall that was stored as soil water prior to planting, plus all of the in-season rainfall.

In order for plants to acquire carbon dioxide to produce plant and seed organic dry matter, the pores in the leaves (known as stomata) must open, allowing water inside the leaf to escape. In effect, plants must exchange water for carbon dioxide. As a general rule, the soybean exchange ratio translates into about 1 acre-inch of water (27,154 gal.) being required for every 3 bu. of seed produced/acre.

Crop water use includes water lost via evaporation directly from the soil, as well as water lost as transpiration from the leaves. Crop water use efficiency can be improved by reducing evaporative water loss as this means more water will be available for transpirational water loss. Early planting helps in this regard, because:

  • The cooler soil and air temperatures prevailing in late April or early May are much less conducive to soil water evaporation than the temperatures in late May and early June.
  • The canopy closes earlier in the season, which reduces the interception of solar radiation by the soil surface, thereby lessening the heating of soil surface that drives soil water evaporation.
  • The  higher humidity that often prevails in a closed (vs. open) soybean canopy minimizes the degree of evaporative soil water loss.

In addition to allowing plants to collect more seasonal solar energy for use in photosynthesis, early planting also increases the yield potential by allowing the crop to use more of the seasonally available water for transpiration because less soil water is lost to evaporation.