An old agronomist (who will remain unnamed) once said "in Pennsylvania we are always 3 weeks away from a drought." This is what we are experiencing now with soil moisture conditions starting to reach the critical level after we just came out of a cold wintery season with plenty of snow. It is always important to remember that the major reason for yield failure in Pennsylvania is lack of moisture during the summer. So what can farmers do to make the most of natural precipitation?
Precipitation can go 6 ways:
- run off
- transpire through living vegetation
- percolate to groundwater
- be stored in the soil
Farmers want to do whatever they can to maximize infiltration and soil water storage and reduce runoff and evaporation, while maximizing transpiration of the economic crop - the water that the crop takes up and is then lost from the crop leaves.
So what can farmers do to maximize water intake, minimize water loss, and maximize water storage? One of the big management decisions farmers face at this moment is to terminate cover crops of rye or to harvest them for forage. Rye takes up a lot of moisture at this moment - water that is taken from the soil and then transpired to the atmosphere. However, if you desiccate rye now with a herbicide and leave the dead rye at the surface, the mulch will actually save water! The mulch helps increase infiltration and also reduces evaporation. Especially on droughty soils that don't store a lot of water, it may make sense to burn the cover crop down and leave it at the surface to conserve water instead of harvesting it.
In the middle of summer, it is not uncommon for a crop like corn or alfalfa to transpire 0.3" per day - 9 inches per month. This is more than twice the average monthly precipitation across most of Pennsylvania, which is only about 4 inches. This means we rely on soil water storage to carry us through the summer. This is where soil differences show up - a deep soil with few rock fragments can store a lot more water than a shallow or rocky soil. But you can also increase moisture storage by increasing the organic matter content of the soil. This is possible by returning organic matter to the soil, adding manure, and reducing disturbance by tillage.
Another decision farmers face is to use no-till or tillage. Tillage leads to more soil water evaporation. First, exposing moist soil to the atmosphere helps to dry the soil out, and after that the bare soil loses more moisture to evaporation. The bare soil also tends to produce more runoff. In contrast, the no-till soil is covered with mulch, which helps to reduce evaporation and increase infiltration. Of course, there are intermediate choices--plowing with a moldboard plow dries soil out most, whereas chisel plowing dries the soil out less. Secondary tillage implements also become important--using a field cultivator brings mulch to the surface, whereas a disk harrow tends to bury it. More mulch means less evaporation and more infiltration.
Other choices farmers have include timely burndown of weeds and cover crops and good weed control in the summer. Remember that weeds rob water from the crop. Optimal pH and soil fertility allow crops to make maximum use of water. One might also think about reducing plant populations--high plant populations usually pay only when plenty of moisture is available. On shallow, rocky soils high plant populations are less likely to pay than on deep soils. Finally, timely planting can help summer crops create deep root systems which helps them access more moisture than late-planted crops.