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Management of Wet Soils, Now and in the Future

Management of Wet Soils, Now and in the Future
By SJOERD WILLEM DUIKER
 
NOAA National Centers for Environmental Information have recorded a large increase in heavy rain events in Pennsylvania and predicts that future increases in winter and spring precipitation will raise the risk of springtime flooding along rivers and streams. The U.S. National Climate Assessment showed 55% increase in extreme rainfall events in the Northeast from 1958 to 2016. One of the reasons for this is that the warming atmosphere holds more water – in fact, for every 1 degree celcius rise in temperature the atmosphere water holding capacity increases 7% - and the mean annual temperature in PA has increased 2 degrees F (about 1 degree C) over the last century. And more water in the atmosphere means greater likelihood that it will come down too. Last year and this year show us what this means, and if our understanding of climate is correct, we can expect more of the same. Our concern – how will we manage our soils to maintain their productivity and environmental function? Our first concern will be how to control soil erosion and water runoff, second will be how to avoid soil compaction and third how to remove excess water and how to deal with dry spells between heavy precipitation events.
 
Soil erosion: The danger of soil erosion increases if heavy rainfall events become more common. The kinetic energy of high-intensity rainfall falling in a short time frame is much higher than a low-intensity event falling over a long period – although the total quantity of rain is the same. To protect our soils from erosion it becomes increasingly more urgent to armor them with crop residue. Soil cover is our first and most important line of defense against soil erosion. The raindrops will now impact the residue or living vegetation and this reduces surface sealing and increases infiltration. Further, it will be more important to improve soil structure to avoid it from collapsing as a result of wetting/drying and freeze/thaw cycles as well as the effects of agricultural activities. Building soil structure basically boils down to stimulating and maintaining soil biological activity. It is a never-ending battle to feed those soil organisms and keep them active by not disturbing the soil and feeding them organic matter. Soil tillage disturbs and breaks up aggregates, leads to loss of soil organic matter, and buries residue, so should be avoided. Keeping residue on top of the soil means preserving the habitat for many important soil organisms (like deep-burrowing earthworms) that contribute to maintaining porosity and structural stability. Adding manure or compost is another way to build soil organic matter and improve soil biological activity. We also need to protect soils from gully formation. If runoff occurs – and it will sometimes - the soil should be stabilized where runoff concentrates. In our annual cropping systems we have times without living roots and that is the time soil is most vulnerable to rill erosion. These rills can become gullies of the situation continues year after year. If you see signs of rill formation it is important to address that to avoid them from growing into gullies. Sometimes vigorous cover crops will be enough to avoid rill formation, but sometimes perennial sod with a more robust root system is called for to protect the soil where runoff concentrates. Other practices to reduce runoff are planting on the contour, growing narrow-spaced crops, and practicing conservation crop rotations.
 
The second issue that will become more important to address is to avoid soil compaction. Soils are most sensitive to compaction when they are in a plastic state – when you can make a ball out of soil in your hand. These conditions are expected to become more common in the spring as well as other times of the year. To avoid causing soil compaction, it is important to limit axle load to no more than 10 tons and to reduce surface stress as much as possible by increasing the foot print of equipment. The latter can be done by using tires inflated to low pressures or by using tracks. New tire technology is available that allows use of variable inflation pressure and change the pressure from road to field. On the other hand, using steel wheels or road tires inflated to 100 psi causes much surface compaction and should be avoided. Planting in wet soils will probably become more common too – so it will be important to have planting equipment set up to avoid side-wall compaction or compaction caused by closing wheels. Surface soil compaction is also a concern for farmers grazing livestock. To avoid compaction during wet periods, graziers should move animals more quickly through paddocks, put the animals on perennials with vigorous, tough root systems such as tall fescue or reed canarygrass, or remove the animals from the pasture until soil dries out. Besides avoiding compaction, it is also important to increase the resilience of soils to compaction by stimulating biological activity, improving their organic matter content, and maximizing living root growth.
 
Finally, more periods with wet weather will increase the need to remove excess water from the fields. I am running into this with a field at our research farm. It has a low-lying area that actually has an alluvial soil type (developed in material deposited by flowing water). It rarely ever had standing water in the past 20 years but in these past two years the ducks were swimming there. We are now looking into improving surface drainage by lowering a farm road that blocks surface water, or installing a pipe underneath the road. Soils in flood plains are often the most productive but the risk of flooding may forestall their profitable use. Although it may also be possible to build bunds or dikes on the edge of the stream to keep the water out of the field. It may also be that tile installation becomes profitable. Changing types of vegetation may be warranted. For example, red clover or grass may be a better choice than alfalfa on soils that sit wet. Warm season grasses like switchgrass or eastern gamma are also plants that handle wet soils better than others. Sorghum handles wet soil better than corn. Interestingly, climatologists also predict more drought periods between heavy precipitation events. So growing crops that can handle drought stress better, improving deep rooting of crops, and practices to reduce water evaporation like mulching would also make sense. Supplemental irrigation may be warranted for high-value crops like vegetables.
 
Because we agriculturalists are so dependent on the weather it will be important to keep the finger on the pulse and adapt to a changing climate.