The Downsides of Overwatering Pecans
Too much, too little, or just right? Much like Goldilocks, pecan growers seek to give their trees the right amount of water to survive and prosper.
But what about overwatering pecans? Pecan trees are much more tolerant of wet soils than just about any other kind of fruit or nut tree species, so overwatering might feel like a kind of insurance policy against underwatering. But just as with underwatering, there are excellent reasons for you to apply the tools of scientific irrigation scheduling—soil moisture monitoring, tracking climate-based evapotranspiration estimates, and plant water status monitoring—to avoid overwatering your pecan orchard.
Obviously, too much water is an unnecessary expense. In just about every instance, applying more water than the orchard’s consumptive water use means increased water losses to deep percolation or evaporation, which will translate into an unnecessary direct or indirect cost to you, the grower. Maybe it will mean higher energy costs (pumping costs), or maybe it will be a kind of “opportunity cost” for you, where you could have used that water for another good profitable purpose in your operation. And then, there is the question of natural resource stewardship. Depending on where your farm is located, these things might be more or less a concern.
A serious concern about overwatering is that it reduces the efficiency of your trees with regard to mineral nutrition. One significant way this happens is that overwatering can leach beneficial mineral nutrients down past the tree’s root zone. Since soil clay particles have a negative charge, this leaching issue is most importantly a concern with negatively charged forms of nutrients such as nitrate, but other nutrients in the solution can be pushed past the trees’ root zone, as well, and represent a real loss too.
Let’s just focus on nitrate because that’s the big one. Nitrate leaching should be your concern even if you use ammonium-based fertilizers. When you apply an ammonium-based nitrogen fertilizer like ammonium sulfate, soil bacteria rapidly begin to convert that ammonium to nitrite (a negatively charged, easily leached nitrogen form) and then to nitrate. Nitrate leaching losses are expensive because your trees aren’t getting all the fertilizer you’re buying for them. And it’s a serious environmental concern impacting human health. Leached nitrate has to go somewhere and is likely to be polluting the groundwater, perhaps the same groundwater that you’re drinking.
The catch-22 here is that on the one hand you have to avoid leaching nitrate and other beneficial nutrients. And on the other hand, you know that you have to apply a leaching fraction (intentional overwatering!) to avoid accumulating salts in the trees’ root zone. Which is it? Can we have it both ways?
The trick here is timing. Applying a leaching fraction for salinity management should typically be minimized during the times of heaviest nitrogen fertilizer application, like during May or June. Rather, it should be concentrated during other parts of the season when nitrogen fertilizer applications are lower. I really like to see wintertime or early spring irrigations for the purposes of leaching salts—ultimately, that can be the most effective for managing both nutrients and salinity.
In heavy clay soils with high water holding capacity and poor drainage, there is another consideration. These kinds of soils stay saturated with water for extended periods so that low oxygen levels in the soils may injure the tree roots—they’re literally drowning in the soil. Pecan tree roots are just like you and me; they need oxygen to function and survive. They can “hold their breath” for a while, but eventually, they need some air.
In a fascinating study conducted a number of years ago here at New Mexico State University (Kallestad et al., 2007), researchers kept mature ‘Wichita’ pecan trees flooded for more than a month, starting in mid-July. Using specialized research equipment, they measured leaf photosynthesis on these trees every other day. These researchers found leaf photosynthesis dropped steadily during the entire time, whereas for the control trees, which were watered and allowed to drain normally, it remained pretty much steady. What does that mean? The drop in photosynthesis in the flooded trees is an indicator of severe physiological stress to the trees caused by the anoxic rootzone conditions. The drop in photosynthesis also means that the tree is losing its ability to produce the food it needs to fill the nut crop. One of the interesting things here is that the damage to the photosynthetic function persisted even after the flooded soils were finally allowed to drain. There was permanent injury to those leaves that season.
I expect that, especially with flood-irrigated immature pecan orchards, the kind of damage seen in that experiment might be fairly commonplace and that it might slow down tree growth and establishment.
When planning your irrigation schedule, don’t forget to consider the downsides of overwatering. As this experiment shows, overwatering can be just as detrimental as underwatering. Applying the tools of scientific irrigation scheduling—soil moisture monitoring, tracking climate-based evapotranspiration estimates, and plant water status monitoring—can help you avoid overwatering your pecan orchard.