Pecan Orchards Improve Soil Quality
Soil quality is too frequently taken for granted, not only in regards to pecan production but in all aspects of agriculture. This is probably because the differences are so subtle and can be hard to see with the naked eye. With that said, soil quality is relative. Our soils in the southeastern Coastal Plain are generally sandy, highly weathered, acidic, and naturally retain few of the nutrients required for optimal plant growth. They don’t hold a candle to the rich soils of the Midwest. But they do respond well to soil amendment-lime, fertilizer, organic matter, etc.
In their native environment, pecan trees prefer soils that have high levels of organic matter, drain well, and are frequently refreshed with a pulse of nutrients in the seasonal flooding of bottomland ridges. Some of the soil quality conditions in the pecan’s native range are a by-product of pecan trees themselves, via the shedding of leaves, shucks, sticks, bark, and other debris.
I’ve thought about this a lot, and based on some orchard soil studies from early in my career, I have had in mind for a long time that pecan orchards in the Southeast had the potential to drastically improve the soil quality of the land they were planted on, which could have multiple direct returns to the grower. A couple of years ago, I tasked my Master’s student, Hunter Slade, with the enormous job of finding out if this was indeed the case. Do pecan trees improve soil?
Over the course of two growing seasons, Hunter pulled a total of 496 soil samples from 41 different pecan orchards and row crop fields across South and Middle Georgia. We pulled most of these samples from a relatively shallow depth of about 6 inches. In 2021, we added an additional sampling depth down to 36 inches to examine any potential long-term differences in soil quality. Our goal was to compare various biological, physical, and chemical soil quality measurements in orchards of different ages and row crop fields. We divided the orchards into different age groups (1st-4th year; 5-10 years; 11-20 years; and over 20 years old). Since most pecan orchards in this region are planted on land that was formerly in row crop production, we wanted to get an idea of how the soil quality of orchards in these different age groups compared with that of annual row crop fields. Therefore, we sampled an adjacent row crop field of the same soil type next to each orchard. This comparison would give us an idea of how the soil quality characteristics of pecan orchards change in the southeastern coastal plain over time. Here’s what we found.
At the 6 inches depth, soil organic matter (SOM) was significantly higher in orchards 11 years and older than in row crop fields in 2020 (Table 1). In 2021, soil analysis showed that organic matter was significantly higher (P < 0.05) in orchards 1 to 4 years old and those 20 years and older than in row crop fields. Variations in the organic matter between 2020 and 2021 are likely related to microbial activity as influenced by variations in temperature and rainfall. In the southeastern United States, organic matter decomposes rapidly due to the warm and wet climate. These conditions are favorable for microbial activity throughout the majority of the year, which prevents extensive accumulation of organic matter. However, even under these conditions, compared with conventionally farmed row crop fields from the same region, pecan orchards do tend to hold a higher percentage of organic matter.
Higher levels of organic matter in pecan orchards are likely the result of the accumulation of plant biomass that is returned to the soil each year. Pecan husks alone account for 25 to 30% of the total mass of the pecan fruit. These husks dry out and fall to the orchard floor each growing season, bringing a considerable amount of plant material back to the soil. Decomposition of leaves and woody plant debris on the orchard floor along with orchard soils being left uncultivated also contribute to the increased levels of organic matter observed in pecan orchards.
Along with soil organic matter, we saw that various measures of soil microbial activity (biologically active carbon, soil microbial respiration, and labile-amino nitrogen or the nitrogen released by micro-organisms) are higher in orchards than in row crop fields. We also observed that, based on these measures, soil microbial activity increases with orchard age. These measurements are associated with an increase in soil fertility.
Although we observed no differences in bulk density or porosity between pecan orchards and row crop fields, aggregate stability did significantly increase in orchard soils with orchard age. Pecan orchards also exhibited a much higher cation exchange capacity (CEC) with age when compared with row crop fields, indicating higher nutrient availability for plants.
Pecans also help to scavenge nitrogen from the soil at greater depths than row crops, resulting in less nitrogen leaching through to the water table. Total nitrogen levels in some orchard soils were higher than that of row crops at 6 inches but similar to that of row crops at 36 inches. This difference indicates that tree roots are likely removing nitrogen from the soil profile. Furthermore, this finding is supported by other studies that suggest that pecan tree roots were able to capture nitrogen in a cotton/pecan alley-cropping system, resulting in lower rates of leaching below the root zone where pecans were grown.
Taken together, the information gained from this study is good news for the pecan industry and provides a good message for consumers about the sustainability and potential environmental benefits of pecan production. Pecan trees not only hold soil in place and enhance the fertility and nutrient availability of the soils on which they grow, but they also actually build soil and filter out pollutants. At least in the southeastern U.S., we can say that the soil quality of land previously used for conventional row cropping systems in the southeastern Coastal Plain can be significantly improved by pecan orchard establishment and can improve with orchard age.
But the benefits of pecan orchards for improving soil quality are only a piece of this puzzle. As the consumer increasingly demands environmental sustainability and climate-smart production systems, the pecan industry needs to gather as much information as possible about the sustainability of pecan production. We are currently involved in research examining methods of growing pecans with less water and fertilizer than we have used in the past. If we can do this, and I believe we can, it will also benefit the producer by lowering production costs, leading to increased profit margins.
Additionally, based on calculations by Kraimer et al. (2001), we know that a mature (approximately 15-year-old) pecan tree has a biomass of 2,795 pounds per tree and accumulates approximately 394 pounds of biomass per year. Roughly half of this biomass is carbon accumulated via photosynthesis. This means that one acre of mature pecan trees can store approximately 50,000 pounds of carbon in the trees alone.
The next step in establishing the environmental sustainability of pecan production will be to determine the net carbon footprint of pecan production through life cycle analysis (LCA). This analysis has been done for almonds, walnuts, beef production, peanuts, and a host of other crops. Because pecans are grown over an entire spectrum of climatic conditions from the humid Southeast to the arid West, we need these studies performed for the various growing regions. The pecan industry would benefit greatly from this information. The results would help us see where we stand as an industry and could serve as a potential marketing tool as we try to further increase the demand for pecans. As sustainability increasingly drives consumer demand, this is the information they will want. There are many great stories to tell about pecans. I believe the potential environmental benefits they offer as a crop are one of those.