I recently was asked to discuss improving soil and root health in pecan orchards at the Comanche County Pecan Workshop in Texas. During my research on this topic, I discovered that soil cover, soil disturbance and plant diversity are often overlooked, but are critical in the production of pecans.
There is a lot of literature focused on the management of the pecan tree, but is this primary focus on the tree the right approach? Over the last couple of years, there has been a lot of interest in soil health, how the health of soils is determined, and methods of improving soil health. There has been a lot of work published that involves improving soil health through organic farming practices and recently in row crops utilizing cover crops and no-till. How can pecan producers use this information to increase efficiency in pecan production and improve the soil health in their orchards?
According to the Natural Resource Conservation Service (NRCS), soil quality is “how well soil does what we want it to do.” Soils should hold water and nutrients like a sponge where they are readily available for plant roots to take up, suppress pests and weeds that may attack plants, sequester carbon from the atmosphere and clean the water that flows through it into rivers, lakes and aquifers.
A healthy soil is defined by the Soil Health Institute as “the continued capacity of the soil to function as a vital living ecosystem that sustains plants, animals and humans.” Some of the objectives of soil health are to the keep the soil surface covered, minimize mechanical soil disturbance, increase plant diversity, keep growing roots in the soil as many days of the year as possible, and mimic nature.
To improve the health of soil we must first understand that soil is organic (living), made up of billions of different organisms from millions of species. Soil is a complex ecosystem that is comprised of air, water, soil, microorganisms, plants and animals. The total weight of living organisms in the top 6 inches of soil per acre is 2,000 pounds to 30,000 pounds (Gugino 2007; Brady 1996). In just one teaspoon of agricultural soil there can be one hundred million to one billion bacteria, 6 to 9 feet of fungal strands put end to end, several thousand flagellates and amoeba, one to several hundred ciliates, hundreds of nematodes, up to 100 tiny soil insects, and 5 or more earthworms. These organisms are essential for healthy growth of your plants.
Soil cover improves soil water relations by preventing crusting of the top soil, holding moisture until it can soak in, and reduces evaporation. Cover also moderates soil temperature, suppresses weeds, and provides food and shelter for microorganisms. In a study of pecan orchards in New Mexico, ground cover reduced the temperature of the soil by approximately 72 percent compared to bare soil. The importance of soil temperature is related to water usage and the survivability of soil bacteria. When temperatures are below 95 degrees F, 100 percent of the moisture is used for plant growth; when temperature increased above 95 degrees F, only 15 percent is used for growth; and when temperatures reach 113 degrees F, 100 percent is lost through evaporation and transpiration.
At 140 degrees Fahrenheit, the soil bacteria die (McEntire, 1956). Cover also helps increase soil organic matter. When it comes to improving the soil, one consideration is management techniques that will increase the organic matter. Organic matter loosens the soil, increases porosity, and stabilizes the soil structure. For every 1 percent increase in soil organic matter, the water-holding capacity of the soil increases 20,000 – 25,000 gallons per acre (Garcia, 2016).
Soil organic matter is both living and dead material. Bacteria, fungi, protozoa, earthworms, tiny insects, and other organisms form the living fraction of soil organic matter. The “dead” portion of soil organic matter is the fresh residues that have been recently added to the soil. These residues are actively decomposing and provide the fuel that the soil organisms need to survive. It will decay into CO2, water, and minerals within a few months to years. This process provides energy (e.g. via respiration) for soil microbes and mineral nutrients for both microbes and plants (e.g. crops). Therefore, the decaying leaves, manure and plant roots provide sugars and carbohydrates for bacteria, fungi, and the soil food web.
Some soil organic matter is very resistant to (further) decay and can last (often bound tightly to clay particles) for hundreds of years. This very stable form of soil organic matter is commonly referred to as humus. In fact, the average humus particle is 1,000 years old. Humus is typically about 70 percent of the total soil organic matter in agricultural soils. Humus, in particular, and soil organic matter, in general, are important in enhancing soil nutrient-holding and water-holding capacities, soil structure and tilth, and general fertility. Organic matter management is an important part of farming, but our understanding of it is quite elementary. We know that soil fertility tends to increase with increasing soil organic matter.
Minimizing mechanical soil disturbance protects surface residue, microorganisms, and preserves microorganisms’ food and habitat. It also conserves water and maintains, or improves, soil structure, and helps reduce weeds. There are times when growers feel that soils need to be aerated to break up a hardpan or to increase permeability of the soil. Typically, the first thought is to use large equipment to accomplish this. It is hard to believe that sometimes the same results can be achieved using simpler biological methods. It may take longer to see the effects, but these methods do not disturb the soil and can actually increase soil health. For example, some producers are using tillage radishes that grow into the soil profile deeper than 12 inches and can have the same effects as deep tillage.
Increasing plant diversity increases the number of different types of roots that are present in the soil. Roots of different plants excrete different substances that soil microorganisms need for survival. Some plants can serve as a habitat for beneficial insects or be used to lure damaging insects out of the trees. Roots excrete sugars that feed microorganisms and build carbon in the soil. Roots provide a habitat for microorganisms and improve the soil structure. As mentioned before, roots of plants can help aerate the soil.
There is still a lot not known about soil health, how to accurately measure it, and the best way to improve it. Universities, the NRCS and the Soil Health Institute are starting to work on these issues and will give us a better understanding of these issues.