The Samuel Roberts Noble Foundation, Inc.
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Soil-Pecan Relationships

November 01, 2000 00:00


In pecan production, the soil is simply a medium in which the tree grows. It comprises particulates (minerals, organic materials, water, air, and living organisms) whose associative complexity is astounding. These associations are the result of weathering that causes rocks to decompose into the elements that can be used as plant nutrients. Nutrient combinations vary according to the chemical composition of the parent material, and external forces such as wind and water can mix minerals to provide a better distribution of available plant nutrients.

Soil has a depth dimension to expand storage beyond the soil surface area. Pecan trees have a massive root system that spreads over large areas and penetrates deeply to obtain moisture and nutrients for growth and production. The roots develop where the soil conditions are conducive to growth. Naturally, water tables can change in soils with good nutrient distributions and cause those nutrients to become unavailable, resulting in a site that discourages pecan tree growth. Soil is a huge, dynamic system that is exceedingly complex to manage correctly.

Pecan trees in Oklahoma grow well on soil sites that provide storage for large volumes of water, adequate levels of the required plant nutrients, good air exchange characteristics, and an abundance of microlife. Those sites are usually found along alluvial plains with deep well-drained soils that developed when soil eroded from other areas was deposited on the site, creating abnormally deep soils with a good distribution of plant nutrients. Where nutrient and moisture levels coalesce in the correct relationships, pecan trees often grow and produce naturally.

Although the right components are present for pecans to grow in certain areas, water and plant nutrients "salts" can exist naturally in excess, their overabundance often expressed as limited nut production or the decline of existing mature trees. There may be a tool to correct the developing or existing limitation; the difficulty is identifying the treatment and determining whether it is economical. It is possible for the limitation to be so large it is impractical to correct. The best decision might be to abandon that area and move your production efforts to a site with better soil characteristics.

Soil Conditions
Most nutrient cycles that support pecan tree growth occur in the presence of oxygen. In fact, limited oxygen availability in the soil can prevent the release of most essential nutrients. Why has tillage been such an important factor in the history of crop production and even that of pecans? Could it be that using tillage to mix the soil with air facilitates the oxidation and release of stored nutrients? Weed control is a primary reason for tillage in pecan orchards, and with the added benefit of nutrient release, it has provided tremendous short-term results.

To take the oxygen infusion factor one step further, what happens to oxygen when the soil is saturated with water from flooding or especially a high water table? Pores in the soil can become filled with water, driving out the oxygen. Absence of oxygen can prune the roots, since they cannot respire, and the nutrient cycles are blocked from mineralizing additional plant nutrients. Nitrogen can be denitrified from the available forms and will then volatilize (evaporate).

Because oxygen is such an important aspect of soil fertility, discussions about fertility are usually prefaced with a discourse on arable soils. Abundant oxygen is a prerequisite for a healthy, productive soil. If air in the soil is important, then how can the soil be full of air and not be tilled? This argument supports using soil aerators or at least some form of tillage. In some soils, tillage can cause plow pans that interfere with moisture, nutrient, and air movement into and out of the soil profile. A healthy soil, which undergoes recycling that supports abundant biology, can become at least as productive as soils with significant physical and cultural inputs.

Recycling is a major component of building a soil and maintaining its health. Tree leaves, forage that grows on the soil surface, trash from the cleaner, and any other carbon-supplying material should be put back onto the soil surface to feed the microlife. As the organic matter decomposes, releasing its nutrients, the mineral cycle is enhanced. Additional plant growth, commonly achieved through seed and fertilizer applications that encourage growth if subsequent management is proper, can further enhance the mineral cycle. The orchard management decides if the cycles will be enhanced or interrupted with haying, tillage, or even intensive aeration.

Develop an understanding of the factors that affect nutrient uptake, and then decide whether to correct soil limitations of pecan production. These limitations usually consist of nutrient deficiencies and an over- or underabundance of water. Use soil tests and leaf analyses to help identify and correct nutrient deficiencies.

Create soil conditions that provide balanced nutrient levels with optimum availability. In most pecan orchards, the process involves developing good surface and internal drainage characteristics. Tillage seldom provides a long-term positive effect on water infiltration and air exchange within the soil.

Annually monitor nutrient levels in the pecan trees' leaves. There is no other way to develop such a vivid image of a tree's nutrition. The results will provide a history that can become a guide for making management decisions.

Add nutrients when necessary. The continued input as guided by the leaf and soil analyses will boost the productivity of the pecan production ecosystem. Protect and enhance nutrient recycling by not removing vegetation that grows in the orchard; it could improve pecan production. Protect the inputs that could enlarge the nutrient pool.

Continued stewardship of the soil is necessary for pecans to stay productive, and annual management decisions could affect the future of your trees greatly. Will your soil, water, and vegetation management increase or decrease pecan production?