Pecan Soil and Production Potential
Irrigation is important, but the water delivery and rate depends on the soil. Soil and water go hand in hand with pecans. Amazingly, we know and have worked with growers with less than ideal soil — but with an excellent understanding of their soil, some have made it work. On the other hand, some growers with good soil fail to understand their soil and produce less than the potential of the soil.
In Texas, a 12-inch soil can bear 500 pounds per acre with good management, a 24-inch soil can bear 1,000 pounds and a 36-inch soil can bear 1,500 pounds. Very deep, well-drained soils with sufficient clean irrigation water can bear more.
Root anchorage depends on soil and roots, enough to withstand high winds and heavy crop loads. When hurricanes or tornados blow trees over, we learn the depth and drainage of our soil. Following a storm, large slabs of soil and roots are pulled up and exposed; thus we see the number and size of the roots, the distribution of the roots, and root type, being a tap root or fibrous root system. The classic Hugo Pape pecan tap root on the bank of the Guadalupe River was one large tap root 15 feet deep with large lateral roots extending out 50 feet. In contrast, when some large 50-year-old ‘Stuart’ trees were blown over at the Texas A&M orchard, the trees had no tap roots, but numerous large lateral roots growing out of the ground-line crown.
Soil/root complex. Why do roots only grow so deep, then fail to develop? Mature trees uprooted by a storm provide the answer. Where there is no water, there are no roots. Where there is no soil air, there are no roots. We all know the importance of water, but many growers fail to appreciate the importance of soil air, with water, on root growth. Root systems should be dynamic, constantly generating new roots. With soil water and air, as young roots die, new roots grow back immediately. However, without water and air, as young roots die, no new roots form.
Soil air. In general, we say the soil should be 50 percent particles (sand, silt, clay), 25 percent water, and 25 percent air (which is 21 percent oxygen). If the soil does not have sufficient oxygen, several negative issues occur: (1) young roots die and water absorption via osmosis stops; (2) with no oxygen, essential nutrient ions such as nitrogen cannot be absorbed because it is an active transport system which requires energy and oxygen; (3) when root tips die due to no soil oxygen, the plant growth regulator, cytokinin, is no longer produced from the root tips and cell division throughout the tree slows or stops; and (4) when there is sufficient oxygen in the soil, the root system can selectively exclude salt absorption. In short, with good soil drainage and sufficient soil air/oxygen, pecan roots can absorb water and nutrients, produce cytokinin and exclude salts.
Soil depth. Most of the water and nutrients absorbed by the tree are in the top 12 inches of soil because oxygen is highest there. We also say that 90 percent of the water and nutrients are absorbed in the top 36 inches of soil. However, when one has superior soil such as those on terraces or ridges of major rivers, absorption can be much deeper, thus more and better. Our major rivers can have 30 feet of drained soil and our smaller lesser rivers can have 10 feet of well-drained soil. Creeks can have deep soil, but they can also have shallow soil. The West Cross Timbers region of Texas has some extremely deep well-drained soil with season-long water-holding capacity and very deep root development. East Texas upland soils are usually less deep with limited water-holding capacity, but with irrigation can do well.
Clay layers in alluvial soils adjacent to our major rivers can stop the upward movement of water during periods of drought and stop the downward movement of water during periods of heavy rain. These clay layers can and do create underground “perched” water tables that prevent new roots from forming due to no soil oxygen. All river bottom orchards have spots or areas where trees fail to grow or die; in most cases, this is due to underground clay layers.
Shallow soils hold less water and saturate faster/easier resulting in no soil oxygen. Growers with shallow soil need to apply only the amount of water the soil can hold. Shallow soils also need to be irrigated more frequently to supply the volume that has been used by the tree.
Soil texture is sand, silt and clay, or the size of soil particles. A loam is a combination of these different particle sizes. Sandy soils drain fast with higher oxygen potential but have poor water- and nutrient-holding capacity. Clay is extremely small particles, at only 2 microns, with slow drainage but high water- and nutrient-holding capacity. Professor Fred R. Brison taught me a good lesson on soil texture; he said, “Young trees grow fast on sandy soil, but old trees bear better on clay soils.” The ideal soil would be a deep, sandy clay loam having both oxygen and water.
Soil structure. Clay can have good soil structure or poor, and there is a big difference. When a clay soil has good structure, the chemistry is such that the clay particles flocculate to form aggregates. Clay soil with good structure drains well; plus it has good water- and nutrient-absorption potential and air can move into and within the soil. Clay soil with good structure will crumble when squeezed in the hand. Clay soil that has poor structure is as hard as a rock when dry. If clay soils are irrigated with high sodium water, the soil becomes deflocculated and it begins to drain poorly and water and oxygen fail to move in the soil. There are millions of acres of good soil in Texas that have no pecan potential because the irrigation water contains too much sodium. Soil scientists use a Sodium Absorption Ration (SAR) to evaluate water for irrigation; an SAR 4 or less is ideal, an SAR of 5 to 7 is acceptable, but will need clean irrigation water or rain to leach sodium out of the soil. An SAR 8 to 12 should not be planted with pecans.
Mature orchards with high SAR and destroyed soil structure can be helped, not corrected, by tilling gypsum (calcium sulfate) into the soil. At El Paso and Las Cruces, some growers with severe sodium issues with no drainage have worked hard to rebuild soil structure between mature pecan tree rows. Gypsum was mixed with the sodium dead soil between rows with a track hoe in trenches 12 feet wide and 6 feet deep. In theory, the calcium from the gypsum will move onto the clay while sodium will move off the clay. Clean irrigation water or rainfall can then leach the now unattached sodium below the potential root zone. Soil or irrigation water with an SAR 13 or higher cannot be used for pecans.
Soil pH is the best and most important information one obtains from a soil analysis. The ideal soil pH is 6.0 to 6.7 for most crops. Pecan do well in a wide range of soil pH. If the soil is too acid, ground limestone (lime) will need to be added; otherwise there will be insufficient calcium in the soil for ideal tree growth and production. Dolomitic limestone with calcium and magnesium is sometimes used. On the other hand, if the soil is too alkaline with a pH 7.9 to 8.4, the pecan root system will not absorb sufficient zinc or iron, regardless of their level in the soil. In the irrigated West where all pecan soils have a high pH, ammonium sulfate fertilizer is used because it helps lower the soil pH. All pecans on acid or alkaline soils benefit from frequent foliar zinc sprays, but with high pH soils zinc sprays are essential.
Compaction of soil can become an issue with repeated heavy equipment use in the orchard. Cultivation, irrigation and equipment traffic over time can move clay particles below the tilled layer and create a plow pan. Growers need to dig holes in their orchard to see if the soil is soft or hard. If the soil is too hard to dig, it is too hard for roots to grow.
Drainage and aeration. In high dollar vineyards of Europe, every vine row has a French drain 36 inches below the soil line to move water out of the soil and prevent water saturation. Golf courses and athletic fields now use plastic drainage pipes to prevent sub-surface soil saturation with water. Low places within an orchard, which trap water for longer than 48 hours, would profit from improved surface drainage or underground drainage tile. River bottom sloughs that never drain should not be planted to pecans. When trees die in poorly drained sites, trees should not be replanted; they will also die.
Soil color in river floodplains can be an indicator of how well the soil drains and how aerated the soil may be. In Texas many years ago, the red soils of the high plains were transported down the Red, Brazos and Colorado rivers to the floodplains of East and South Texas. These are excellent soils as they tend to be deep, well-drained, and fertile. These river bottoms have ridges or terraces that are higher than the normal floor and they are also usually a sandy loam, two reasons that make them outstanding pecan soils.
A special thank you goes to Jason and Jennifer Pape for a very generous gift to the Texas A&M Pecan Orchard in honor of their dad and friend of all pecan growers, the late Harold Pape. We all miss Harold, he was a wealth of pecan knowledge, and we know no one loved pecans more he. Harold was a true pecan hero.