Availability of residual phosphorus fertilizer for loblolly pine
Introduction
Phosphorus fertilization of slash pine (Pinus elliottii Engelm.) and loblolly pine (Pinus taeda L.) plantations has been an operational practice on the Coastal Plain of the United States since the late 1960s. At least 81,000 ha/year were fertilized with P at establishment in the southeastern United States between 1996 and 2004 (Albaugh et al., 2007, Fox et al., 2007a, Fox et al., 2007b). Which pine stands to fertilize and when to apply phosphorus fertilizer continue to be important research and operational questions, due to the dramatic recent increases in fertilizer prices. Phosphorus applied in the first rotation may not be sufficient to support rapid growth of the subsequent rotations. Harvest removals of phosphorus, immobilization of P in soil organic matter and adsorption of P to soil iron and aluminum compounds are all factors which may influence the availability of residual P fertilizer to second and third rotation stands.
Historically, procedures used to identify phosphorus deficient sites were based on foliar analyses, soil chemical analyses and/or land classification. Presently, land classification plus knowledge of previous fertilizer additions (amounts and dates) are used to identify stands to be fertilized at establishment (Fox et al., 2007b). Geologic regions where P is applied at establishment include the Citronelle terrace on the uplands of the Gulf Coast, as well as poorly drained sites on the Lower Coastal Plain of the Atlantic and Gulf Coasts (Albaugh et al., 2007, Fox et al., 2007a, Fox et al., 2007b).
Foliar analyses are a direct measure of trees’ phosphorus status, and are usually the best predictor of response to P fertilization. Widely recognized critical levels for foliar P concentrations are 0.09% P for slash pine and 0.10% P for loblolly pine (Ballard, 1980, Ballard and Pritchett, 1975b, Pritchett, 1968, Wells et al., 1973, Wells et al., 1986). One expects a response to P fertilization when the foliar P concentration is initially less than the critical level. The principal limitation to using foliage samples for deciding what sites to fertilize is that the technique can not be used unless pine trees are present which are suitable for sampling. In the 1970s and 1980s some companies delayed P fertilization for 1 or 2 years and collected foliage samples from planted seedlings. Normally in the 1970s and 1980s, a decision to fertilize at time of planting was based either on soil chemical analyses or on land classification rather than sampling mature trees, from the previous stand, prior to harvest.
The most commonly used soil chemical analysis is HCl–H2SO4 extractable P (0.05N HCl + 0.025N H2SO4) also known as the dilute double acid or Mehlich 1 soil test. Critical levels reported for HCl–H2SO4 extractable P are 3 μg/g P (Wells et al., 1973) and 5 μg/g P (Ballard and Pritchett, 1975b) for 0–20 cm depth soil samples. Wells et al. (1973) found 75% of their Lower Coastal Plain study sites were correctly classified when HCl–H2SO4 extractable P was used to predict whether or not loblolly pine would respond to P fertilization. Similarly, Ballard and Pritchett (1975b) found that slash pine response to P fertilization, or the lack thereof, was correctly predicted by the Mehlich 1 soil test for 65% of their study sites. HCl–H2SO4 extractable soil P has been evaluated more often than other soil chemical analyses because several of the soil testing laboratories in the southeastern United States have used it as a routine procedure for many years. The HCl–H2SO4 extractable P method has compared favorably to other soil tests in several studies investigating southern pine response to P fertilization (Ballard and Pritchett, 1975a, Ballard and Pritchett, 1975b, Comerford and Fisher, 1982, Wells et al., 1973, Wells et al., 1986).
Information gained from land classification can be as useful as soil test results for making decisions on what sites to fertilize with phosphorus (Ballard and Pritchett, 1975b, Comerford and Fisher, 1982, McKee and Wilhite, 1986, Kushla and Fisher, 1980). The effects of soil parent material (e.g. geologic formation or terrace) on subsoil P status, soil drainage class, and depth to a spodic horizon, argillic horizon or mottles are all factors related to pine response to P fertilization on the Lower Coastal Plain. Conventional field data collected as part of a land classification program can be combined with laboratory soil chemical analyses to improve P-fertilizer recommendations on specific sites. Comerford and Fisher (1982) found soil drainage class could be used to correctly classify study sites as responsive to P fertilization 60% of the time. Use of HCl–H2SO4 extractable P in conjunction with soil drainage class increased the percentage of sites correctly classified to 80%.
Southern pines grown on poorly drained soils need phosphorus to compensate for anaerobic soil conditions and promote root growth (DeBell et al., 1984, McKee et al., 1984). Good responses to P fertilization are observed on poorly drained soils because P is frequently limiting, but ample nitrogen and water are available for tree growth once phosphorus is supplied. Some of the most responsive sites in the southeast are poorly drained clayey soils which are classified as aquults (Kushla and Fisher, 1980). Substantial responses to relatively low P-fertilizer rates (22–67 kg/ha P) have been observed on these soils (Gent et al., 1986, McKee and Wilhite, 1986, Pritchett and Comerford, 1982, Wells et al., 1986), but in some cases application rates as high as 157 kg/ha P have produced an additional pine growth response (Pritchett and Comerford, 1982).
The effectiveness of residual phosphate fertilizer as a source of phosphorus for crop growth has been studied extensively in agriculture (Barrow, 1980) and to a limited extent in forestry of New Zealand (Ballard, 1978, Comerford and Skinner, 1989, Comerford et al., 2002), Australia (Flinn et al., 1982, Gentle et al., 1986, Turner, 1982, Turner and Lambert, 1985, Turner and Lambert, 1986, Turner et al., 2002), and southern Africa (Crous et al., 2007). Little information is available as to the effectiveness of residual P fertilizer on the growth of second or third rotation plantations in the southeastern United States (Comerford et al., 2002, Harding and Jokela, 1994, Torbert and Burger, 1984). Working on the Coastal Plain, Comerford et al. (2002) conducted a greenhouse pot study and found no significant difference in P content of seedlings grown in soil from field plots which received 0–70 kg/ha P 29 years before the soil samples were collected. Comerford et al. (2002) did find that P content of the F layer of the forest floor was greater in the 35 and 70 kg/ha P treatments than in the 0 and 17.5 kg/ha P treatments, and concluded that the bioavailable P from the forest floor would be sufficient to meet the needs of 1-year-old seedlings of the second rotation.
Section snippets
Methods
MeadWestvaco's second rotation phosphorus fertilization trial was located near Andrews, South Carolina. The study involved reestablishing a P fertilization trial following harvest of the first rotation loblolly pine stand. The soil at the Andrews study site is classified as a Typic Albaquult, clayey, mixed, thermic (Bladen Series), having poor internal soil drainage and clayey subsoil with mixed mineralogy. Clayey aquults are common on the Lower Coastal Plain, and the Andrews study site is
Results
At the end of the first rotation (age 22), the control plots (0–0–0) averaged 19.4 m in dominant and codominant tree height, and the 56 kg/ha P in 1967 plots (56–0–0) averaged 22.2 m, a difference of 2.8 m (Everett and Patterson, 1991). This difference showed the effects of P fertilization lasted throughout the first rotation. Statistical analyses, for age 5 in the second rotation, showed highly significant interactions between tree response to phosphorus applied in 1967 and response to
Conclusion
Loblolly pine growth was substantially better in the second rotation than in the first rotation. We attribute these differences between rotations to differences in competition control and improved genetics between 1967 and 1991. These results indicate that pine plantations on clayey aquults in the southeastern United States are sustainable, and that management differences have resulted in large increases in pine growth from one rotation to the next.
Mineralization of P in old litter incorporated
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