Phosphate Sorption of Calcined Materials Used as Components of Soilless Root Media Characterized in Laboratory Studies

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  • 1 Department of Horticulture, Forestry, and Recreation Resources, Kansas State University, 2021 Throckmorton Hall, Manhattan, KS 66506-5506
  • | 2 Department of Agronomy, Kansas State University, Manhattan, KS 66506

Calcined materials may contribute enhanced phosphate (PO4-P) retention to soilless root media used in container production. Properties of nutrient retention vary greatly depending on the parent clay and calcining treatment. This research characterized PO4-P sorption of various calcined clay products, including low and regular volatile material (LVM and RVM) 2:1 attapulgite, montmorillonite, and illite clays at various particle sizes; 1:1 kaolin clays in powder form; and diatomaceous earth. Extractable PO4-P, initial pH, PO4-P sorption isotherms, amount of P sorbed as a function of solution pH at a fixed total concentration, and degree of phosphorus saturation were determined. Initial pH of the clays ranged from 3.7 to 8.7. Sorption isotherms were conducted with initial adsorbate concentrations ranging from 0 to 200 mg·L−1 PO4-P from KH2PO4. The calcined materials varied in their ability to sorb PO4-P and generally yielded L-type isotherms, indicating that the adsorbate had relatively high affinity for the calcined material sample surface at low surface coverage. Some 2:1 calcined clays exhibited substantial PO4-P retention, but 1:1 calcined clays and diatomaceous earth did not. Clays with less moisture (LVM) resulted in greater PO4-P sorption than those calcined at lower temperatures (RVM). Terra Green montmorillonites had higher PO4-P sorption than Terra Green attapulgites. Laboratory results indicated potential for substantive PO4-P retention by several of the calcined clay materials when used in container production. For most materials, PO4-P sorption did not show pronounced pH dependence, which suggests that PO4-P retention is not influenced by pH-dependent charge within the pH range of container production.

Abstract

Calcined materials may contribute enhanced phosphate (PO4-P) retention to soilless root media used in container production. Properties of nutrient retention vary greatly depending on the parent clay and calcining treatment. This research characterized PO4-P sorption of various calcined clay products, including low and regular volatile material (LVM and RVM) 2:1 attapulgite, montmorillonite, and illite clays at various particle sizes; 1:1 kaolin clays in powder form; and diatomaceous earth. Extractable PO4-P, initial pH, PO4-P sorption isotherms, amount of P sorbed as a function of solution pH at a fixed total concentration, and degree of phosphorus saturation were determined. Initial pH of the clays ranged from 3.7 to 8.7. Sorption isotherms were conducted with initial adsorbate concentrations ranging from 0 to 200 mg·L−1 PO4-P from KH2PO4. The calcined materials varied in their ability to sorb PO4-P and generally yielded L-type isotherms, indicating that the adsorbate had relatively high affinity for the calcined material sample surface at low surface coverage. Some 2:1 calcined clays exhibited substantial PO4-P retention, but 1:1 calcined clays and diatomaceous earth did not. Clays with less moisture (LVM) resulted in greater PO4-P sorption than those calcined at lower temperatures (RVM). Terra Green montmorillonites had higher PO4-P sorption than Terra Green attapulgites. Laboratory results indicated potential for substantive PO4-P retention by several of the calcined clay materials when used in container production. For most materials, PO4-P sorption did not show pronounced pH dependence, which suggests that PO4-P retention is not influenced by pH-dependent charge within the pH range of container production.

Small volumes (2% to 20%) of a variety of calcined clay-type products are being used as components of soilless root media because of their potential to increase nutrient retention, air space, water retention, and bulk density of mixes used for container production in the greenhouse and nursery industries. Properties imparted by these materials are inherited from the parent clay or source, particle size distribution, and calcining process. High calcining temperatures result in expansion of the crushed clay to form a porous structure that is physically and chemically stable. The resulting granules provide aeration to the root medium and hold water internally within their pore structure.

Traditionally, the soil component of root media used for crop production in containers imparted significant PO4-P retention to the root medium. Marconi and Nelson (1984) showed that over 33% of the total P applied was leached from a soilless root medium containing 1 peatmoss:1 vermiculite, but less than 5% was leached from a 1 sand:1 soil:1 peatmoss mix in a simulated plant-watering scheme. This difference, they concluded, was related to differences in P sorption by the various root medium components. Use of soil in container root media has fallen out of favor with commercial producers because it is relatively expensive compared with other components, variable from source to source, and must be pasteurized before use (Nelson, 2005).

Many types of calcined clay amendments may enhance PO4-P and water retention of soilless root media. Incorporation of 13% (by volume) of a calcined clay has been shown to reduce the amount of PO4-P leached from pine bark-based container media by 73% in production of Loropelatum chinense var. rubrum R. Br. ‘Blush’ over a 16-week period (Ruter, 2003). Reduction in fertilizer and water use in poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch) with incorporation of calcined clays has been shown (Catanzaro and Bhatti, 2005; Catanzaro et al., 2004). Owen et al. (2003) found that amending a pine bark root medium with 8% (by volume) calcined clay led to increased nutrient retention and water-buffering capacity in production of container-grown Cotoneaster dammeri C.K. Schneid ‘Skogholm’. A root medium of 3 peat:5 compost:2 Turface (calcined arcillite clay; v/v) reduced PO4-P leaching by 70% compared with a control of 3 peat:5 compost:2 sand in production of black-eyed susan, Rudbekia hirta L. (Bugbee and Eliot, 1998). Because use of calcined clays offers potential to reduce environmental impact from PO4-P leaching during container production, PO4-P retention and other properties of a variety of calcined materials should be characterized.

Clay surfaces bear an electrical charge that is a function of permanent charge, variable charge, and inner and outer surface charge. The resultant particle charge is either created through isomorphic substitution, protonation and deprotonation reactions or partial charge as expressed through the polarity of atoms at the clay surface (Essington, 2004). The silicate minerals present in clays develop a pH-dependent charge at the broken edges of the layer structure, which is a phenomenon that occurs in both 1:1 and 2:1 clay soils (Havlin et al., 2003).

Sorption of PO4 by clay minerals has been attributed to a pH-dependent reaction of PO4 with calcium, iron (Fe), and aluminum (Al) in the clay (Havlin et al., 2003). Much of the P adsorption by soils can be attributed to chemisorptions involved in ligand exchange between PO4-P and hydroxyl ions at Fe and Al oxyhydroxides (Parfitt, 1978). pH-dependent PO4-P sorption by calcined clays has not been systematically investigated and it is not clear how the process of calcining changes the PO4-P sorption characteristics of clays.

The commercial calcining process results in low volatile material (LVM) or regular volatile material (RVM) moisture contents, depending on calcining temperatures. Calcining, generally, reduces the amount of water trapped between the clay's silicate sheets and also produces a very hard granule that, even when fully saturated with water, does not break apart easily. The first stage of calcining, known as fluid bed drying, reduces the clay moisture level from 40% to 45% down to 6% to 9% using temperatures of ≈120 to 176 °C, resulting in RVM clays. The LVM clays have even lower moisture content of 0% to 3%, which is achieved by secondary calcining at temperatures in the range of 460 to 800 °C (Moll and Goss, 1997). Heating the clays causes aggregation of particles that optimizes sorbtivity by creating a stable porous internal structure. Calcining reduces the exchange capacity of the clays, but surface binding and internal pores maintain some nutrient retention capacity. X-ray diffraction studies on a 2:1 Ca-montmorillonite clay showed that heat treatment of 200 to 400 °C completely collapsed the interlayer, incorporating interlayer cations into the tetrahedral or unoccupied octahedral sheets (Bray et al., 1998). Noyan et al. (2006) evaluated the specific surface area, specific micromesopore volume, total surface acidity, and adsorption equilibrium constant of original and heated samples of a bentonite clay from Turkey. The specific area, micromesopore volume, and total surface acidity stayed constant as temperature increased to 450 °C and then decreased. Total surface acidity, in general, declined with increasing temperatures. The most acidic sites, however, increased with heating and especially at temperatures of dehydration (100 to 550 °C) and dehydroxylation (550 to 700 °C). Calcining temperatures for maximizing sorbtivity, generally, do not exceed the dehydration interval. Noyan et al. (2006) concluded that the decomposition of the 2:1 layers of the clay and collapsing of micro- and mesopores by intra- and interparticle sintering caused a rapid decrease in specific surface area and specific micromesopore volume as the temperature increased.

Diatomaceous earth (DE) is not derived from clay, but consists of granules mined from sedimentary rock deposits resulting from accumulation of amorphous silica comprising the cell walls of dead, single-celled aquatic organisms called diatoms (Handreck and Black, 2002). The usual commercial function of DE is as an absorbent. Although some commercial growers are using this material as a root medium component, it is not expected to sorb PO4-P because it lacks charged sorption sites.

Phosphate retention properties of clays after calcining are generally unknown, and the role of pH-dependent edge charge in the PO4-P retention of calcined materials is untested. In addition, the calcined materials themselves may provide PO4-P and other nutrients as fertilizer if natural levels are high. Therefore, this research characterizes several calcined clay materials and diatomaceous earth as a basis to evaluate their potential to reduce PO4-P leached during container production when used as components of soilless root media. Specifically, we 1) determined inherent PO4-P by extraction and total P present in the mineral; 2) determined initial pH of unaltered samples; 3) characterized PO4-P sorption at natural pH; 4) developed PO4-P adsorption envelopes under variable pH conditions to establish the role of pH-dependent edge charge on PO4-P retention; and 5) determined the initial degree of phosphorus saturation of calcined materials.

Materials and Methods

The tested calcined materials are commercially available (Table 1). The test materials included various particle sizes of 2:1 clays of attapulgite (Attasorb; Engelhard Corp., Quincy, FL) calcined at two temperatures, RVM and LVM: LVM montmorillonite mined in Mississippi and LVM attapulgite mined in Georgia (Terra Green; Oil-Dri Prod. Co., Ripley, MS) and montmorillonite + illite (Turface; Profile Products LLC, Buffalo Grove, IL); calcined 1:1 clays (Thiele Kaolin Co., Sandersville, GA) in powder form; and diatomaceous earth (Diatomite Eagle-Picher Minerals, Inc., Reno, NV). For ease of reference, the test materials were coded using trade name, temperature treatment, and particle size as mesh size designated by the manufacturer (Table 1). The materials were tested as received from the manufacturer.

Table 1.

Calcined material trade names, sample codes, and descriptions of test materials provided by their sources.

Table 1.

Particle size distribution.

Particle size distribution was determined by dry sieving. The sieving process was carried out using woven analytical precision sieves (USA standard testing sieves, ASTME specification; Fisher Scientific Co., Columbia, MD). Dry test materials were weighed to 1000 g and passed through a stack of eight sieves ranging from 5 mesh (4.0-mm nominal sieve opening) to 80 mesh (180-μm sieve opening) that were arranged in order of size with the smallest sieve at the bottom of the stack. Samples were uniformly shaken on a circular motion shaker set at 2.2 gn for 5 min. Sample recovered in each sieve was weighed and the proportions calculated as a percentage of the whole test sample (Table 2) averaged across three replications.

Table 2.

Particle size distribution of the 16 test materials.

Table 2.

pH measurements.

The pH was measured on a slurry consisting of 5 cm3 test material to 10 mL deionized water. Mixtures were thoroughly stirred and allowed to settle for 10 min before reading pH (Oyster pH meter; Extech Instr. Waltham, MA) at 27 °C. The data were analyzed using PROC GLM in SAS ver. 9.1 (SAS Institute, 2002) with three replicates per material.

Phosphorus extraction.

Naturally occurring P present in the calcined materials was evaluated with three methods: dilute acid extraction, Mehlich III (M3) extraction, and salicylic–sulfuric digestion. For dilute acid extraction, the weight of 5 cm3 of each test material was determined and measured into 50-mL Erlenmeyer flasks. Thirty milliliters of acid extractant consisting of 0.5 N HCl and 0.2 N H2SO4 was added. The flasks were placed on a circular motion shaker at 2 gn for 10 h at 26.0 ± 2 °C, and after 10 h, the supernatant was transferred into 50-mL centrifuge tubes and centrifuged at 310 gn for 5 min. Solution pH was adjusted by drop-wise addition of 5 M NaOH using p-nitrophenol as the indicator until the color of the sample just changed from colorless to yellow (Bender and Wood, 2000). Phosphate in the supernatant was then analyzed using the colorimetric method of Murphy and Riley (1962) on an ultraviolet/VIS spectrophotometer (Perkin-Elmer, Norwalk, CT). The experimental design was completely random (CRD) and each treatment was replicated three times. The data were analyzed using PROC GLM in SAS ver. 9.1 (SAS Institute, 2002).

Mehlich III (M3) extraction solution contained 0.2 N acetic acid, 0.25 N NH4NO3, 0.015 N NH4F, 0.013 N HNO3, and 0.001 N EDTA (Sen Tran and Simard, 1993). Mehlich III extractable P (1:5 test material:solution ratio) was analyzed with a Lachat-FIA system (Lachat FIA 800 series, Loveland, CO) The M3 procedure uses a strong acid extractant and estimates available P in acid soils. Two replications of each test material were run.

Finally, total P of the test materials was analyzed by colorimetric procedures after salicylic-sulfuric acid digestion (Bremner and Mulvaney, 1982; U.S. Environmental Protection Agency, 1984). The extract was analyzed by colorimetric procedures using the Technicon Auto Analyzer II (Technicon Industrial Systems, Tarrytown, NY). Two replications of each test material were run.

PO4-P sorption isotherms.

To characterize and compare P sorption of the calcined products, sorption isotherms were created for each test material. An amount equal in weight to 5 cm3 volume was measured into a 50-mL Erlenmeyer flask. Samples were equilibrated for 18 h with 30 mL of solution containing initial adsorbate concentrations of 0, 10, 25, 50, 100 and 200 mg·L−1 PO4-P prepared from KH2PO4 using deionized water. Treatment structure was factorial with three replicates of each material for each P concentration and experimental design was CRD. Flasks were placed on a circular motion shaker at 2 gn for 10 h at 25 ± 2 °C and then centrifuged at 310 gn for 5 min. Equilibrium P concentration and pH of the supernatant were determined using colorimetric procedures (Chapman and Pratt, 1961; Murphy and Riley, 1962) and the pH meter described previously. The amount of P adsorbed by the clay sample was calculated as follows:
DE1
where Q is the amount of PO4-P sorbed (mg·kg−1 P calcined material), v is the liquid sample volume (L), Ci is the initial concentration of PO4-P in the solution containing adsorbate (mg·L−1), Cf is the final (equilibrium) concentration of PO4-P in the supernatant solution (mg·L−1), and m is the amount of the test material (kg, dry weight basis; Essington, 2004). Phosphate sorbed, Q, was presented as g·m−3 for ease of application to greenhouse container production.
The Freundlich equation, although considered purely empirical in nature, has been extensively used to describe ion adsorption by soils (Aslam et al., 2000; Chaudhry et al., 2003; Obaid-ur-Rehman et al., 2004; Sposito, 1980). The sorption isotherms were examined by modified Freundlich equations proposed by Le Mare (1982). The form of the modified Freundlich model is as follows:
DE2
where P is quantity of sorbate (g) per unit volume (m3) of adsorbent, C is equilibrium solution concentration (mg·L−1) of the adsorbate, “a” is the amount of PO4-P sorbed (g·m−3) when the concentration C is 1 mg·L−1, and “b” is the buffer power defined by the slope of the sorption curve at the point where P/C = 1 L·m−3.
The main advantage of this equation is that “a” and “b” are the amount of P sorbed and buffer capacities, respectively, at the same point on the curve where C = 1 mg·L−1, and this point is the same for all the test materials. In our research, the parameters “a” and “b” were estimated by regression of the logarithmic form of the data obtained from sorption isotherms. Therefore, a plot of log P (y-axis variable) against log C (x-axis variable) yields a straight line with slope b/a and y-intercept of log a.
DE3
From the Freundlich equation the parameter, “a” could be considered as a capacity factor and was referred to as P sorption capacity; this implies that a material having a larger “a” value has a larger sorption capacity than one with a smaller “a” value. Therefore, “a” value estimates were used to differentiate the P sorption capacities of calcined materials. The Freundlich equation does not predict or include a maximum adsorption capacity, but it is reliable with low solution P concentrations (Havlin et al., 2003). R values were determined for each Freundlich equation.

Adsorption envelopes.

Adsorption envelopes were created to determine the amount of P adsorbed as a function of solution pH at the fixed total P concentration of 600 mg·L−1. An amount equal in weight to 5 cm3 volume of eight test materials, A-LVM-8/16, A-RVM-24/28, T-A-24/48, T-M-24/28, T-M-5/20, K-LSA-P, Turface, and Diatomite (Table 1), were equilibrated for 10 h at 25 °C with 25 mL KNO3. A 2.5-mL aliquot of 600 P mg·L−1 from KH2PO4 was added and pH adjusted to ranges of 2 to10 at intervals of one unit using additions of 0.1 M HNO3 or 0.1 M KOH. Samples were centrifuged as described previously, decanted, filtered, and analyzed for PO4-P concentration as described previously. The experimental design was CRD with three replications of each treatment. Eight test materials were subjected to nine pH levels.

Degree of phosphorus saturation.

Degree of P saturation (DPS) estimates how close the test material is to being saturated with PO4-P (Sharpley, 1995). The DPS of the calcined materials was determined to compare the level of initial saturation with PO4-P of each material's exchange sites. A DPS index (Pote et al., 1999) was created from soil test phosphorus (STP) determined by M3 extraction and a phosphorus sorption index (PSI) calculated from a single point isotherm (Bache and Williams, 1971; Sims et al., 2002). To establish the PSI, 30 mL of a 500 mg·L−1 PO4-P sorbate solution from KH2PO4 was added to 5 cm3 of test material, equilibrated on a shaker for 18 h at 25 ± 2 °C, and centrifuged as described previously. Phosphate was determined as described previously (Murphy and Riley, 1962). The PSI was calculated using the equation:
DEU1
where q is the amount of P sorbed (mg·kg−1) and C is the equilibrium solution P concentration (mg·L−1). The second step involved estimating DPS STP using the ratio of STP to (PSI + STP) and multiplying by 100: DPSSTP (%) = {STP(mg·kg −1)/[PSI + STP(mg·kg −1)]} × 100.

Results and Discussion

Particle size distribution, pH, and bulk density.

Characterization of particle size distribution of the test materials is summarized in Table 2. Results follow U.S. standard mesh sizes.

Attasorb materials were alkaline with pH ranging from 8.2 to 8.7 (Table 3). Terra Green montmorillonites were acidic with pH ranging from 3.8 to 4.3, whereas Terra Green attapulgites were less acidic with pH ranging from 5.6 to 6.2. Turface and diatomite had pH values of 6.1 and 5.1, respectively (Table 3). The various calcined materials exhibited a wide range of pH. It is important for a grower to be aware of the pH levels that root medium components contribute to a mix because they may influence nutrient retention and management during production.

Table 3.

Bulk density, pH, amount of P extracted from test materials from three different extraction procedures, degree of phosphorus saturation, P sorption index, and P sorption capacity, and buffer capacity calculated from the modified Freundlich equation.

Table 3.

Calcined materials have lower bulk density (less than 1 g·cm3) compared with most mineral soils, which have bulk densities between 1 and 2 g·cm3 (Hillel, 2004). Lower bulk densities after calcining indicate increased porosity of the test material. Compared with Attasorb and Terra Green materials, DE exhibited greater porosity (Table 3). Compared with clay soils, the granular test materials have the advantage of adding stable pore space to soilless root media, which contributes to beneficial physical properties in container production.

Phosphorus content of calcined materials.

Total native P present in the materials was relatively high in the Attasorb and Terra Green attapulgites (700 to 2800 mg·kg−1; Table 3). This significant amount of PO4-P may provide some P fertilizer during production and/or remain in the container after the production cycle concludes. Kaolin test materials contained between 580 and 670 mg·kg−1 P, whereas diatomite and Terra Green montmorillonite had the least native P at between 250 and 370 mg·kg−1 P (Table 3). High levels of native P in the calcined materials might be expected to reduce PO4-P sorption capacity.

Phosphorus was extracted from the test materials using two procedures, an acid extraction and the M3 test, which was developed in North Carolina for routine analysis of P, potassium, calcium, magnesium, sodium, and micronutrients in acid soils. The two extraction methods yielded substantially different P contents (Table 3). Different chemical extractants are designed for soils depending on their chemical characteristics. Attapulgite test materials from two sources, which have pH ranging from 5.6 to 8.7, resulted in greater P extracted by 0.5 N HCl + 0.2 N H2SO4 than the M3 procedure, but the opposite was true for kaolinite test materials, which had pH of 4.5 to 5.3. The evolution of carbon dioxide during acid extraction of the Attasorb materials indicated a possible presence of free CaCO3, which, in combination with the higher pH, suggested that the M3 test is not ideal for these calcareous clays. Results of the M3 test showed the influence of particle size on available PO4-P with smaller sizes of the same material yielding more PO4-P. Kaolinite materials KLSA and K-P yielded the most PO4-P, 460 and 116 mg·kg−1 PO4-P, respectively, from the M3 procedure. Attasorb RVM materials yielded more available PO4-P compared with LVM materials (45 to 92 mg·kg−1 P and 6 to 14 mg·kg−1 P, respectively).

PO4-P isotherms.

The isotherms, which are L-type according to classification of sorption isotherms by shape, indicate monomolecular adsorption of PO4-P (Figs. 1A, 2A, 3A, and 4A). Using the parameter “a” from the Freundlich equation as a capacity factor, Terra Green montmorillonites had the highest PO4-P sorption (425 to 700 g·m−3). Terra Green attapulgites had lower PO4-P sorption (210 to 250 g·m−3) as shown in Table 3 and Figure 1A. Information obtained from the manufacturer of the Terra Green calcined materials indicated differences in the clays mined in Georgia versus Mississippi. The montmorillonites from Mississippi contained more Fe, whereas the product mined in Georgia had higher magnesium and calcium content, which were capable of forming surface or solution precipitates with oxyanions. Differences in surface area may have influenced PO4-P sorption; however, the specific surface area provided by the suppliers ranged from 98 m2·g−1 for Attasorb materials and 102 to 122 m2·g−1 for Terra Green materials based on the standard Brunauer, Emmett and Teller procedure (Brunauer et al., 1938), which did not appear to be different enough to explain difference in PO4-P sorption.

Fig. 1.
Fig. 1.

PO4-P isotherms for Terra Green test materials (A) and pH of solution at various equilibrium P concentrations (B).

Citation: HortScience horts 44, 2; 10.21273/HORTSCI.44.2.431

Fig. 2.
Fig. 2.

PO4-P isotherms for Attasorb test materials (A) and pH of solution at various equilibrium P concentrations (B).

Citation: HortScience horts 44, 2; 10.21273/HORTSCI.44.2.431

Fig. 3.
Fig. 3.

The PO4-P isotherms for kaolin test materials (A) and pH of solution at various equilibrium P concentrations (B).

Citation: HortScience horts 44, 2; 10.21273/HORTSCI.44.2.431

Fig. 4.
Fig. 4.

PO4-P isotherms for diatomaceous earth and Turface test materials (A) and pH of solution at various equilibrium P concentrations (B).

Citation: HortScience horts 44, 2; 10.21273/HORTSCI.44.2.431

Phosphate sorption levels of Terra Green materials increased with increasing initial pH. The T-M-16/30 and T-M-5/20 had the highest sorption with the initial pH range of 3.8 to 4.0, whereas T-A-16/30, T-A-24/28, and T-A-5/20 had comparatively lower PO4-P sorption with initial pH ranging from 5.6 to 6.2 (Table 3). The high PO4-P sorption under lower pH could have involved the dissolution of Al in the clay lattice in the formation of AlPO4 crystals. In acidic solutions, the mineral surface has a net (+) charge, although both (+) and (–) sites exist. The predominance of (+) charges readily attracts H2PO42- (Havlin et al., 2003).

The Attasorb LVM materials had higher PO4-P sorption (“a” = 320 to 495 g·m−3) than Attasorb RVM materials (“a” = 220 to 230 g·m−3) as shown in Table 3 and Figure 2A. Particle size played a part in enhancing PO4-P sorption in Attasorb LVMs with the smallest particle size having higher sorption than larger particle sizes. It was observed that RVM materials had a tendency to disintegrate into smaller particles when in solution compared with the LVM materials; therefore, the difference between the eventual surface area of RVM clay particles was not as great, which explained the lack of effect of particle size on PO4-P sorption. The LVM materials demonstrated better aggregate stability than RVM materials and would be preferred for use as soilless root medium components. Owen et al. (2007) compared the use of two temperature treatments (RVM and LVM) of a Georgian palygorskite–bentonite calcined clay at 8% by volume of a bark-based root medium; the mix with LVM clay leached 35% less PO4-P than the mix containing RVM clay.

The Terra Green attapulgites that originated from Georgia, T-A-5/20, T-A-16/40, and T-A-24/48, resulted in PO4-P sorption capacity, “a”, of 250, 240 and 210 g·m−3, respectively, showing no influence across these particle sizes on amount of PO4-P sorbed (Table 3). Ruter (2003) investigated the influence of three particle sizes of calcined clay (Sud-chemie, Meigs, GA) on PO4-P retention. He concluded that particle size had limited influence on amount of PO4-P sorbed when particle sizes were greater than 3.36 mm and less than 1.00 mm (24/48 US mesh) and within the range of 1.00 to 3.36 mm (5/20 and 16/30 US mesh), 61%, 76%, and 74% reduction of PO4 leached occurred, respectively.

The slopes of the isotherms are indicators of buffer capacity with steeper slopes indicating higher buffering capacity (Ozanne and Shaw, 1968). Such plots can be used to estimate the quantity of P to be applied to maintain the soil solution concentration at the desired level (Fox and Kamprath, 1970). Material with high PO4-P sorption like Terra Green montmorillonites continued to sorb PO4-P at even the highest levels of added PO4-P (200 mg·L−1; Fig. 1A). The higher PO4-P buffer capacity of these materials would necessitate greater P addition to achieve 0.2 mg·L−1 soluble PO4-P, which is considered optimum for most plants (Beckwith, 1965), but ensures that the level of soluble PO4-P is maintained for a longer period. As PO4-P is removed from solution by the plant, there would be a less dramatic change in intensity (root medium solution PO4-P concentration) in the more highly buffered media.

The buffer index from the slope of the Freundlich equation indicated that the kaolinite materials had very low buffer capacity (25 to 53 L·m−3; Table 3 based on Fig. 3A); they also had the lowest bulk density (Table 3). The kaolins are processed to achieve a low permeability, a property that would curtail sorption capability. Among the kaolin samples, K-HAS-P had the highest bulk density (0.26 g·cm−3) as shown in Table 3.

The relatively low PO4-P sorption exhibited by Turface (Fig. 4A) could be as a result of high calcining temperatures of over 550 °C leading to the collapse of micro- and mesopores. Calcining temperatures play an important role in determining the sorptivity of the test materials.

Diatomaceous earth resulted in the lowest PO4-P sorption. If diatomaceous earth increased PO4-P retention after incorporation into soilless root media, the mechanism would not appear to be of a chemical nature.

pH.

The pH of the supernatant after sorption remained fairly stable above the equilibrium concentration of ≈0.5 mg·L−1 PO4-P for all test materials (Figs. 1B, 2B, 3B, and 4B). This was an indication that the calcined clays could help stabilize pH of soilless root media, which often contribute to problems of pH drift because of their poor buffer capacity. In peat-based soilless root media, lack of cation exchange capacity and base saturation of peatmoss had been implicated as contributors to pH drift phenomena (Rippy et al., 2005). However, calcined clays would need to comprise a significant volume of the total root medium to stabilize problems with pH drift during production.

Adsorption envelopes.

The Attasorb RVM material sorbed more PO4-P with increasing pH with 0% sorbed at pH 2 to 3 and up to ≈90% sorbed at pH 10; ≈40% to 60% was sorbed at a pH range of 5 to 8 (Fig. 5B). The Attasorb LVM material, on the other hand, had 60% to 80% sorption over the pH range of 2 to 10, and the fraction generally increased as pH increased (Fig. 5B). The LVM materials (A-LVM-8/16, T-A-24/28) compared with an RVM material (A-RVM-24/28) showed yet another desirable characteristic of pH stability: sorption increased with increasing pH up to 6, and no significant change in PO4-P sorption was observed from pH 6 to 9. Three possible mechanisms are proposed to explain the increasing PO4-P sorption with increasing pH exhibited by Attasorb: precipitation of Ca phosphates, Ca-induced P sorption, or coadsorption of Ca and H2PO4 or HPO42– as ion pairs or complexes (Essington, 2004).

Fig. 5.
Fig. 5.

Adsorption envelopes of PO4-P in suspensions containing 50 PO4-P mg·L−1 in 0.01 M KNO3 solution for the various test materials (A–B). Sorption is expressed as the fraction of the average PO4-P adsorbed (mg·kg−1) and maximum total PO4-P concentration (mg·kg−1).

Citation: HortScience horts 44, 2; 10.21273/HORTSCI.44.2.431

Terra Green montmorillonite of 5/20 mesh had a stable, high PO4-P sorption of 98% over the entire pH range, whereas the smaller particle size of this material, 24/28 mesh, had a high sorption fraction of 85% to 90% between pH 2 and 8 with a sharp decrease in sorption after pH 8 to approximately only 20% PO4-P sorption at pH 10 (Fig. 5B).

Turface had a stable PO4-P sorption of ≈60% over the pH range of 3 to 10 (Fig. 5A). Diatomaceous earth had a small fraction of PO4-P sorbed (15% to 30%), which decreased with increase in pH. Low surface area metakaolin (K-LSA-P) had a high PO4-P sorption of 98% at pH 2 and 3, which decreased with increasing pH to 40% PO4-P adsorbed at pH 10 (Fig. 5A).

The adsorption envelopes indicate that pH-dependent charge of the calcined materials is not a critical mechanism of PO4-P sorption in greenhouse production. Laboratory results from various calcined clays demonstrate that the mechanism by which PO4-P attaches to the surface of a calcined material is generally not pH-dependent within the pH ranges of 5.0 to 7.0 maintained during container production.

Degree of phosphorus saturation.

The DPS was calculated using the ratio of M3 PO4-P to the experimentally determined PSI (Bache and Williams, 1971; Pote et al., 1999; Sims et al., 2002; Table 3). The DPS of the calcined materials generally suggested that they possess significant capacity to sorb PO4-P with DPS less than 7.7% for all materials except two of the 1:1 kaolinites (Table 3). The low STP and high PSI values associated with several of the calcined materials, namely Attasorb LVM and Terra Green attapulgites and montmorillonites, suggest that a large quantity of vacant sites exist for PO4-P sorption.

In summary, several of the calcined materials that we characterized could be used as a component of soilless media to decrease PO4-P leaching. Results of isotherm and DPS calculations indicated that the most promising materials were Terra Green montmorillonites and attapulgites and Attasorb LVM attapulgites. The least promising materials were diatomaceous earth and kaolinites. The PO4-P adsorption envelopes indicated that PO4-P sorption was not strongly pH-dependent within the pH range of commercial production, indicating that pH-dependent charge is not a critical means of PO4-P retention during greenhouse production. Calcining temperature affected optimal PO4-P sorption: LVM attapulgites had better sorption than the RVM attapulgites that were calcined at lower temperatures, but Turface, which is also a 2:1 clay and calcined at a very high temperature, did not match the PO4-P sorption potential of LVM Attasorb and LVM Terra Green attapulgites. Finally, the high level of native total P in some of the materials may provide some PO4-P as fertilizer to crops.

Benefits of using calcined materials beyond PO4-P retention may include increased water retention because the calcined products have significant moisture sorption properties, which are related to their primary commercial use as absorbents. The temperatures of calcining are crucial in determining the extent to which the hardened aggregates maintain internal porosity.

Limitations of using calcined products as soilless root medium components include the high cost of purchase and shipping, especially if used at high enough percentages in soilless root media to impart adequate benefit such as buffering against pH drift. Because the materials are mined, natural variability occurs at different mines and even mining depths within the same mine; this would contribute to lack of uniform and consistent quality attributes in the materials. Finally, if calcined materials with very high PO4-P sorption capacity are used, the materials themselves may remove PO4-P from root medium solution when used in tandem with very low rates of PO4-P fertilization.

Future research should investigate optimal percentages of incorporation of calcined products into soilless media, especially under conditions of greenhouse production. The mechanism of PO4-P sorption by the calcined materials is ambiguous and further research could elucidate these mechanisms. Phosphate desorption of the materials should be further evaluated. Finally, the economic benefits of reduced PO4-P runoff in greenhouse effluent and reduced water use during production should be evaluated to help growers make decisions about using calcined materials.

Literature Cited

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    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
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Contributor Notes

This research was funded by The Gloeckner Foundation and the Kansas Agricultural Experiment Station (KAES).

This manuscript has been assigned contribution No. 08-380-J from the KAES.

We gratefully acknowledge the following companies for providing calcined clay samples: Oil-Dri Production Co., Ripley, MS 38663; Engelhard Corp., Quincy, FL 32352; and Thiele Kaolin Co., Sandersville, GA 31082. We thank James Higgins for statistical advice.

The use of trade names in this publication does not imply endorsement by the KAES of products named nor criticism of similar ones not mentioned.

Graduate Research Assistant.

Professor.

To whom reprint requests should be addressed; e-mail kwilliam@ksu.edu.

  • View in gallery

    PO4-P isotherms for Terra Green test materials (A) and pH of solution at various equilibrium P concentrations (B).

  • View in gallery

    PO4-P isotherms for Attasorb test materials (A) and pH of solution at various equilibrium P concentrations (B).

  • View in gallery

    The PO4-P isotherms for kaolin test materials (A) and pH of solution at various equilibrium P concentrations (B).

  • View in gallery

    PO4-P isotherms for diatomaceous earth and Turface test materials (A) and pH of solution at various equilibrium P concentrations (B).

  • View in gallery

    Adsorption envelopes of PO4-P in suspensions containing 50 PO4-P mg·L−1 in 0.01 M KNO3 solution for the various test materials (A–B). Sorption is expressed as the fraction of the average PO4-P adsorbed (mg·kg−1) and maximum total PO4-P concentration (mg·kg−1).

  • Aslam, M., Zia, M.S., Rahmatullah, & Yasin, M. 2000 Application of Freundlich adsorption isotherm to determine phosphorus requirement of several rice soils Intl. J. Agr. Biol. 2 286 288

    • Search Google Scholar
    • Export Citation
  • Bache, B.W. & Williams, E.G. 1971 A phosphorus sorption index for soils J. Soil Sci. 22 289 301

  • Beckwith, R.S. 1965 Sorbed phosphate at a standard supernatant concentration as an estimate of the phosphate needs of soils Aust. J. Exp. Agr. Anim. Husb. 5 52 58

    • Search Google Scholar
    • Export Citation
  • Bender, M.R. & Wood, C.W. 2000 Total phosphorus in residual materials 77 82 Pierzynski G.M. Methods of phosphorus analysis for soils, sediments, residuals, and waters Southern Cooperative Series Bull. No. 396

    • Search Google Scholar
    • Export Citation
  • Bray, H.J., Redfern, S.A.T. & Clark, S.M. 1998 The kinetics of dehydration in Ca-montmorillonite: An in situ x-ray diffraction study Mineral. Mag. 62 647 656

    • Search Google Scholar
    • Export Citation
  • Bremner, J.M. & Mulvaney, C.S. 1982 Salicylic acid thiosulfate modification of the Kjeldhal method to include nitrate and nitrite 621 Miller R.H. & Keeney D.R. Methods of soil analysis. Part 2 Amer. Soc. Agron Madison, WI

    • Search Google Scholar
    • Export Citation
  • Brunauer, S., Emmett, P.H. & Teller, E. 1938 Adsorption of gases in multimolecular layers J. Amer. Chem. Soc. 60 309 319

  • Bugbee, G.J. & Eliott, G.C. 1998 Leaching of nitrogen and phosphorus from potting media containing biosolids compost as affected by organic and clay amendments Bull. Environ. Contam. Toxicol. 60 716 723

    • Search Google Scholar
    • Export Citation
  • Catanzaro, C.J. & Bhatti, S.M. 2005 Incorporated substrate amendments to reduce water use in poinsettias Proc. S.N.A. Res. Conf. 50 37 38

  • Catanzaro, C.J., Bhatti, S.M. & Kamake, R. 2004 Incorporation of clay amendments to reduce fertilizer use in poinsettias Proc. S.N.A. Res. Conf. 49 34 36

    • Search Google Scholar
    • Export Citation
  • Chapman, H.D. & Pratt, P.F. 1961 Methods of analysis for soils, plants, and waters Univ. Calif., Div. of Agr. Sci 169 170

  • Chaudhry, E.H., Ranjha, A.M., Gill, M.A. & Mehdi, S.M. 2003 Phosphorus requirement of maize in relation to soil characteristics Intl. J. Agr. Bio. 5 625 629

    • Search Google Scholar
    • Export Citation
  • Essington, M.E. 2004 Soil and water chemistry CRC Press New York, NY

  • Fox, R.L. & Kamprath, E.J. 1970 Phosphate sorption isotherms for evaluating the phosphate requirements of soils Soil Sci. Soc. Amer. Proc. 34 903 906

    • Search Google Scholar
    • Export Citation
  • Handreck, K. & Black, N. 2002 Growing media for ornamental plants and turf 3rd Ed UNSW Press Sydney, Australia

  • Havlin, J.L., Beaton, J.D., Tisdale, S.L. & Nelson, W.L. 2003 Soil fertility and fertilizers: An introduction to nutrient management 7th Ed Pearson/Prentice Hall Upper Saddle River, NJ

    • Search Google Scholar
    • Export Citation
  • Hillel, D. 2004 Introduction to environmental soil physics Elsevier Academic Press Amsterdam, The Netherlands

  • Le Mare, P.H. 1982 Sorption of isotopically exchangeable and non-exchangeable phosphate by some soils of Colombia and Brazil, and comparisons with soils of Southern Nigeria J. Soil Sci. 33 691 707

    • Search Google Scholar
    • Export Citation
  • Marconi, D.J. & Nelson, P.V. 1984 Leaching of applied phosphorus in container media Sci. Hort. 22 275 285

  • Moll, W.F. & Goss, G.R. 1997 Mineral carriers for pesticides—Their characteristics and uses Standard Technical Publication 943. Amer. Soc. Testing and Materials West Conshohocken, PA

    • Search Google Scholar
    • Export Citation
  • Murphy, J. & Riley, J.P. 1962 A modified single solution method for the determination of phosphate in natural waters Anal. Chim. Acta 27 31 36

  • Nelson, P.V. 2005 Greenhouse operation and management 6th Ed Prentice Hall Upper Saddle River, NJ

  • Noyan, H., Müserref, Ö. & Sarikaya, Y. 2006 The effect of heating on the surface area, porosity and surface acidity of a bentonite Clays Clay Miner. 54 375 381

    • Search Google Scholar
    • Export Citation
  • Obaid-ur-Rehman, A.M., Ranja, R., Gil, M.A. & Mehdi, S.M. 2004 Phosphorus requirement of wheat using modified Freundlich model in Rasulpur soil series Pak. J. Agr. Sci. 41 1 2

    • Search Google Scholar
    • Export Citation
  • Owen, J.S., Warren, S.L. & Bilderback, T.E. 2003 Amending pine bark substrate with clay to increase nutrient and water efficacy The North Carolina Association of Nurserymen November/December.

    • Search Google Scholar
    • Export Citation
  • Owen, J.S., Warren, S.L., Bilderback, T.E. & Albano, J.P. 2007 Industrial mineral aggregate amendment affects physical and chemical properties of pine bark substrates HortScience 42 1287 1294

    • Search Google Scholar
    • Export Citation
  • Ozanne, P.G. & Shaw, T.C. 1968 Advantages of the recently developed phosphate sorption test over the older extractant methods for soil phosphate Transactions of the 9th International Congress of Soil Science Adelaide, Australia Intl. Soc. of Soil Sci. 2 273 280

    • Search Google Scholar
    • Export Citation
  • Parfitt, R.L. 1978 Anion adsorption by soils and soil materials Adv. Agron. 30 1 50

  • Pote, D.H., Daniel, T.C., Nichols, D.J., Sharpley, A.N., Moore, P.A., Miller, D.M. & Edwards, D.R. 1999 Relationship between phosphorus levels in three ultisols and phosphorus concentrations in runoff J. Environ. Qual. 28 170 175

    • Search Google Scholar
    • Export Citation
  • Rippy, J., Nelson, P. & Bilderback, T. 2005 Factors affecting pH establishment and maintanance in peat moss based substrates HortScience 40 1124 (abstr.).

    • Search Google Scholar
    • Export Citation
  • Ruter, J.M. 2003 Calcined clay reduces phosphorus losses from pine bark substrate Proc. S.N.A. Res. Conf. 48 84 87

  • SAS Institute 2002 SAS/STAT, Version 9.1 SAS Institute, Inc Cary, NC

  • Sen Tran, T. & Simard, R.R. 1993 Mehlich III-extractable elements 43 49 Carter M.R. Soil sampling and methods of analysis Lewis Boca Raton, FL

  • Sharpley, A.N. 1995 Dependence of runoff phosphorus on extractable soil phosphorus J. Environ. Qual. 24 920 926

  • Sposito, G. 1980 Derivation of the Freundlich equation for ion exchange reactions in soils Soil Sci. Soc. Amer. 44 652 654

  • Sims, J.T., Maguire, R.O., Leytem, A.B., Gartley, K.L. & Paulter, M.C. 2002 Evaluation of Mehlich 3 as an agri-environmental soil phosphorus test for the mid-Atlantic United States of America Soil Sci. Soc. Amer. J. 66 2016 2032

    • Search Google Scholar
    • Export Citation
  • U.S. Environmental Protection Agency 1984 Methods for chemical analysis of water and wastes. EPA-600/4-79-020 U.S. Environmental Protection Agency Cincinnati, OH

    • Search Google Scholar
    • Export Citation
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