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a USDA-ARS, Northwest Irrigation and Soils Research Lab., 3793 North 3600 East, Kimberly, ID 83341-5076 USA
crobbins{at}magiclink.com
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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800000 ha of south-central Idaho silt loam soils because of topsoil removal. Phosphorus availability is a known production problem after topsoil removal. This study evaluated the effect of three P sources on soil P solubility by three standard methods for calcareous soils. A long-term study was initiated on a Portneuf silt loam (Coarse-silty, mixed, superactive, mesic Durinodic Xeric Haplocalcid) by removing the surface 0.3 m of topsoil from strips between undisturbed topsoil strips. Phosphorus treatments applied across all strips were conventional fertilizer (applied according to soil test), dairy manure, and cheese whey. All treatments increased the freshly exposed subsoil bicarbonate extractable ortho-P concentrations up to or greater than the topsoil concentrations, which were more than adequate for economical crop production. The high-whey and manure treatments increased the subsoil saturation paste and 0.01 M CaCl2 extractable ortho-P concentrations up to or greater than the untreated topsoil ortho-P concentrations. The initial topsoil ortho-P solubility was along the ß-tricalcium phosphate (ß-TCP) isotherm and the initial subsoil was well below the ß-TCP isotherm. The ortho-P solubility of the subsoil monocalcium phosphate (MCP) treatment remained just below the ß-TCP isotherm. The cottage cheese whey treatment increased subsoil P solubility up to the ß-TCP isotherm and the manure treated subsoil ortho-P solubilities were between the ß-TCP and octacalcium phosphate (OCP) isotherms. Most subsoil ortho-P concentrations by all three extraction methods decreased from spring to fall and then increased over winter in the subsequent spring samples. Soil solution ortho-P concentrations decreased with time in the subsoil treatments except immediately following treatment applications. The topsoil ortho-P extract concentrations by all three methods varied among samplings but remained about the same during the study period.
Abbreviations: EC, electrical conductivity • F, fall • HA, hydroxyapatite [Ca5(PO4)3OH] • MCP, monocalcium phosphate • OC, organic C • OCP, octacalcium phosphate [Ca4H(PO4)3·2.5H2O] • S, spring • STP, soil test phosphate (0.5 M NaHCO3 extractable ortho-P) • ß-TCP, beta tricalcium phosphate [ß-Ca3(PO4)2]
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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In the spring of 1991, Robbins et al. (1997) established a long-term study to find a method(s) to increase freshly exposed calcareous subsoil productivity to the levels of adjacent uneroded soils. Fertilizer treatments were applied to Portneuf silt loam subsoil plots in 1991 and crop rotation variables were applied for 3 yr (Fig. 1 , Table 1) . In 1994, all plots were planted to bean (Phaseolus vulgaris L., cv. Viva) as a test crop. Soil and whole bean plant N, P, K, Cu, Mn, Fe, and Zn availability and uptake were measured to determine why dairy manure increased bean yields to that of the topsoil, while acid cottage cheese whey and conventional fertilizer applications did not. Zinc uptake and soil organic C (OC) increases were the only measured factors that appeared to be associated with increasing subsoil bean yields to levels of the topsoil yields. Soil test and whole bean plant uptake of all other measured nutrients were more than adequate for maximum bean seed yield (Robbins et al., 1997).
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Phosphorus availability and mobility in calcareous soils can be evaluated by several methods. In general, for soils with free lime, the standard extractant for relative plant available P is 0.5 M NaHCO3 (Watanabe and Olsen, 1965). Water-soluble ortho-P gives an indication of potential ortho-P movement with leaching and is sometimes used as a measure of plant-available P (Nishimoto et al., 1977) and can be estimated with a dilute CaSO4 solution after contact for 30 min, or in the saturation paste extract after 18 h of equilibrium. Alternatively, soil solution P concentrations can be estimated using 0.01 M CaCl2 solutions equilibrated for 24 h with continuous, mild shaking or with intermittent shaking for 6 d. This latter estimate is also used to compare soil P solubility relationships with known mineral solubilities and for developing adsorption isotherms (Lindsay, 1979). Both the saturated CaSO4 and 0.01 M CaCl2 solutions have Ca concentrations approximately equal to calcareous soil solutions.
In this paper, we report 0.5 M NaHCO3, saturation paste, and 0.01 M CaCl2 extractable ortho-P concentration and ortho-P solubility changes across 7 yr caused by treating topsoil and subsoil plots with monocalcium phosphate (MCP), cheese whey, and fresh dairy manure.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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18% down to 1.3 m. The clay content decreases from 20% in the surface to 15% at 1.3 m on uneroded sites (Robbins, 1977). The topsoil contained 80 g CaCO3 equivalent and the subsoil contained 240 g CaCO3 equivalent kg-1 soil. The plot area has a 1.1% slope and was surface irrigated for more than 90 yr. There is a slight but definite redistribution of topsoil from the top to the bottom of the field. In 1991, topsoil was removed from three long narrow strips, along the full length of the field, between undisturbed strips of topsoil plots to create freshly exposed subsoil plots (Fig. 1). The topsoil was spread on an adjacent field. The strip arrangement was necessary because the plot area is furrow irrigated and the study was to last for at least 8 yr. Each plot is 9 m wide and 21 m long. Crop rotations were randomized across the field (Fig. 1); however, crop rotations did not significantly affect STP or P uptake by subsequent crops (Robbins et al., 1997), consequently crop rotations are not discussed here. The four fertilizer treatments were applied only to the subsoil strips in 1991. Phosphorus treatments were not applied to any topsoil plots until after the 1994 cropping season (Table 1). Following the 1994 cropping season, manure was applied to two topsoil and three subsoil plots in each replication, and whey was applied to two topsoil and three subsoil plots in each replication (Fig. 1, Table 1). Four topsoil plots in each replication were left untreated for the entire study.
The four 1991 fertilizer treatments were conventional (fertilized according to soil tests for calcareous soils), manure, high-whey, and low-whey. Only the P fertilizer rates and results will be discussed here. Please refer to Robbins et al. (1997) for N, K, and Zn application rates. The conventional subsoil plots received 135 kg P ha-1 as MCP on 23 Apr. 1991 (Table 1). The following day, fresh dairy manure was applied at an air-dry rate of 44 Mg ha-1. It contained 0.6 g water g-1 of wet manure and 8.6 g total P kg-1 of air-dry manure, providing 380 kg total P ha-1. The subsoil plots were then furrowed with large shovels on 0.50-m centers and 0.25 m deep to make large ridges. This covered the manure and commercial fertilizer and provided the maximum surface possible for applying the whey treatments. Berms were made at each end of the whey plots for whey containment. Cottage cheese whey (acid) was surface applied to the whey plots through gated irrigation pipe. The acid whey was the byproduct of cottage cheese produced by adding 3 kg of H3PO4 to each 1000 kg of milk and contained 1050 mg P kg-1 of whey (as opposed to sweet whey from hard cheeses coagulated by biological culture as applied in the fall of 1994). The low-whey plots received 230 m3 whey ha-1 (equivalent to 23-mm depth of whey) and the high-whey plots received 920 m3 whey ha-1 (92-mm depth). Total and ortho-P applications are shown in Table 1. Based on initial soil test concentrations (Watanabe and Olsen, 1965), P was not added to the topsoil plots (Tables 1 and 2) . The entire plot area was moldboard plowed 0.3 m deep after the fertilizer treatments were applied.
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After the 1994 bean harvest, 74 Mg ha-1 of fresh dairy manure (dry weight basis) was applied to the original subsoil low-whey plots and the topsoil plots in the same rows (Fig. 1). The high-whey subsoil plots and the topsoil plots in the same rows received 500 m3 (50-mm depth) of sweet whey (whey from cultured cheese) on 8 and 9 Nov. 1994 (Table 1). The 1991 manure plots received no additional treatments.
Ortho-P concentrations in the manure were measured in 1:20 manure/0.5 M NaHCO3 extracts and in water dilutions of the whey (Watanabe and Olsen, 1965). Total-P was determined by ashing manure or whey samples at 500°C, dissolving the ash in 10 mL of 1.0 M HNO3, heating for 10 min just below boiling and then determining total P in an appropriate-sized aliquot (Kitson and Mellon, 1944). No attempt was made to identify P forms beyond ortho- and total-P.
Soil samples were taken each spring from 1991 to 1998, except spring 1995, and each fall from 1991 to 1995. Topsoil samples were not taken in the fall of 1991 because of unusually early adverse weather (Table 2). All soil samples were taken before treatments were applied each spring or fall. The entire plot area was moldboard plowed each spring following soil sampling to 0.30 m, except in the spring of 1997. Five soil samples 75 mm in diameter by 300 mm deep (plow depth) were taken from each plot, combined, and thoroughly mixed. A 5-kg subsample was air dried and saved for chemical analysis.
Bicarbonate extractable (STP) (Table 2) and water-soluble (saturation paste extract) (Robbins and Wiegand, 1990) (Table 3) ortho-P was measured on all soil samples (Watanabe and Olsen, 1965). Ortho-P concentrations in 0.01 M CaCl2 (Table 4) were measured each spring from 1991 to 1997 except for the spring of 1995 (the spring following the second treatment application year) and in the fall of 1994 and 1995 by shaking 2.0 g of air-dried ground soil in 40 mL of 0.01 M CaCl2 for 22 h at one cycle per second at 20°C (Fox and Kamprath, 1970). The samples were centrifuged and the pH and electrical conductivity (EC) of the supernatant were measured immediately (Robbins and Wiegand, 1990) and solution ortho-P was determined (Watanabe and Olsen, 1965). Sulfate and F were determined by ion chromatography (Robbins, 1989), and Ca and Mg were determined by atomic absorption spectrophotometry in the 0.01 M CaCl2 extracts.
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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The MCP-treated subsoil conventional treatment soil STP increased from 9 to 31 mg ortho-P kg-1 (S1991–S1992) after the second phosphate application (Table 2). With no additional P application for the next 7 yr, STP varied between 21 and 36 mg P kg-1 and showed no long-term change during the sampling period. The saturation paste ortho-P concentrations increased from 0.05 mg L-1 in the freshly exposed subsoil samples (S1991) to a maximum 0.59 mg L-1 in the S1992 samples after a second MCP application, then decreased with time (Table 3). The 0.01 M CaCl2 extract ortho-P concentrations followed the same trends, although samples were analyzed from fewer sampling dates (Table 4).
The high-whey subsoil treatment STP increased from 9 to 48 (S1991– fall [F] 1991), then to 71 mg ortho-P kg-1 soil by the S1992 sampling (Table 2). It steadily decreased to 35 mg P kg-1 in the F1994 sampling. Additional whey applied after the F1994 sampling increased the STP to 77 (F1995), to 82 (S1996), and then it decreased to 57 mg P kg-1 in S1998. Saturation paste extract ortho-P increased from 0.05 mg L-1 in the fresh subsoil to 1.65 in the S1992 samples and decreased to 0.29 prior to additional whey application (Table 3). It increased to 1.51 mg ortho-P kg-1 after the fall 1994 whey application and to 1.71 by S1996, followed by a substantial decrease the S1997 and S1998 samples. The 0.01 M CaCl2 extract concentrations increased from 0.05 in S1991 to 0.34 mg ortho-P L-1 in S1992, and decreased to 0.12 in F1994. Adding more whey increased it to 0.37 in F1995 then it decreased to 0.21 by S1998. (Table 4).
The low-whey results followed the same pattern as the high-whey but with less intensity with respect to STP, saturation paste extract, and the 0.01 M CaCl2 extractable ortho-P (Tables 2, 3, and 4). Following the F1994 soil sampling, manure was applied to the low-whey subsoil plots and the topsoil plots in the same rows. For the remainder of the study they were designated the Manure 94 plots (Table 1). The STP increased to 118 in F1995, 137 in S1996, 142 in S1997, and decreased to 126 mg ortho-P kg-1 soil in the S1998 samples (Table 2). The saturation paste and 0.01 M CaCl2 extractable ortho-P concentrations also increased following the manure additions (Tables 3 and 4).
The subsoil manure 1991 STP increased with each sampling and manure application from S1991 to S1993, starting at 9 and peaking at 134 mg P kg-1 in the S1993 samples (Table 2). The STP then slowly decreased with time across the next seven samplings. The saturation paste and 0.01 M CaCl2 extract ortho-P concentrations in the manure 1991 samples were the highest of the 1991 subsoil plot treatments and remained very high over the study period (Tables 3 and 4).
Adding 450 kg P ha-1 in manure and 225 kg P ha-1 in whey to the topsoil after the F1994 sampling increased the STP, saturation paste, and 0.01 M CaCl2 extractable ortho-P in the F1995 and S1996 samples followed by slight decreases in the S1997 samples (Tables 2, 3, and 4).
Using the data used to produce Tables 2, 3, and 4, linear regressions were calculated among bicarbonate, saturation paste, and 0.01 M CaCl2 extractable ortho-P for all data, subsoil data, and topsoil data (Table 6) . These data suggest that good relationships exist among the three extraction methods for the topsoil and the subsoil, but that the relationships are different for the topsoil and the subsoil due to the differences in the slopes of the regressions.
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The freshly exposed subsoil ortho-P solubilities fell along or slightly above a 1:1 slope isotherm between the HA and ß-TCP isotherms (Fig. 2). The initial topsoil P solubilities were about equally spaced above and below the ß-TCP isotherm. The ortho-P solubilities in the MCP-treated subsoil conventional treatment moved closer to and up along the ß-TCP isotherm by the S1992 sampling, then moved back down but remained below the ß-TCP isotherm during the sampling period (Fig. 3). The high-whey S1992 P solubility moved up to and along the ß-TCP isotherm past the conventional treatment subsoil and higher than most of the topsoil treatment samples for the same date (Fig. 2, 3, and 4). By S1994, the P solubility had moved back down along the ß-TCP isotherm. Whey addition in the fall of 1994 moved the S1996 solubility up and above the ß-TCP isotherm. The S1997 solubilities then moved to a slightly lower position and onto the ß-TCP isotherm. Each low-whey P solubility for a particular sampling was down the isotherm from the corresponding high-whey concentration but remained near the ß-TCP isotherm (Fig. 4 and 5). With the fall 1994 manure addition to the low-whey soil, the ortho-P solubility increased to well above the ß-TCP isotherm in S1996 and S1997 (Fig. 5). Ortho-P solubility in the subsoil manure 1991 treatments moved up along and above the ß-TCP isotherm in the S1992 samples (Fig. 6). The S1993 through S1997 solubilities moved closer to the OCP isotherms and slightly down along the isotherm. All untreated topsoil ortho-P solubility means were near to or below the ß-TCP isotherm (Fig. 7). With whey or manure additions, the phosphate solubilities moved above and slightly up along the ß-TCP isotherms, but remained below the OCP isotherm (Fig. 7).
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Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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The untreated freshly exposed subsoil STP mean was 9 mg P kg-1, as compared with 23 mg P kg-1 in the topsoil at the beginning of the study. The first 135 kg P ha-1 added to the subsoil as MCP increased the STP by 3 mg P kg-1 in the F1991 sampling, which was not significant at P < 0.05. The high P immobilization was probably due to freshly exposed lime minerals (240 g CaCO3 equivalent kg-1) that reacted with the added P. The second 130 kg P ha-1 added to the same plots after the F1991 soil sampling increased the STP by 19 mg kg-1, or 6.8 kg P ha-1 added for each mg P kg-1 increase in STP by the S1992 sampling (Table 2). When Westermann (1992) added MCP to other Portneuf soils that were deep-mixed 18 yr earlier, or to Portneuf topsoil treated with CaO or CaCO3, more MCP was required to increase the STP one unit than for the topsoil, which required about 7.1 kg P ha-1 for each mg STP kg-1. This same comparison is hard to make with the whey and manure treatments since the whey contained H3PO4 and both contained ortho-P, as well as organic P, which mineralizes at various rates, depending on composition and soil conditions. The very high P application rates with the whey and manure increased the STP well above that needed for economical crop production and remained adequate for the duration of the study.
Organic materials added with the whey and manure would be expected to coat the reaction surfaces of the freshly exposed subsoil particles and prevent or delay P sorption, thereby increasing P solubility. There was evidence that this was occurring. The regression line slope between STP and both the saturation paste and the 0.01 M CaCl2 extractable ortho-P was 1.42 times greater for whey than for MCP (conventional) applied ortho-P. For the manure 1991 treatment, the regression line slope was 1.69 and 2.73 times greater than for the MCP treatment for the saturated paste and 0.01 M CaCl2 extract estimates. The addition of 25 Mg OC ha-1 in 1991 by the whey did not affect the subsoil OC content, while the addition of 41 Mg OC ha-1 from the manure tripled the subsoil OC in 1992 and decreased to double the original subsoil OC after 7 yr, or slightly higher than the topsoil (9 g OC kg-1). The organic materials in whey are primarily sugars and simple proteins (the fats and long chain proteins end up in the butter and cheese) and decompose rapidly (Jones et al., 1993). These observations suggest that P sources increase solution ortho-P concentrations in the order: manure > acid-whey > MCP.
We speculate that the STP, saturation paste, and 0.01 M CaCl2 extractable ortho-P spring to fall decrease and the fall to spring increase were caused by P cycling from crop roots and residues. Plants take up P during the growing season, decreasing the extractable P. During and between growing seasons, plant roots, incorporated plant material, and residual organic matter (previously added plant material or manure) decompose and liberate extractable P. There was no seasonal saturation paste pH change that would explain this cycling. It is also possible that there was some conversion of mineral P to labile P, which is partially extracted by NaHCO3 and influences solution-P concentrations (Westermann, 1992).
Initially the subsoil ortho-P solubility concentrations were along or slightly above a 1:1 slope isotherm lying between the HA and ß-TCP isotherms (Fig. 2). The 1:1 slope of these data points suggests that with time an ortho-P mineral, with a different Ca:H:PO4 ratio than HA or ß-TCP, formed in the subsoil below the native vegetation root zone, probably before the soils were irrigated and cultivated. The ortho-P solubility in the subsoil–conventional treatment did not increase significantly and remained below the ß-TCP isotherm as the extractable H2PO-4 concentration increased after 265 kg P ha-1 was added as MCP in 1991 (Fig. 3).
The initial topsoil P solubilities were astride the ß-TCP isotherm (Fig. 2) and the untreated topsoil P solubilities remained unchanged throughout the study (Fig. 7). The untreated topsoil STPs showed the annual tendency for the spring–fall–spring cycles in all but the F1993 to S1994 samplings. These P changes were not as significant as in the treated subsoils, but the trend was occurring. The only topsoil treatment with P solubilities nearer the OCP than the ß-TCP isotherm line was that receiving manure (Fig. 7).
The horizontal shifts in the subsoil solubility points were primarily due to pH changes in the CaCl2 extracts. The Ca activities changed very little among samplings and treatments for the topsoil and the subsoil, due primarily to the calcareous nature of the soils and buffering within the extract. The topsoil pH values changed very little with time and treatment. The vertical movement was a combination of pH and H2PO-4 changes in the subsoil and primarily H2PO-4 changes in the topsoil.
We would remind the reader that comparing calculated H2PO-4 solubility from CaCl2 extract data does not imply that the soil solution is in equilibrium with the listed minerals. The precipitation–dissolution reactions are very slow and solution and surface contaminations further perturb the soil P equilibrium compared with pure solutions in contact with a pure mineral surface.
With the second whey and manure application in the fall of 1991, the total P applied was slightly higher in the high-whey than in the manure treatment (Table 1). The STP, saturated paste, and 0.01 M CaCl2 ortho-P concentrations in the subsoil high-whey treatment were significantly lower than those in the subsoil manure 1991 from S1992 through F1994 (Tables 3–5). This difference was probably caused by release of appreciable amounts of soluble Ca2+ from H3PO4 neutralization by soil lime in the whey treatments. This would tend to precipitate additional P as the soil pH returned to that of the matrix. An additional factor that may contribute to higher P availabilities after manure is its higher portion of organic P compounds compared with the acid whey (44 vs. 15%). These organic compounds will mineralize with time, releasing additional ortho-P.
Both the 1994 whey and manure applications increased the 0.01 M CaCl2 ortho-P concentrations several fold in the subsoil and topsoil plots (Tables 3 and 4) and were appreciably greater than the 0.2 to 0.3 mg P kg-1 required for optimum plant growth (Nishimoto et al., 1977).
Soil P solubility was generally near equilibrium with ß-TCP when the STP concentration was <100 mg P kg-1 (Table 2 vs. Fig. 2–7). As the STP approached and exceeded 100 mg P kg-1, the soil P solubilities approached the OCP isotherm line. In addition, ortho-P concentrations in the 0.01 M CaCl2 extract generally exceeded 1 mg L-1 when the STP was >100 mg P kg-1, both more than sufficient for optimum plant growth on calcareous soils and at concentrations that suggest that ortho-P would move deeper into the soil profile with leaching water.
Received for publication February 11, 1998.
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