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USDA-ARS, Conservation and Production Research Laboratory, P.O. Drawer 10, Bushland, TX 79012
Corresponding author (pwunger{at}tcac.net)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: LSD, protected least significant difference • MWD, mean weight diameter • NT, no-tillage • R+, residue retained • R-, residue removed • ST, sweep tillage • WSF, wheat–fallow–sorghum–fallow rotation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The above results clearly showed that soil water storage and, consequently, crop yields could be increased by using crop residues as a mulch on the soil surface. Dryland crops, however, often do not produce sufficient residues (straw, stover, etc.) to result in large increases in water storage. Also, crop residues often are used as animal feed, fuel, etc. Where residues are limited for whatever reason, a readily available, cheap, alternative mulching material is needed to achieve greater soil water storage and, in turn, crop yields. One such material is waste paper that currently is mostly discarded and constitutes about 40% of the solid waste material deposited in landfills each year in the USA. Space in landfills, however, is limited and waste paper no longer is accepted for disposal in some landfills (Edwards et al., 1993). Hence, disposal of paper on land is a possibility, either as a surface mulch or incorporated as a soil amendment.
For waste paper to become acceptable for disposal on land, it must be in a form that is not dispersed by wind and does not cause the land to have a trashy appearance. One such form is paper pellets. Results of a laboratory study showed that paper pellets applied as a surface mulch reduced evaporation compared with that from bare soil (Unger, 1995b). The objective of this study was to determine the effect of a paper pellet mulch under field conditions on soil water storage; grain sorghum growth, yield, and yield components; pellet decomposition rates; pellet movement due to wind or water; and soil total C concentration and aggregation.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The study had a randomized block, split-split plot design, with treatments replicated three times. Whole plot treatments were wheat residue condition (residue retention or removal, R+ and R-, respectively). Whole plots were 9.2 m wide and 18.4 m long. Residues were raked from the R- plots before tillage (split plot) treatments were applied. Tillage treatments were no-tillage (NT) and sweep tillage (ST). Tillage strips were 4.6 m wide and 18.4 m long. No-tillage involved no soil disturbance other than opening a slot for seed placement. The ST treatment involved one sweep tillage 7 to 10 cm deep to loosen the surface soil after applying the residue treatments, but before applying the paper pellet treatments. After imposing the residue condition and tillage treatments, paper pellets were applied at rates of 0 (check), 5, 10, and 15 Mg ha-1 to 4.6 m by 4.6 m plots as the split-split plot treatments. Pellets were provided by Tascon1, Houston, TX. The diameters of pellets applied to Areas 1, 2, and 3 were 19, 10, and 8 mm, respectively. (Pellet size was not a treatment.) Pellets were spread on the surface as uniformly as possible by hand. Total surface cover provided by pellets plus wheat straw was determined at five different points on each plot by randomly placing a wire grid on the surface, counting grid intersections underlain by pellets or residues, and converting that number to a percentage cover. Throughout the rotation, effects of wind and water on the position of pellets within plots were noted, but not measured.
Before pellets were applied, one access tube was installed in each plot to determine soil water contents by the neutron attenuation method. Determinations were made from the time of pellet application (usually in July on each area) until harvesting grain sorghum the following year. Determinations were made to a depth of 1.8 m at 0.30-m increments at about 14- or 28-d intervals, depending on weather conditions and precipitation frequency (less frequent in winter or when no precipitation occurred). Precipitation was measured within 0.25 km of the plots.
Because ST plots were tilled only once, a tank mix of atrazine [6-chloro-N-ethyl-N'(1-methylethyl)-1,3,5-triazine-2,4-diamine] at 340 mg a.i. m-2 and 2,4-D [(2,4-dichlorophenoxy)acetic acid] at 110 mg a.i. m-2 was applied soon after wheat harvesting to all plots for weed control. Applications of glyphosate [N-(phosphonomethyl)glycine] at 60 mg a.i. m-2 provided additional weed control when needed during the fallow period. Terbutryn [N(1,1-dimethylethyl)-N'-ethyl-6-(methylthio)-1,3,5-triazine-2,4-diamine] was applied at 220 mg a.i. m-2 to all plots at sorghum planting for growing season weed control. No fertilizer was applied because grain sorghum in rotation with winter wheat under dryland conditions has not responded to applied nutrients on Pullman soil (Eck and Jones, 1992).
A John Deere Max-Emerge planter (Deere & Co., Moline, IL) was used to plant grain sorghum hybrid `383Y' (Northrup King) on 1 July 1996 (Area 1) and `DK 46' (DeKalb) on 6 June 1997 (Area 2) at a 0.75-m row spacing. Plantings were at a rate needed to obtain about 96000 plants ha-1. Plant populations were determined after sorghum establishment. A drought in 1998 resulted in inadequate soil water for seed germination, and sorghum was not planted that year (Area 3).
Sorghum grain and forage yields were based on hand-harvested panicle and total plant samples obtained from 3 m long sections of the two center rows of each plot when the sorghum was physiologically mature. Panicle samples were cut at the base of panicles, oven dried at 60°C, then threshed to obtain grain weights to calculate grain yields. After removing panicle samples, plants were cut 1 to 2 cm above the soil surface; weighed; and subsamples taken, weighed, oven dried at 60°C, and weighed again to determine stover yields. Grain and stover yields are reported on an oven dry weight basis. Sorghum final plant height, number of panicles per hectare, grain test weight, and seed weight were determined also.
Amounts of pellets remaining on the soil surface were determined at sorghum planting time by weighing pellets removed from five 0.25-m2 areas of each plot. In addition, a pellet subsample was ashed by total combustion in a muffle furnace operated at 500°C for 4 h to correct for soil adhering to the pellets.
Bulk soil samples were taken to the 5-cm depth from four positions in each plot with a flat-bottomed spade when sorghum was planted and again when the subsequent wheat crop in the rotation was planted. These samples were composited and passed through a 12.7-mm screen with square holes, air dried, and stored in closed containers until the size distribution of water-stable aggregates was determined on duplicate subsamples according to Kemper and Rosenau (1986). From those results, the mean weight diameter (MWD) of water-stable aggregates and percentage of the aggregates <0.25 mm in diameter were calculated. A portion of the dried bulk soil also was sieved to obtain 1- to 2-mm diameter aggregates. The stability in water of duplicate subsamples of those aggregates was determined by Kemper's (1965) procedure.
The bulk soil sampling at wheat planting occurred about 820 d after pellet application. Samples obtained at that time were also used to determine soil total C concentration. After grinding and passing a portion of the soil through a sieve with 0.18-mm openings, C concentration of duplicate subsamples was determined by a high-temperature combustion method using a Leco CNS-2000 analyzer (Leco Corp., St. Joseph, MI).
Values for duplicate subsamples were averaged before statistically analyzing the data by the analysis of variance technique (SAS Inst., 1989). Data for each area were analyzed separately. When the F value was significant at the P 
0.05 level, the protected least significant difference (LSD) was used to separate the means.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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5 mm in 15 of the 53 mo, with none in 5 mo. The total was almost three times the long-term average in July 1996 and was five times the long-term average in October 1998. High variability is a characteristic of precipitation in semiarid regions such as the southern Great Plains. Prolonged low precipitation periods strongly influenced the results in this study (soil water contents at sorghum planting and crop yields, which are discussed in later sections). Total precipitation from October 1995 through May 1996 was only 57 mm, which resulted in low soil water contents at sorghum planting in June 1996 on Area 1 (Table 2). From October 1996 through March 1997, the total was even lower (39 mm), but precipitation in April and May 1997 resulted in greater soil water contents at sorghum planting (Area 2) than in the previous year (Area 1). Precipitation was generally favorable from the time of pellet application in July 1997 through March 1998. However, much below average precipitation occurred from April through September 1998. Because of low precipitation in April, May, and June, the seed-zone soil water content was inadequate for seed germination and sorghum was not planted in 1998 as planned (Area 3). Hence, sorghum results are available only for Areas 1 and 2.
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The residue condition x tillage method interaction effect was significant for soil water contents on Area 3 at sorghum planting time (not planted due to drought) (data not shown). With residues retained, the water content was greater with NT than with ST (178 vs. 174 mm), which is consistent with results of numerous studies that showed use of no-tillage improves water conservation when adequate residues are present on the soil surface. With residues removed, the content was lower with NT than with ST (167 vs. 171 mm), showing that no-tillage is not a good practice when crop residue amounts on the soil surface are low. The cover provided by paper pellets did not substitute for the cover removed when wheat straw was removed from NT plots. The residue condition x pellet application rate interaction effect was not significant.
At sorghum planting on Area 1 (1996), soil water contents were greater with NT than with ST and progressively lower with increasing rates of pellet application. The greater water contents with NT are similar to results obtained in previous studies where greater amounts of wheat straw were on the soil surface during the noncrop period (Unger, 1978, 1984; Unger and Wiese, 1979).
Water content differences on pellet rate treatment plots on Area 1 at sorghum planting closely paralleled the differences at pellet application, which were significant at the P = 0.07 level. Increases in water contents from the initial determination until sorghum planting were 26, 18, 22, and 20 mm for the 0, 5, 10, and 15 Mg ha-1 pellet rate treatments, respectively. These results show that a one-time application of paper pellets as a mulch did not result in greater soil water storage and may have decreased it as compared with that under bare soil conditions. Multiple applications may have given different results, but that was not investigated in this study. On Areas 2 and 3, soil water contents at sorghum planting did not differ and increases from pellet application to sorghum planting time [not planted in 1998 (on Area 3)] were similar with all mulch rates (116–127 mm on Area 2 and 96–99 mm on Area 3). The greater soil water contents at sorghum planting time on Areas 2 and Area 3 than on Area 1 resulted from more favorable precipitation during the 1996–1997 and 1997–1998 fallow periods (Table 1). Based on the means, water contents tended to decrease with increases in pellet application rates (Table 2), but mean increases from the initial determination until sorghum planting time were almost identical (79–82 mm), again showing that paper pellet applications did not result in increased soil water storage.
For this study, the soil water contents at planting and the increases that occurred during the period before planting were contrary to expectations, based on results of a laboratory study for which evaporation was lower with a pellet mulch than with bare soil (Unger, 1995b). The mulches, however, were placed on the surface after water had been applied to the soil, and the pellets did not intercept the applied water. In contrast, pellets intercepted precipitation under field conditions, thus resulting in reduced water entry into soil and increased evaporative losses directly from the pellets. Precipitation interception under field conditions apparently increased when the pellets disintegrated, which resulted in almost a complete mat-like cover on the surface with the 15 Mg ha-1 pellet treatment. As a result, little or no water storage occurred from small precipitation amounts that are typical of storms in semiarid regions.
Evaporative losses greatly influence soil water storage from precipitation in the semiarid southern Great Plains because many storms provide only a small amount of water (Fig. 1). Under such conditions, much of the water likely was intercepted by the pellets or it only wet the soil surface, which resulted large evaporative losses. Also, the pellets may have served as a wick to transport water from the soil surface to evaporation sites on the pellets. Storage in soil occurred only from storms that provided sufficient water to wet the pellets and additional water to infiltrate into the underlying soil.
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The residue condition x pellet application rate interaction effect was significant on Areas 2 and 3. On Area 2, surface cover was 39 percentage units lower with the R- as compared with the R+ treatment for the 0 Mg ha-1 rate treatment, but only 28, 21, and 18 units lower for the 5, 10, and 15 Mg ha-1 rate treatments, respectively. These results indicate some overlapping of cover (wheat straw and pellets) occurred on R+ plots, especially at the higher rates of pellet application. Results regarding this interaction on Area 3 were similar to those for Area 2.
The tillage method x pellet rate interaction effect on surface cover was significant on Area 3. Without applied pellets, cover was greater on ST than on NT plots (35 vs. 28%), indicating the tilling operation flattened some wheat straw, thus resulting in greater percent cover than on NT plots for which the straw remained standing. With applied pellets, cover was 4 percentage units greater on NT than on ST plots, indicating that overlapping of straw and pellets was greater with ST than with NT.
Residues of recently harvested wheat contributed to surface cover after spreading the paper pellets (Table 3). Wheat residue was removed from the R- plots, but not all was removed and average cover on those plots without applied pellets was 13% (data not shown). On R+ plots without applied pellets, average cover was 50% (data not shown). The mean value for the 0 Mg ha-1 pellet rate (29%, Table 3) corresponds closely with the average of the above values for the R- and R+ treatments. Hence, the pellet treatments resulted in cover of about 10, 16, and 27% with the 5, 10, and 15 Mg ha-1 rates, respectively. Tillage methods had no effect. Although sweep tillage undercut the surface and was used only once, it incorporated some residues with soil and flattened some residues that remained on the surface. All residues (mostly standing) remained on the surface of NT plots. Pellets applied to Areas 2 and 3 were 10 and 8 mm in diameter, respectively, which explains the greater surface cover on those areas than on Area 1 for which pellet diameter was 19 mm.
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The tillage method x pellet application rate interaction effect was significant for pellets recovered at sorghum planting time only on Area 3. Recovery amounts (data not shown) were similar for both tillage methods with the 0 (none recovered) and 5 Mg ha-1 application rates. However, as the rates increased, increasingly more pellets were recovered from ST than from NT plots, indicating that pellet decomposition was greater under NT conditions. This is attributed to the soil surface of NT plots remaining wet longer because more residues were on the surface. This was the case, even though the cover percentages were similar (47 for NT and 46 for ST, Table 3). The flattened straw with ST provided for a cover percentage similar to that with NT for which the straw was standing, but less effective for providing for soil cover. Precipitation from the time of pellet application in 1997 through March 1998 generally was more uniformly distributed than in other years (Table 1), thus resulting in a wetter surface more frequently than in other years. A similar trend occurred on Area 2 for which the interaction effect was significant at the P = 0.06 level.
The amount of pellets recovered at sorghum planting time (300–330 d after application) was not affected by the residue treatments on any area, but was affected by tillage treatments on Area 3. Although sorghum was not planted on Area 3 in 1998, measurements were made at the normal planting time. The amount recovered on Area 3 was less from NT than from ST plots (Table 3), indicating that pellet decomposition was greater on NT plots, as discussed previously. Although not significant, the trend toward less recovery of pellets from R+ plots in 1998 supports the reasoning that more wheat residue on the surface contributed to greater pellet decomposition than where less residues were present. Even greater decomposition may have occurred where more residues were present if N fertilizer had been applied, especially in the wet year.
Pellet recovery amounts differed on each area due to amounts applied (Table 3), which is logical, but recovery percentages were similar for all application rates and averaged 60, 57, and 68% with the 5, 10, and 15 Mg ha-1 rates, respectively.
Sorghum Establishment, Growth, Yield, and Yield Components
Most variables describing sorghum growth and yield were not affected by the residue condition, tillage method, and pellet application rate treatments. Hence, results are given in Table 4 only for some variables. For others, mean values are given and discussed in the text as appropriate.
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The final plant height averaged 97 cm on both areas, with no differences due to treatments. A height of about 100 cm is typical for dryland grain sorghum in the region (Unger, 1991a; 1994).
The tillage method x pellet application rate interaction effect was significant for the number of panicles harvested from Area 1. The number was greater on NT plots with 5 and 15 Mg ha-1 of applied pellets, but not different with other treatments. The results, however, showed no consistent trends. The number of panicles harvested was greater from R+ than from R- plots on Area 1 
, possibly because the soil water content tended to be greater (P = 0.06 level) on R+ plots at planting time. As a result, more water was available for development of secondary panicles. The initial plant populations were similar with both treatments (55000 and 62000 ha-1 for R+ and R-, respectively). For other treatments, the number of panicles did not differ on Area 1 and averaged 107000 ha-1. The average was 96000 ha-1 on Area 2, with no differences due to treatments.
Grain yields (Table 4) differed due to residue condition treatments, with yield being greater on R+ plots of both areas. Tillage method and pellet rate treatments did not affect grain yields on either area. The reason for greater yields with the R+ treatment is not apparent because residue treatments did not influence soil water contents at planting. Soil water contents at planting strongly affect sorghum grain yields under dryland conditions in the semiarid southern Great Plains (Jones and Hauser, 1975; Unger and Baumhardt, 1999). For example, sorghum grain yields increased 17 kg ha-1 for each additional 1 mm of water in soil at planting time under conventional (sweep) tillage conditions (Jones and Hauser, 1975). For this study, the average increase (330 kg ha-1) was greater than suggested by the greater average soil water content at planting with the R+ treatment (8 mm, Table 2), indicating that some other factor contributed to the greater yield. One possibility is more residues being on the surface during the growing seasons. For example, grain yields were greater in 2 of 3 yr on plots mulched with wheat straw when sorghum was planted than on bare soil plots (Unger and Jones, 1981). For the 3 yr, average increases over the bare soil treatment were 0.18, 0.21, and 0.24 Mg ha-1 with 2, 4, and 8 Mg ha-1 of applied wheat straw, respectively. Undoubtedly, slightly more water in soil at sorghum planting time combined with more surface residues during the growing season were responsible for the yield increases. The presence of more residues on the surface during the growing season possibly increased water infiltration and decreased evaporation, which could have supplied more water to the sorghum without contributing to the soil water content determined only at sorghum planting and harvesting. Another possibility is a more favorable soil temperature regime where more residues were on the soil surface.
Sorghum grain test weights, which are indicators of grain quality, were not affected by treatments, but averaged 747 and 892 g L-1 for on Areas 1 and 2, respectively. The greater test weights were associated with the greater yields, suggesting grain from Area 2 was of higher quality than that from Area 1.
Besides having greater test weights, grain from Area 2 also had greater seed weights, averaging 17.6 mg seed-1 as compared with 12.7 mg seed-1 for grain from Area 1. These results further suggest that grain from Area 2 was of higher quality than that from Area 1. Seed weights were not affected by residue or tillage treatments on either area, but differed due to pellet application rates on Area 1 (Table 4). They were similarly high on 0 and 15 Mg ha-1 pellet rate plots and lowest on the 10 Mg ha-1 treatment plot. Grain yield followed the same trend, but did not differ significantly due to pellet application rates.
Sorghum stover yield was greater on R+ than on R- plots of Area 1 (Table 4). Stover yield trends paralleled grain yield trends on Area 1. Overall, stover yields averaged 1.1 and 3.2 Mg ha-1 on Areas 1 and 2, respectively. Average grain yield also was greater on Area 2 than on Area 1.
Soil Total Carbon Concentration
Soil total C (mainly organic C for Pullman soil) concentration was determined at wheat planting time, which averaged about 820 d after pellet application when the previous wheat crop in the rotation was harvested. Most pellets had disintegrated and decomposed by that time, and possible effects of the paper pellets on soil C should have developed by then. Soil C concentrations differed due to the tillage method x pellet application rate interaction effect on Area 3 (data not shown), and due to tillage methods on Area 1 and pellet application rates on Area 3 (Table 3).
For the tillage method x pellet application rate interaction 
on Area 3, C concentrations were similar for both tillage methods with no applied pellets (5.4 and 5.5 g kg-1 on NT and ST plots, respectively). With pellets, concentrations were 5.4, 6.2, 7.1, and 7.7 g kg-1 with the 0, 5, 10, and 15 Mg ha-1 application rates, respectively, on NT plots, and 5.5, 5.7, 6.0, and 6.3 g kg-1 with the respective rates on ST plots. Greater increases with NT indicate pellet decomposition was greater, probably because of slower soil and pellet drying under the no-tillage conditions where more straw remained on the surface.
On Area 1, C concentration was greater on NT than on ST plots. Because pellet application rates were identical, these results suggest the difference was due to tillage method per se. Soil C (organic matter) contents were greater under no-tillage than sweep tillage conditions in other studies on Pullman soil (Unger, 1991b). In this study, however, ST plots were tilled only once, and that occurred before pellet application. Subsequently, ST and NT plots were managed the same. Area 1 plots were established on fields managed without tillage for the last 16 yr. The total C results, therefore, suggest the difference possibly resulted from a decrease due to the initial sweep tillage operation. Because the C concentration under no-tillage conditions was greatest at or near the soil surface (Unger, 1991b), even one sweep tillage operation may have altered the C distribution with soil depth, thus leading to the lower concentration with the ST treatment. Decomposition of crop residues resulting from the mixing action of tillage may have been involved also. In contrast, continued use of no-tillage maintained the original C concentration of NT plots, or possibly even increased it due to pellet applications (concentrations were not determined when plots were established). A similar increase may have occurred on ST plots, but the difference resulting from the initial tillage operation was maintained.
Carbon concentrations as affected by tillage methods were lower on Areas 2 and 3 than on Area 1, probably because Areas 2 and 3 had been managed by the no-tillage method for fewer years when pellets were applied. Effects of previous long-term management may have been involved also.
Total C concentrations tended to increase with increasing pellet application rates on all areas, but they differed at the P = 0.05 level only on Area 3. On Area 1, concentrations differed at the P = 0.10 level.
Soil C increases with increases in pellet application rates are logical because paper contains C. The full benefits of the applied C (paper pellets) with regard to soil C, however, may not have been achieved when the soil was sampled. The pellets were applied about 820 d before sampling and had disintegrated by that time, but the material had not fully decomposed. With further decomposition, the materials should become incorporated with soil by cultural operations and natural processes, thereby further increasing soil C concentrations. Also, multiple pellet applications undoubtedly would increase soil C concentrations.
Pullman soil at the study site has a relatively low organic C concentration (
9.2 g kg-1 at the 0- to 10-cm depth; Unger, 1991b), and any C increase could improve its long-term productivity. Under dryland conditions, crops often produce relatively small amounts of residues. Thus, increases in soil C are low, even under no-tillage conditions. Applying paper pellets, especially multiple applications, could increase the soil C concentration and provide an alternative means of disposing waste paper that may be no longer accepted for disposal in land fills.
Soil Aggregation
Soil aggregate determinations were made on samples taken at sorghum harvesting and wheat planting time. Data for samples taken at sorghum harvesting did not differ in most cases and were not as complete (sorghum not planted on Area 3) as those taken at wheat planting. Therefore, results are presented and discussed only for samples taken at wheat planting.
The residue condition x pellet application rate interaction effect on the stability in water of 1- to 2-mm diameter aggregates was significant (data not shown). However, no consistent trends occurred, except that stability was lower where pellets were applied to R+ plots. On R- plots, aggregate stability was lower with 5 Mg ha-1 of applied pellets than with other rates. Except for treatments that resulted in the lower percentages, stabilities were similar for other treatments. Because the results were inconsistent, the differences probably were of little importance.
The stability was greater for 1- to 2-mm diameter aggregates from ST plots than for those from NT plots on Area 3 (Table 3). Stability of aggregates from Areas 1 and 2 did not differ at the P = 0.05 level, but tended to be greater on ST than on NT plots (P = 0.08 level, based on a paired t-test). The reason for the difference is not known because ST plots were tilled only once when the plots were established. After tilling, management was identical for the ST and NT plots. Stabilities did not differ due to the residue condition and pellet application rate treatments.
The residue condition x pellet application rate interaction effect on aggregate mean weight diameter (MWD) was significant 
on Area 2, with the MWDs being identical (0.9 mm) on R+ and R- plots without applied pellets. On R+ plots, MWDs were greater in all cases with than without applied pellets (1.4–1.7 vs. 0.9 mm). These results show that paper pellets helped stabilize soil aggregates, apparently through their decomposition and, therefore, their effect on soil C concentration. On R- plots, only the 10 Mg ha-1 pellet application rate resulted in an increased MWD (1.8 vs. 0.9 mm). The reason for the inconsistent results where residues were removed is not apparent, but pellet decomposition may have been slower because the surface soil was drier without wheat residues being present.
The MWD was greater for water-stable aggregates from R- than from R+ plots on Area 1 (Table 3), but the difference was small (0.1 mm) and probably of little consequence. Greater differences resulted from pellet treatments. The MWDs were lowest and highest with the 0 and 15 Mg ha-1 rate treatments, respectively, and roughly paralleled the soil total C concentrations, even though C values did not differ significantly in most cases. Unger (1997) found a highly significant relationship between aggregate MWDs and soil organic C concentrations of Pullman soil. For this study, however, the relationship was not significant.
The percentages of water-stable aggregates <0.25 mm in diameter were not affected by residue or tillage treatments, but differed due to pellet treatments on all areas (Table 3). In addition, the mean was greater for the no-pellet treatment than for all other treatments, based on a paired t-test.
Percentages of <0.25-mm diameter aggregates have implications regarding surface seal development and water infiltration (Loch, 1989) because such aggregates are easily transported by water and, therefore, can clog soil pores and retard water infiltration. Although not determined, aggregates <0.125 mm in diameter have an even greater effect on surface seal development and water infiltration because they are more easily transported by water (Loch, 1989). On Pullman soil, runoff was greater with no-tillage when surface residue amounts were low and the surface seal was not disrupted than where sweep tillage was used to loosen the surface soil (Jones et al., 1994).
Percentages of <0.25-mm diameter aggregates were lower with than without applied pellets in this study. Soil water storage, however, was not increased, even though soil conditions seemingly were better where pellets were applied. Possibly contributing to these results were the small percentage differences on all areas (9 units maximum on Area 2), which may have been inadequate to result in soil water storage differences. Also, water absorption by pellets resulted in high evaporation losses of water from many rains, thus negating the benefits of fewer small aggregates where pellets were on the surface.
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Soil total C concentration when the next wheat crop was planted in the rotation was greater in pellet covered than in bare soil on one of three areas. Although the pellets had disintegrated, some pellet material remained on the soil surface, suggesting that its further decomposition would further increase soil C concentrations. Pellet applications resulted in greater mean weight diameters of water stable aggregates and lower percentages of <0.25 mm diameter water stable aggregates. These C and aggregate results suggest that applying paper pellets will improve the physical condition of a soil, which, in turn, should increase its productivity.
Paper pellet application apparently had no harmful effect on the soil or on crop production. Hence, it is concluded that waste paper in a suitable form (e.g., pellets) can be disposed of on cropland. However, shallow incorporation, which would hasten its decomposition and result in more rapid improvements in soil conditions, may be a better practice than surface applications as used in this study.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Received for publication April 19, 2000.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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