Agronomy Journal 92:106-114 (2000)
© 2000 American Society of Agronomy
TROPICAL SOIL MANAGEMENT
Stover and Potassium Management in an Upland Rice–Soybean Rotation on an Indonesian Ultisol
Thomas S Dierolfa and
Russ S. Yostb
a Jasa Katom, Jalan Kehakiman No. 283, Bukittinggi, West Sumatra, Indonesia 26136
b Dep. Agronomy and Soil Sci., Univ. of Hawaii, Honolulu, HI 96822 USA
katom{at}bukittinggi.wasantara.net.id
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ABSTRACT
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Fertilizer and stover management greatly determine the extent of K deficiency on weathered, low-K soils in the humid tropics. This study quantified the effects of K fertilization and stover management on soil properties and crop yields on a Typic Kanhapludult in Indonesia. Cowpea [Vigna unguiculata (L.) Walp. subsp. unguiculata]–cowpea–rice (Oryza sativa L.)–soybean [Glycine max (L.) Merr.]–rice–soybean were grown during a 2-yr period. Fertilizer KCl was applied as 70 and 250 kg K ha-1 to the first crop only, and as a total of 250 and 600 kg K ha-1 applied to several crops. The effect of stover removal or return was examined for each K rate. Critical soil K levels were 0.14 cmolc kg-1 for the final rice crop and 0.14 and 0.16 cmolc kg-1 for the two soybean crops. By returning stover, a single application of 70 kg K ha-1 to the first crop was adequate to maintain soil K above the critical level for all six crops. When stover was removed, a total of 250 kg K ha-1 applied during several crops maintained soil K above critical levels. A one-time 250 kg K ha-1 application to the first crop, however, resulted in yield declines by the fifth crop. A maintenance rate of about 45 kg K ha-1 per crop was required when stover was removed. Stover removal also hastened soil Mg depletion, and thus a maintenance rate of about 6 kg Mg ha-1 crop-1 is recommended.
Abbreviations: DAP, days after planting • ECEC, effective cation exchange capacity
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INTRODUCTION
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HIGHLY WEATHERED, leached Ultisols and Oxisols are often used for the expansion of agricultural production in Indonesia (Donner, 1987). Although soil acidity and low soil P are the major fertility constraints in these soils, K usually becomes limiting to crop growth under continuous cultivation (von Uexküll, 1985). This is primarily because both available and reserve K are usually low in these soils (Sparks and Huang, 1985). Soil and crop management factors also contribute to the occurrence of K deficiency. For example, in the move to intensify crop production on highly weathered soils, K application usually ranks far behind N and P (Pushparajah, 1985). K uptake by plants is similar to that of N, however, and is usually an order of magnitude greater than that for P (Cooke, 1985). Additionally, removing crop stover from the field hastens the depletion of soil K (Pieri, 1982; Vilela and Ritchey, 1985). Through a combination of the above factors, the soil eventually becomes depleted of available K. Although K can be replenished through fertilization, excessive fertilization can result in leaching losses (Ritchey, 1979; Gill and Kamprath, 1990) and in losses from the luxury consumption of K (Brady, 1984) when stover is not returned.
Potassium and stover management are critical to the uplands of Sitiung, Indonesia, where the predominant cropping system is upland rice followed by soybean or peanuts (Colfer, 1991). Rice stover (straw) in this system is usually burned in piles after threshing. Even if farmers rotate the location of the rice burn piles each season, incorporation of burnt stover generally results in an uneven distribution of nutrients, which can hasten nutrient depletion. The large concentration of cations in one area may exceed the soil effective cation exchange capacity (ECEC) and render the nutrient cations susceptible to leaching losses (Ritchey, 1979). The objectives of this experiment were to (i) quantify the effects of varying rates and timing of K applied as fertilizer KCl or cattle manure and crop stover return or removal on soil base cation status; and (ii) relate the effect on base cation status to crop yields for a 2-yr period during which six crops were grown.
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Materials and methods
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Site Characteristics
Field experiments were conducted at a site near the village of Sitiung 1Á, West Sumatra, Indonesia (102° E, 1° S), from 1989 to 1991. The rainforest originally covering the site was cut and cleared by bulldozer in 1976. After three seasons of rice, the field was fallowed and became covered with alang-alang grass [Imperata cylindrica (L.) P. Beauv.]. In 1989, the alang-alang was cut and removed after it was sprayed with a glyphosate herbicide. The surface 15 cm was then plowed and roots of alang-alang were removed. The pedon at the site was classified as a clayey, kaolinitic, isohyperthermic Typic Kanhapludult by staff at the Center for Soil and Agroclimate Research, Bogor. Selected soil chemical and physical data with depth are shown in Table 1
. Rain ranges from 2500 to 3000 mm yr-1 and averages more than 200 mm per month except from June to September, when monthly rain ranges from 100 to 200 mm. Total rain during the 24-month experimental period was 6870 mm, of which 5440 mm fell while crops were growing in the plots.
Experimental and Treatment Design
Treatment, fertilization, and planting information are shown in Table 2
. A six-crop rotation of cowpea (cv. local), cowpea (cv. local), rice (cv. Tondano), soybean (cv. Lokon), rice (cv. Laut Tawar), and soybean (cv. Wilis) was grown from May 1989 to May 1991. Treatments consisted of combinations of rates and timing of KCl fertilizer application and the return or removal of crop stover. An additional treatment examined the replacement value of cattle manure for KCl fertilizer. The field experiment consisted of nine treatments in a randomized complete block design with four replications. Plots were 42 m2 with a 12-m2 harvest area. Calcitic lime was applied at the beginning of the experiment to reduce acid saturation to 25%. An additional 2 t ha-1 of calcitic lime was applied before crop 6 (soybean) to reduce the possibility that acidity would limit soybean growth. All lime and fertilizer applications were incorporated into the surface 15 cm. Plant spacing was 25 by 25 cm for the first five crops and 20 by 25 cm for the final crop. Legumes were thinned to 2 plants hill-1 at 30 days after planting (DAP). The first soybean crop was inoculated with rhizobium. Urea was side-dressed to the rice crops (Table 2) and 10 kg ha-1 of ZnSO4 and 15 kg ha-1 of borate were applied as supplements to crop 5 (rice). Each crop was hand-weeded at least twice and pesticides were applied several times per crop to control insects and rice blast disease (Pyricularia oryzae).
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Table 2 Treatment codes, fertilization rates, planting and harvesting dates for cowpea (Cp), rice (Rc), and soybean (Sy). An S in the code indicates that the treatment was split over several crops. Rainfall is shown for period between planting and harvesting of each crop#
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Crop 1 (cowpea) was harvested by collecting the entire pod and separating the seeds after air-drying. Pods were not included in the grain weight, but were used to calculate nutrient removal when pods were not returned to the plot. No grain was harvested for crop 2 (cowpea) because the entire biomass was harvested prematurely to allow for timely planting of the subsequent rice crop. Rice and soybean plants were cut near ground level, sun-dried, and hand-threshed to harvest grain. Returned stover was cut into 10-cm pieces and returned to the appropriate plots. Stover and grain subsamples were taken for chemical analyses and moisture content determination. Grain yields are reported at 140 g kg-1 moisture for all crops. Stover weights are reported on an oven-dry basis. Harvest fraction samples were dry-ashed at 550°C and nutrients were determined by inductively coupled plasma spectrometry.
Potassium and Mg uptake in grain and stover were calculated by multiplying the subsample cation concentration by the respective biomass. Because tissue cation concentrations are based on oven-dried samples, grain biomass was multiplied by 0.90 to adjust from 140 g kg-1 moisture to an average oven-dry weight of 40 g kg-1. Nutrient removal for the stover-returned treatments was calculated by summing the uptake in grain (and pod for the first cowpea crop) for all crops and the stover for crop 6 (soybean). Nutrient removal for the stover-removed treatments was considered equivalent to nutrient uptake, because all aboveground biomass was removed. Nutrient analyses for crop 3 (rice) were not available and were estimated by multiplying the average nutrient concentrations in crop 5 (rice) grain (2.4 g K kg-1 and 1 g Mg kg-1) and stover (12.5 g K kg-1 and 1 g Mg kg-1) with the grain and stover yields of crop 3 (rice).
Grain discoloration from predominantly Helminthosporium sp. (J. Klap and J. Castano, 1991, personal communication) was measured in rice (crop 3) at 90 DAP by randomly selecting 15 panicles per plot. Individual grains were visually inspected and considered discolored if >10% of the grain was darkened. The percentage of grains not discolored was related to extractable K determined from soil samples taken 35 d after grain sampling. These were the soil samples, of those available, that most closely represented the soil K status at panicle sampling for grain discoloration.
Critical Soil Potassium Levels
The critical soil K level is defined as the exchangeable soil K level above which no further grain yield increase is expected. The critical soil K for a crop was determined from the relationship between relative yield and exchangeable soil K for the respective crop. Soil samples were taken from the surface 15 cm for crop 1 at 0 months (before fertilization), for crop 3 at 5 months, for crop 4 at 9 months, for crop 5 at 16 months, and for crop 6 at 24 months (after harvest). Soils were sampled prior to fertilization to avoid the high variablity in measured exchangeable soil K, which our previous experience had indicated was the result of sampling immediately after fertilization on these low-K soils. Cations from the soil samples were extracted with 1 M NH4OAc. Extracts were then analyzed for K, Mg, and Ca by atomic absorption spectrophotometry. Relative yields for soybean (crops 4 and 6) and rice (crop 5) were based on the maximum treatment yield for the respective crop. Soil samples used to determine relative yield-exchangeable K relationships were taken prior to fertilization for crops 4 (soybean) and 5 (rice), and after harvest for crop 6 (soybean).
To more accurately reflect soil K during crop growth, measured soil K was corrected to include fertilizer K for crops 4 and 5, and K taken up in the grain and stover for crop 6. Exchangeable soil K values were corrected for crops 4 (soybean) and 5 (rice) by the equation
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and for crop 6 (soybean) by the equation
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where KSoil is exchangeable soil K after fertilization (cmolc kg-1), KTest is exchangeable soil K determined from the soil sample (cmolc kg-1), KFert is the amount of fertilizer applied to either crop 4 or 5 (kg ha-1),
in the surface 15 cm of soil per cmolc K kg-1,
, the soil bulk density in the surface 15 cm,
, the proportion of applied K recoverable in this soil by 1 M NH4OAc extraction (Dierolf, 1992), and KCrop is the amount of K taken up in grain and stover for crop 6.
Statistical Analyses
An analysis of variance was used to test treatment effects on crop yield and nutrient uptake for each harvest date, and on exchangeable K and Mg for each soil sampling date. An analysis of variance was conducted to test treatment and time effects on the change in soil organic carbon. In all cases, an F-protected LSD
was calculated when the analysis of variance indicated a significant variable effect (P < 0.05). Relative yield-exchangeable K relationships were estimated using several models, with the most suitable being a linear response and plateau model,
 | (3) |
where Y is the relative grain yield (%), ß1 is the plateau yield, ß2 is the change in relative yield with respect to extractable K when extractable K was less than the critical K level, KCritical is the critical soil K level (cmolc kg-1), and X is extractable soil K (cmolc kg-1) (Systat, 1992). A similar model was used to describe the relationship between grain discoloration and exchangeable K, except that Y was grains not discolored (%).
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Results and discussion
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Grain Yields
Potassium fertilization responses were affected by stover management (Table 3)
. When stover was returned, there were no grain yield responses to K fertilization for any of the six crops (with one exception in crop 5). However, when stover was removed, responses to K fertilization began to appear by crop 4 (soybean) at the lowest K rate (70-). Relative yield for this treatment continued to decrease for the remainder of the experiment. When 250 kg K ha-1 (250-) was applied as a single dose before planting the first crop, relative yield did not decrease until crop 5 (rice), and the decrease continued through crop 6 (soybean). In comparison, yields did not decline when the 250 kg K ha-1 was applied over several crops (250/S-). For any given K treatment, stover removal generally resulted in lower yields, starting with crop 4 (soybean) (Table 3).
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Table 3 Grain (140 g kg-1 moisture content) and oven-dried stover yields for cowpea (Cp), rice (Rc), and soybean (Sy)
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Critical Soil Potassium Levels
Critical soil K levels were 0.14 cmolc kg-1 for rice (crop 5) (Fig. 1a)
and 0.14 and 0.16 cmolc kg-1 for the two soybean crops (Fig. 1b). No critical soil K levels were established for cowpea because there was no grain yield response to soil K levels in the first crop and the second crop was harvested before maturity. Ritchey (1979) reported that critical K levels for food crops ranged from 0.13 to 0.20 cmolc kg-1 for several Ultisols and Oxisols in South America and Puerto Rico. A critical level of 0.13 cmolc K kg-1 was recommended for cereal crops in the Brazilian humid savannas (cerrados) (Vilela and Ritchey, 1985).
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Fig. 1 Relationship of exchangeable soil K in the surface 15 cm and relative grain yield for (a) rice, crop 5; and (b) soybean, crops 4 and 6
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Higher critical levels for rice (0.16 to 0.20 cmolc kg-1) than for soybean (0.10 to 0.18 cmolc kg-1) were reported in an Indonesian Ultisol (Gill, 1988). Gill also found that values of critical K levels for soybean increased with higher lime rates and critical levels for both upland rice and soybean crops were affected by the soil extraction method used. Based mostly on the work by Gill (1988), Wade et al. (1988) recommended critical K levels of 0.20 for upland rice and 0.16 for soybean in the Sitiung area. In contrast, Cox and Uribe (1992a, 1992b) reported higher critical K levels for soybean (0.19) than for rice and cowpea (0.10). The authors noted, however, that the coefficients of determination for the linear response and plateau models for rice
and cowpea
were low in comparison to soybean
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Rice panicles in crop 3 exhibited grain discoloration caused predominantly by Helminthosporium sp. Figure 2
shows the relationship between exchangeable soil K and the percent of grains not discolored that was measured at 90 DAP. The critical K level was 0.135 cmolc kg-1. Field observations indicated that the severity of discoloration increased up to harvest (109 DAP), with treatment 70- most adversely affected. These results confirm other reports of the positive effect of K in reducing grain discoloration by Helminthosporium sp. (Huber and Arny, 1985).
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Fig. 2 Relationship of exchangeable soil K in the surface 15 cm and percentage of grains rated as not discolored from fungal infection (predominantly Helminthosporium sp.) at 90 DAP for rice (crop 3). Soil samples were taken 35 d after grain discoloration was rated
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Stover Management Effects on Soil Potassium
Figures 3a and 3b
show the effect of stover management on soil K levels in the surface 15 cm. The soil K levels in Fig. 3a and 3b underestimated the soil K status at planting for each crop, since soil sampling was completed before basal fertilization and planting of the subsequent crop (indicated by arrows). For example, the soil K levels at planting for both the 250/S- and 250/S+ treatments would be about 0.08 cmolc K kg-1 greater (to account for the addition of 45 kg K ha-1) than those shown in Fig. 3a and 3b for crops 3, 4, and 5. Soil samples at 5, 9, and 24 months were taken within 1 to 2 wk after harvest. The short time interval between the return of stover and soil sampling probably did not allow sufficient time for all of the K to be released from the decomposing stover. This would result in an additional underestimation of soil K for the residue-returned treatments.
Returning the stover always resulted in the maintenance of soil K above the critical level. Thus, the lack of grain yield response to K fertilization when stover was returned (Table 3) was a direct result of the soil K for each treatment remaining above the critical K levels (Fig. 3b). (Note that the drop below the critical level in the last crop for the 70+ treatment is because stover from the final harvest was not returned to the soil at the termination of the experiment. Therefore, the quantity of K in the stover is not reflected in the soil analyses.)
Stover removal for the 70- treatment decreased soil K to below the critical level by the time crop 3 (rice) was planted (Fig. 3a). Although this did not have a significant effect on yield between treatments 70+ and 70- (Table 3), the 70- treatment had the highest rate of grain discoloration (data not shown). Even with a one-time application of 250 kg K ha-1 and stover removal, the 250- treatment could not maintain the soil K above the critical level beyond crop 4 (soybean). This is reflected in the lower yield for 250- as compared to 250+ for crop 5 (Table 3).
The split-application of 250 kg K ha-1 (250/S-) maintained soil K above the critical level even when stover was removed (Fig. 3a). (As mentioned earlier, soil samples for crops 1, 3, 4, and 5 were taken prior to fertilization. Thus, the exchangeable soil K values shown in Fig. 3a and 3b do not include the fertilizer that was applied to that crop. For example, for the 250/S- treatment, 45 kg K ha-1 was applied to crop 5 and increased the measured prefertilization soil K by approximately 0.08 cmolc kg-1.) In contrast, soil K decreased to below the critical level by crop 5 for the one-time application of 250 kg K ha-1 when stover was removed (250-). Thus, the yield decline that occurred in the one-time application of 250 kg K ha-1 (250-), but did not occur when the same amount of fertilizer was split over several treatments (250/S-), is related to the maintenance of soil K above the critical soil K level.
These results are similar to those reported by Cox and Uribe (1992b) on a Peruvian Ultisol recently cleared of an 18-yr-old secondary forest. Returning stover maintained soil K above the critical level of 0.10 cmolc K kg-1 for 12 crops of rice and cowpea. Removing stover required a maintenance rate of 40 kg K ha-1 to maintain the soil K above the critical level. Wade et al. (1988) recommended slightly higher rates of 60 kg K ha-1 for rice and 40 kg K ha-1 for soybean when stover was removed from Ultisols in Sitiung, Indonesia.
Removing the stover in the 600/S- treatment resulted in significantly lower yields for crop 4 as compared with the one-time application of 70 kg K ha-1 and stover return treatment (70+). The exchangeable soil K was not the limiting factor in this case (Fig. 3a), and indicates that stover removal may have detrimental effects on soybean grain yield that can not be corrected by K replenishment with fertilizer KCl.
Stover Management Effects on Soil Magnesium
Yields for the stover-removed treatments in crop 4 (soybean) were, in general, lower than for the stover-returned treatments (Table 3). Extractable soil K was above the critical K level for soybean except when stover was removed for the 70- treatment (Fig. 3a). A yield response to increased soil K may not explain all of the yield increase due to stover return, since after applying the maintenance rate of 45 kg K ha-1 to the 250/S- treatment, the available soil K should be higher than that for the 70+ treatment (Fig. 3a and 3b). However, grain yield for crop 4 was higher for the 70+ treatment than for the 250/S- treatment (Table 3).
A possible explanation for the relatively lower yields of the stover-removed treatments in crop 4 (soybean) may be the reduction in exchangeable soil Mg. Figure 4
shows the change in soil Mg for the 250/S treatments. Magnesium was not applied after the initial application for crop 1 to crop 5 (Table 2). The gradual decrease in soil Mg up to crop 5 was hastened by stover removal (Fig. 4). The soil Mg values for both treatments were near the critical limits of 0.24 cmolc Mg kg-1 for soybean and 0.21 cmolc Mg kg-1 for rice recommended by Wade et al. (1988). After Mg was reapplied to crops 5 (rice) and 6 (soybean), there were no significant differences between stover treatments for the 600/S+ and 600/S- treatments (Table 3). Although yield differences due to stover management for the 250/S+ and 250/S- treatments were still evident for crop 5, they were no longer significantly different for crop 6 (Table 3).
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Fig. 4 Change in exchangeable soil Mg in the surface 15 cm with time as affected by stover return (250/S+) or removal (250/S-) for the split-application of 250 kg K ha-1 over several crops. Vertical bars indicate one SE of the mean
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Cattle Manure as Substitute for Potassium Chloride Fertilizer
One ton of air-dried cattle manure contained about 9 to 10 kg K ha-1, in addition to the 150 kg K ha-1 that was applied as KCl fertilizer, and resulted in a total application of 215 kg K ha-1 (215/S-) for the experiment (Table 2). The amount and timing of K in the cattle manure treatment (215/S-) was most similar to the 250/S- treatment (Table 2). Grain yields in Table 3 were not significantly different between these two treatments. Exchangeable soil K for the two treatments was also very similar throughout the experiment (Fig. 3). These results indicate that 1 t air-dried cattle manure ha-1 could effectively replace about 18 kg KCl fertilizer ha-1.
Stover Management Effects on Soil Organic Carbon
We also considered the effect of stover management on organic carbon content, and some of the indirect effects of organic carbon content on soil fertility. There were no treatment differences after 24 months, although there was an overall decrease
in organic carbon between the beginning and the end of the experiment (Table 4)
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Cation Removal in Biomass
The effect of soil cation loss through crop biomass (grain and stover) removal is shown in Table 5
. Calcium loss through biomass removal is not discussed because of the relatively low percentage of crop uptake of Ca compared with that applied (Dierolf, 1992). Average K depletion through biomass removal was 40 kg K ha-1 when both stover and grain were removed and 18 kg K ha-1 when only grain was removed. The replenishment rate when stover was removed is similar to the maintenance rate of 45 kg K ha-1 that was applied to crops 3 to 6 in the 250/S+ and 250/S- treatments.
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Table 5 Uptake and removal of K and Mg in grain (G) (includes pod for cowpea) and stover (S). Nutrient analyses were not obtained for crop 3 (rice) and were estimated to calculate nutrient uptake (see text). The total uptake in biomass and total removal (in biomass not returned to the treatment) includes estimates from crop 3 (rice) and are shown as `six–crop estimate'. Stover for the sixth and final crop was not returned for any treatment and was considered removed  §
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Magnesium lost through biomass removal would have reduced Mg in the surface 15 cm by about 0.12 cmolc Mg kg-1 even when stover was returned, and by about 0.21 cmolc kg-1 when stover was removed. Considering that initial Mg levels were about 0.25 cmolc kg-1, it seems that Mg can be rapidly depleted through stover removal.
When stover was removed, the one-time application of 250 kg K ha-1 (250-) resulted in higher K removal in the biomass for crops 1 and 2 (actual plant uptake data were not available for crop 3), but the split-application (250/S-) resulted in higher K removal for crops 4, 5, and 6. This trend is similar to the relative soil K levels for the two treatments shown in Fig. 3. However, because soil K in both treatments remained above the critical level during the first three crops, there was no grain yield increase from the one-time application (Table 3). Thus, a partial explanation for the depletion of soil K below the critical level in the 250- treatment is the luxury uptake and removal of K in the biomass of early crops. Another example of luxury uptake is that the 600/S- treatment removed 84 kg K ha-1 more in the biomass than did the 250/S- treatment (Table 5), but there was no significant yield difference (Table 3) between the two treatments. Thus, there is probably no reason to maintain soil K much higher than the critical soil K level because the uptake of additional K not needed for yield increase is susceptible to removal from the soil if the biomass is not returned. The incidence of grain discoloration in rice also disappeared at about 0.13 cmolc K kg-1, which is near the critical soil K level for rice grain yields.
Recommendations for Potassium, Magnesium, and Stover Management
Our results suggest that stover should be returned to the field when possible. Returning crop stover after harvest can substantially reduce the need for K fertilizer inputs in a rice–soybean cropping system. When the stover is returned, a one-time application of 70 kg K ha-1 (70+) can sustain up to six sequential plantings of cowpea, soybean, and rice over a period of 24 months. When stover is removed, however, an additional 35 to 45 kg K ha-1 per crop is required to sustain soil K above the critical levels for rice and soybean. In systems where stover is removed because of its usefulness for other purposes, applying smaller rates of K to each crop rather than a large single amount to one crop can reduce luxury K uptake. Cattle manure can be used to replace some of the KCl fertilizer at a rate of about 1 ton ha-1 for every 18 kg ha-1 of KCl fertilizer (9.5 kg K ha-1).
Stover removal also hastened the depletion of soil Mg; about 6 kg Mg ha-1 is required to replace Mg removed in the grain and stover of each crop. These recommendations also apply to systems where stover is not removed from the field but may be gathered and not equally redistributed across the field.
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NOTES
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This work was supported by the Center for Soil Research and Agroclimate (CSAR), Bogor, Indonesia, and the Soil Management Collaborative Research Support Program (USAID).
Received for publication August 7, 1998.
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REFERENCES
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- von Uexküll, H.R. 1985. Improvement and maintenance of soil fertility in tropical upland farming systems. p. 233–250. In Potassium in agricultural systems of the humid tropics. Int. Potash Inst., Bern, Switzerland.
- Wade, M.K., D.W. Gill, H. Subagjo, M. Sudjadi, and P.A. Sanchez. 1988. Overcoming soil fertility constraints in a transmigration area of Indonesia. TropSoils Bulletin 88-01. Soil Manage. Collaborative Support Progr., North Carolina State Univ., Raleigh.
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ABSTRACT
INTRODUCTION
