Effect of long-term intensive rice cultivation on the available silica content of sawah soils: Java Island, Indonesia

Abstract

The dramatic increases in rice productivity and cultivation intensity through the implementation of green revolution (GR) technology using high yielding varieties (HYVs) of rice and chemical fertilizers were not long lasting in Indonesia. The stagnancy of rice productivity in recent years without any scientific reasons presents a challenge for agronomists and soil scientists in Indonesia. This study describes the effects of long-term intensive rice cultivation on the change in available silica (Si) in sawah soil. The term sawah refers to a leveled and bounded rice field with an inlet and an outlet for irrigation and drainage. Soil samples collected by Kawaguchi and Kyuma in 1970 and new samples taken in 2003 from the same sites or sites close to the 1970 sites were analyzed and compared. From 1970 to 2003, the average content of available Si decreased from 1,512 ± 634 kg SiO₂ ha⁻¹ to 1,230 ± 556 kg SiO₂ ha⁻¹ and from 6,676 ± 3,569 kg SiO₂ ha⁻¹ to 5,894 ± 3,372 kg SiO₂ ha⁻¹ in the 0–20 cm and 0–100 cm soil layers, respectively. Cultivation intensity differences between seedfarms planted with rice three times a year and non-seedfarms rotating rice and upland crops appeared to affect the changing rates of available Si within the study period. In the 0–20 cm soil layer, the average content of available Si decreased from 1,646 ± 581 kg SiO₂ ha⁻¹ to 1,283 ± 533 kg SiO₂ ha⁻¹ (−22%) and from 1,440 ± 645 kg SiO₂ ha⁻¹ to 1,202 ± 563 kg SiO₂ ha⁻¹ (−17%) in seedfarms and non-seedfarms, respectively. Differences in topographical position also influenced the decreasing rate of available Si in this study. Using similar management practices and cultivation intensity, upland sampling sites lost more Si compared with lowland sites. Planted rice under a rain fed system with no Si addition from rain water in an upland position may be a reason for the higher loss of Si, particularly in non-seedfarms. The Si supply from irrigation water might have contributed to the slowdown in the decreasing rate of available Si in Java sawah soils.

Key words:

• available silica
• cultivation intensity
• Java
• sawah
• topographical position

INTRODUCTION

Rice is the major caloric intake for a large portion of the earth's population. This food grain is produced in at least 95 countries around the globe. China is leading the world rice producing countries, followed by India and Indonesia. In 1999, these three countries contributed approximately 36%, 21% and 8% of the world's rice production, which was equal to 187.9, 128.3 and 52.4 metric tons, respectively (Coats 2003).

The implementation of new rice cultivation systems, known as green revolution (GR) technology, using high-yielding varieties (HYVs) of rice and chemical fertilizers in the mid-1960s increased rice productivity in all South-East Asian countries. In the case of Indonesia, rice production increased dramatically from the end of 1960s to the end of 1990s. During that period, rice productivity increased sharply from 1.8 to 4.5 Mg ha⁻¹ (approximately 250%). It was the highest among South-East Asian countries, including the Philippines (230%), India (223%), Bangladesh (165%) and Thailand (121%) (Food and Agriculture Organization 2002; Indonesia Ministry of Agriculture 2001; Otsuka 2000).

Although GR technology achieved great things, there were several critics of this technology. A number of studies in India found that the application of GR technology caused significant soil degradation and reduced the availability of fertile soil genetic diversity of crops (Singh 2000). Similar conclusions were also drawn by researchers from Bangladesh (Rahman 2003), China (Zhang et al. 2003) and Latin America (Redclift 1989). For Indonesia itself, Darmawan et al. (2006) reported that the intensive use of chemical fertilizers over three decades had caused acidification and accumulation of phosphorus in sawah soils in Java. The term sawah refers to a leveled and bounded rice field with an inlet and an outlet for irrigation and drainage (Wakatsuki et al. 1998). As the soil pH value is still in a suitable range to support rice growth (5.8 at average) (Darmawan et al. 2006; RDA 1999 cited in Lee et al. 2006), the stagnancy of rice productivity at approximately 4.5 Mg ha⁻¹ reported recently (Indonesia Ministry of Agriculture 2001; Otsuka 2000) might be the result of other problems.

Increasing rice cultivation intensity will also accelerate the flow rate of nutrients out of sawah through the harvesting processes and drainage, particularly silica (Si) as the dominant element in the rice husk. As a non-essential nutrient, Si was not considered by government and farmers to be important, indicated by no artificial addition of Si to date. The Si supplies for rice growth only come from natural sources, such as irrigation water, soil weathering and the decomposition of plant residues (Gascho 2001). As the amount of Si supplied varied with the parent material, the river basin geology and the plant species (Ma and Takahashi 2002), the demand for Si always exceeds that which can be supplied by those sources (Gascho 2001). The purpose of the present study was to determine the effects of long-term intensive rice cultivation on changes in available Si content in the soils, with respect to cultivation intensity and differences in topographical position.

MATERIAL AND METHODS

Description of study area and sampling sites

Soil samples taken by Kawaguchi and Kyuma in 1970 and new samples taken from the same sites or close to the original sites were analyzed and compared. From a total of 46 sampling sites in 1970, four sites (site numbers 2, 4, 5 and 40) were not sampled in 2003 because of land-use changes to non-agricultural purposes. Site numbers 15 and 30, which are not used as sawah, were also excluded from this study. The topographical position of the sites ranged from 11 to 825 m a.s.l. To examine the effect of cultivation intensity, sampling sites were grouped into seedfarms, where rice was planted throughout the whole year, and non-seedfarms, where rice and vegetables were planted in a rotation pattern. To study the impact of topographic position on changes in the pattern of available Si, the sampling sites were divided into upland (higher than 100 m a.s.l.) and lowland (lower than 100 m a.s.l.) sites.

Among the 40 sites compared in this study, 18 of them were located in seedfarms while the remaining 22 were in non-seedfarms (Darmawan et al. 2006). The lowest site recorded in seedfarm Bulakamba (site number 23) was located at 11 m a.s.l., while the highest was a non-seedfarm in Warung Kaweni (site number 16) at 825 m a.s.l. Based on topographical position, 25 sites were considered to be located in a lowland position, of which 12 were seedfarms and the other 13 were non-seedfarms. The remaining 15 sites were upland sites, six of them belonged to seedfarms and the others to non-seedfarms (Fig. 1). Site numbers 1–17 were located in the western part of Java (five sites are seedfarm), site numbers 18–29 in central Java (three sites are seedfarm), and the remaining site numbers from 30 to 46 in the eastern part of Java (ten sites are seedfarm) (Table 1). To ensure the reliability of samples taken in 1970, our data were compared with the original data from Kyoto University. The result showed that both samples were similar with less than 5% difference (data not shown).

Soil sampling and interview

Soil samples were collected from each horizon in a profile at the respective sites by using 100 cm³ core samplers to determine the bulk density of soil. Composite soil samples from each horizon were also collected for chemical analyses. To obtain the latest information about the changes in rice cultivation systems and productivity in seedfarms and non-seedfarms from 1970 to 2003, we interviewed the seedfarm staff and the farmers on the respective sites assisted by an interpreter.

Laboratory analyses and calculation

The available Si content in the soil was measured using acetate buffer proposed by Imazumi and Yoshida (1958). The air-dried soil was extracted with 100 mL acetate buffer pH 4.0 in a flask with a ratio of 1:10. The mixture was shaken occasionally for 5 h at 40°C. After filtration with dry filter paper, the concentration of Si in the filtrate was determined using the colorimetric molybdenum blue method.

Calculation

To examine the effects of intensive rice cultivation on the available Si content in the soils, samples taken in both 1970 and 2003 were analyzed and compared. The

Figure 1  Map of Java island showing the distribution of sampling sites in lowland and upland positions.

available Si content calculated in mg SiO₂ kg⁻¹ soil was estimated on a per hectare basis (kg SiO₂ ha⁻¹) using Eq. 1 and the observations of each profile were summed up to the top 100 cm layers to eliminate any errors associated with differences in the sampling depth. For an individual profile with n horizons, the available Si content on a volume basis was as follows:

(1)

Where S _d is the content of available Si (kg Si ha⁻¹) at a depth d, bi is the bulk density (kg m⁻³) of horizon i, Si is the available Si (mg Si kg⁻¹) in horizon i, di is the thickness of the horizon (cm). To convert the value of available Si from a weight to volume basis, the bulk density of samples taken in 2003 (Darmawan et al. 2006) were used in this calculation.

RESULTS AND DISCUSSION

From 1970 to 2003, rice cultivation intensity increased in all sites examined. Sampling sites located in upland positions were dominated by non-seedfarms and were mostly planted with rice and upland crops in some rotation pattern. In contrast, seedfarm sites were mostly located in the lowland and were planted with rice the whole year round (Table 1).

Available Si content varied greatly from site to site. In 1970, available Si content in the 0–20 cm soil layer ranged from 131 kg SiO₂ ha⁻¹ to 2,912 kg SiO₂ ha⁻¹. The lowest content was observed at site number 8 (non-seedfam at Pasir Gombong, West Java), while the highest was at site number 43 (seedfarm at Yosowilangun, East Java) (Table 2). In 2003, the lowest available Si content was still found at site number 8, but the highest content was observed at site number 41 (non-seedfarm Maron Kulon, East Java). Land management and soil type differences might be the main reason for this result. As a seedfarm, site number 43, which had been planted with rice three times a year, transported Si more than site 41. During the period 1970 to 2003, the content at site number 43 decreased to 373 kg SiO₂ ha⁻¹ (−13%), which was slightly higher than that at site number 41. The low available Si content at site number 8 may be correlated with the land-use system at this site. Darmawan et al. (2006) reported that the top soils at site numbers 7 and 8 were often removed and never put back because the soil was also used as the raw material to make bricks for house construction.

Soil type differences between the sites may also influence the available Si content. According to Epstein (2001), the content of available Si in the soil also correlates with soil type or weathering processes. Young soils such as Molisols and Vertisols have a higher available Si content compared with weathered old soils such as Inceptisols and Ultisols. As site number 8 is a Ultisol and site numbers 41 and 43 are Vertisols, the large difference in available Si content between these sites is understandable.

Land management and soil type differences clearly play an important role in the decreasing rate of available Si observed in this study. As shown in Table 1, the eastern part of Java is dominated by Vertisols, with a higher available Si content compared with the western part of Java, which is mostly Inceptisols. The average available Si content in the eastern part was 2,002 ± 558 kg SiO₂ ha⁻¹, which was much higher compared with the west, 1,040 ± 562 kg SiO₂ ha⁻¹, and

Table 1 Topographical position, elevation and changes in land-use pattern from 1970 to 2003 at each sampling site

Profile no.	Site name	Seedfarm/non-seedfarm	Elevation (m)	Land-use pattern		USDA Taxonomy	Note
				1970^†	2003
In-1	Kedung Halang, Bogor	Non-seedfarm	213	rice-upland crop	upland crop	Aeric Epiaquepts	B-NS
In-3	Bendungan Ciawi, Bogor	Seedfarm	529	rice-rice	rice-rice-upland crops	Aeric Epiaquepts	A-SF
In-6	Kebun Percobaan Singamerta, Ciruas	Seedfarm	26	rice-rice	rice-rice-rice	Typic Epiaquepts	A-SF
In-7	Petung Sentul, Kragilan Serang	Non-seedfarm	31	rice-upland crop	rice-rice-upland crops	Typic Halaquepts	B-NS
In-8	Pasir Gombong Lemahabang, Bekasi	Non-seedfarm	31	rice-upland crop	rice-rice-upland crops	Typic Kanhapludults	B-NS
In-9	Palawad, Karawang	Non-seedfarm	32	rice-rice	rice-rice-upland crops	Vertic Epiaquepts	B-NS
In-10	Balitpa Sukamandi, Subang	Seedfarm	31	rice-rice	rice-rice-rice	Aeric Endoaqualfs	A-SF
In-11	LPPP Pusakanegara, Subang	Seedfarm	22	rice-rice	rice-rice-rice	Vertic Epiaquepts	A-SF
In-12	Sudikampiran, Sliyeg Indramayu	Non-seedfarm	22	rice-rice	rice-rice-upland crops	Vertic Endoaquepts	B-NS
In-13	Sampora, Cilimus Kuningan	Non-seedfarm	452	rice-upland crop	rice-rice-upland crops	Typic Dystropepts	B-NS
In-14	Pamoyanan, Ketapang Bandung	Non-seedfarm	685	rice-rice	rice-rice-upland crops	Typic Endoaquepts	B-NS
In-16	Warungkaweni Cipageran, Cimahi	Non-seedfarm	825	rice-upland crop	upland crop	Mollic Fragiaquepts	B-NS
In-17	LPPP Ciheya, Ciranjang, Cianjur	Seedfarm	209	rice-rice	rice-rice-rice	Aeric Epiaquerts	A-SF
In-18	Medini, Undaan Kudus	Non-seedfarm	22	rice-upland	rice-rice-rice/upland crops	Vertic Endoaquepts	A-NS
In-19	Mayong Lor, Mayong Jepara	Non-seedfarm	25	rice-upland crop	rice-rice-upland crops	Aquic Eutropepts	B-NS
In-20	Katonsari, Demak	Non-seedfarm	17	rice-upland crop	rice-rice-upland crops	Typic Calciaquepts	A-NS
In-21	Kartoharjo, Buaran Pekalongan	Non-seedfarm	14	rice-upland crop	rice-rice-upland crops	Aeric Epiaquerts	A-NS
In-22	Sirandu, Pemalang	Non-seedfarm	25	rice-upland crop	rice-upland crop	Aeric Epiaquerts	A-NS
In-23	Seedfarm Bulakamba, Brebes	Seedfarm	11	rice-rice	rice-rice-upland crops	Typic Natraquerts	A-SF
In-24	Bojong, Purbolinggo	Non-seedfarm	45	rice-upland crop	rice-rice-upland crops	Typic Endoaquepts	B-NS
In-25	Lajer Ambal, Kebumen	Non-seedfarm	22	rice-upland crop	rice-rice-upland crops	Vertic Endoaquepts	A-NS
In-26	Seed farm Wonocatur, Bantul	Seedfarm	118	rice-rice	rice-rice-rice	Aeric Epiaquepts	A-SF
In-27	Humo Seed farm, Semangkak	Seedfarm	159	rice-rice	rice-upland crop	Aeric Epiaquepts	A-SF
In-28	Jumapolo, Karanganyar	Non-seedfarm	339	rice-upland crop	rice-rice-upland crops	Typic Dystrustepts	B-NS
In-29	Papahan, Tasikmadu Karanganyar	Non-seedfarm	182	rice-upland crop	rice-rice-rice/upland crops	Typic Epiaquerts	A-NS
In-31	LPPP Ngale, Paron Ngawi	Seedfarm	68	rice-rice	rice-rice-upland crops	Typic Calciaquerts	A-SF
In-32	BPMD Sukodadi, Lamongan	Seedfarm	26	rice-upland crop	rice-rice-upland crops	Typic Epiaquerts	A-SF
In-33	BPMD Brenggolo, Bojonegoro	Seedfarm	37	rice-upland crop	rice-rice-upland crops	Aeric Endoaquerts	B-SF
In-34	Kresek Wungu, Madiun	Non-seedfarm	277	rice-upland crop	rice-rice-upland crops	Aeric Epiaquepts	B-NS
In-35	Banjarsari, Dagangan Madiun	Non-seedfarm	214	rice-upland crop	rice-rice-rice	Typic Calciaquerts	B-NS
In-36	Patang, Nglames Madiun	Non-seedfarm	74	rice-rice	rice-rice-upland crops	Typic Epiaquerts	A-NS
In-37	Pelem, Paree Kediri	Non-seedfarm	113	rice-upland crop	rice-rice-upland crops	Typic Epiaquepts	B-NS
In-38	Seed farm Waung, Baron Nganjuk	Seedfarm	56	rice-upland crop	rice-rice-upland crops	Aeric Epiaquepts	A-SF
In-39	LPPP Mojosari, Mojokerto	Seedfarm	33	rice-upland crop	rice-rice-rice	Aeric Epiaquepts	A-SF
In-41	Maron Kulon, Maron Probolinggo	Non-seedfarm	78	rice-upland crop	rice-rice-rice/upland crops	Typic Epiaquepts	A-NS
In-42	Labruk Kidul, Lumajang	Seedfarm	89	rice-rice	rice-rice-rice/upland crops	Typic Epiaquerts	A-SF
In-43	BPMD Yasowilangun, Lumajang	Seedfarm	30	rice-upland crop	rice-rice-rice	Aeric Endoaquerts	A-SF
In-44	Balai benih Srimurni, Arjasa Jember	Seedfarm	181	rice-upland crop	rice-rice-rice	Fluvaquentic Epiaquep	A-SF
In-45	LPPP Genteng, Banyuwangi	Seedfarm	159	rice-rice	rice-rice-rice/upland crops	Aeric Epiaquepts	A-SF
In-46	Seed farm Sukorejo, Banyuwangi	Seedfarm	93	rice-upland crop	rice-rice-rice	Typic Calciaquerts	A-SF
^†Data from Kawaguchi and Kyuma (1977). A, original sites; B, close to original sites; SF, seedfarms; NS, non-seedfarms.

Table 2 Changes in available silica (kg SiO2 ha−1) during the period between 1970 and 2003 in 0–20 cm and 0–100 cm soil layers and the Si concentration in irrigation water (mg SiO2 L−1) at each sampling site

Profile no./site name	Location	Available Silica						SiO2 in irrigation water^†
		0–20 cm			0–100 cm
		1970	2003	% change	1970	2003	% change
1. Kedunghalang Bogor	WJ	964	753	−22	3721	3267	−12	7.96
3. Bendungan Ciawi	WJ	1570	1103	−30	6077	5292	−13	12.77
6. Singamerta Ciruas	WJ	505	397	−21	3359	2969	−12	5.64
7. Petung Sentul	WJ	195	141	−27	1099	1024	−7	3.99
8. Pasir Gombong	WJ	131	110	−16	657	568	−14	2.34
9. Palawad Karawang	WJ	860	843	−2	3860	3411	−12	7.74
10. Balitpa Sukamandi	WJ	1101	801	−27	3991	3363	−16	10.31
11. LPPP Pusakanegara	WJ	1214	792	−35	3831	3146	−18	11.54
12. Sudikampiran Sliyeg	WJ	763	632	−17	4040	3688	−9	14.55
13. Sampora Cilimus	WJ	1646	1424	−13	8299	7893	−5	11.14
14. Pamoyanan Ketapang	WJ	964	657	−32	3872	2500	−35	14.40
16. Warungkaweni Cipageran	WJ	1864	1590	−15	6181	5864	−5	18.30
17. LPPP Ciheya	WJ	1738	1668	−4	12498	11669	−7	19.80
Mean WJ		1040	839	−20	4730	4204	−13	10.81
SD		562	497	10	3069	2953	8	5.25
18. Medini Undaan, Kudus	CJ	1771	1496	−16	4248	3571	−16	11.00
19. Mayong Lor Jepara	CJ	1289	1090	−15	4010	3464	−14	8.34
20. Katonsari Demak	CJ	1757	1413	−20	6037	5172	−14	8.61
21. Kartoharjo Buaran	CJ	1538	1444	−6	5187	4814	−7	12.86
22. Sirandu Pemalang	CJ	1766	1580	−11	8446	7543	−11	9.53
23. Bulakamba Brebes	CJ	1211	922	−24	3165	2735	−14	8.64
24. Bojong Purbolinggo	CJ	1644	1447	−12	7038	6232	−11	12.98
25. Lajer Ambal, Kebumen	CJ	1239	1068	−14	6299	5884	−7	10.60
26. Wonocatur Bantul	CJ	828	575	−31	3824	3125	−18	15.56
27. Semangkak Klaten	CJ	1405	806	−43	5576	4090	−27	19.57
28. Jumapolo Karanganyar	CJ	1622	1359	−16	6266	5540	−12	23.21
29. Tasikmadu Karanganyar	CJ	1211	903	−25	4646	4036	−13	17.96
Mean CJ		1440	1175	−19	5395	4684	−14	13.24
SD		293	324	10	1517	1437	5	4.86
31. LPPP Ngale	EJ	2572	1869	−27	13294	11425	−14	20.99
32. BPMD Sukodadi	EJ	2481	1931	−22	8341	6935	−17	19.54
33. BPMD Brenggolo	EJ	1832	1371	−25	3919	3198	−18	14.09
34. Krese Wungu	EJ	1746	1322	−24	4853	4130	−15	23.21
35. Banjarsari Dagangan	EJ	2310	1980	−14	9464	8208	−13	23.00
36. Patang, Nglames	EJ	2156	1595	−26	7142	6363	−11	17.18
37. Pelem Paree	EJ	1327	1047	−21	7570	6385	−16	14.91
38. Waung Baron Nganjuk	EJ	2260	1907	−16	13114	12159	−7	22.41
39. LPPP Mojosari	EJ	1383	1170	−15	7387	6503	−12	13.41
41. Maron Kulon	EJ	2904	2568	−12	12723	11871	−7	17.18
42. Labruk Kidul Lumajang	EJ	1861	1688	−9	10136	9815	−3	15.52
43. BPMD Yosowilangun	EJ	2912	2539	−13	17575	16434	−6	18.19
44. Srimurni Arjasa	EJ	1497	1138	−24	9161	8186	−11	10.34
45. LPPP Genteng	EJ	1495	1094	−27	9485	8294	−13	22.41
46. Sukorejo Bangorejo	EJ	1289	977	−24	6659	5010	−25	8.24
Mean EJ		2002	1613	−20	9388	8328	−13	17.37
SD		558	516	6	3579	3512	5	4.66
Mean Java		1512	1230	−18	6676	5894	−12	14.00
SD		634	556		3569	3372		5.56
t-test			**			*
^†Sample taken in 2003. CJ, Central Java; EJ, East Java; WJ, West Java; SD, standard deviation.
*P = 0.05;
**P = 0.001.

the central part of Java was between these values (1,440 ± 293 kg SiO₂ ha⁻¹) (Table 2).

Although from a total of 15 sites located in eastern Java, 10 of the sites are in seedfarms, where rice was the only crop, the intensive mining of Si by rice did not differ greatly from the central and western part of Java. Table 2 shows that, during the period 1970–2003, the available Si in the whole of Java decreased approximately 20%. The Si content in irrigation water, 17.37 ± 4.66 mg SiO₂ L⁻¹, 13.24 ± 4.86 mg SiO₂ L⁻¹ and 10.81 ± 5.25 mg SiO₂ L⁻¹ in east, central and western Java, respectively, may have contributed to slowing down the decreasing rate of available Si in Java (Table 2).

We estimate that, from 1970 to 2003, the average available Si content significantly decreased from 1,512 ± 634 kg SiO₂ ha⁻¹ to 1,230 ± 556 kg SiO₂ ha⁻¹ (−18%) and from 6,676 ± 3,569 kg SiO₂ ha⁻¹ to 5,894 ± 3,372 kg SiO₂ ha⁻¹ (−12%) in the 0–20 cm and 0–100 cm soil layers, respectively (Table 2). The statistical analyses from data presented in Table 2 also indicated that the decrease in available Si was mostly found in the plow layer of sawah soils. The decreasing rates of available Si were significant at P = 0.01 and P = 0.05 in the 0–20 cm and 0–100 cm soil layers, respectively.

The harvesting management difference might contribute to the trend changes in available Si between seedfarms and non-seedfarms. Darmawan et al. (2006) reported that because of high cultivation intensity, there was no time for plant residues to decompose at seedfarm sites. To avoid the accumulation of organic matter (plant residues) in the soil surface, all plant residues in seedfarms were burned several days after harvest. This type of management may affect the lost of Si in ash form through drainage water. In contrast, in non-seedfarms, where the farmers just bring out the rice grain and let the rest decompose at the site or use the plant residue as organic mulch for the upland crop (Lansing et al. 2001), the Si was kept in the plant residues to be mixed with the soil and to prevent it being washed by drainage water.

Effects of rice cultivation intensity on the decreasing availability of silica

The effects of rice cultivation intensity in seedfarms and non-seedfarms on the decreasing rate of available Si during the period 1970 to 2003 in Java sawah soils is presented in Table 3. Available Si in seedfarms decreased at a higher statistical level (P < 0.01) than in non-seedfarms (P < 0.05). It is clear that the seedfarm sites lost more available Si compared with non-seedfarms. In the 0–20 cm soil layer, the average content of available Si in seedfarms decreased from 1,646 ± 581 kg SiO₂ ha⁻¹ to 1,283 ± 533 kg SiO₂ ha⁻¹ (−22%), while in non-seedfarms it decreased from 1,440 ± 645 kg SiO₂ ha⁻¹ to 1,202 ± 563 kg SiO₂ ha⁻¹ (−17%) (Table 3). Within the 0–100 cm soil layer, the change rates of available Si in both seedfarms and non-seedfarms were similar. In the 0–100 cm soil layer the average available Si in seedfarms decreased from 7,853 ± 4,187 kg SiO₂ ha⁻¹ to 6,906 ± 4,024 kg SiO₂ ha⁻¹ (−14%), while in non-seedfarms it decreased from 5,710 ± 2,700 kg SiO₂ ha⁻¹ to 5,063 ± 2,528 kg SiO₂ ha⁻¹ (−12%) (Table 3).

Although the available Si content in Java sawah soils is the highest among the South-East Asian countries (Kawaguchi and Kyuma 1977), intensive rice cultivation has mined Si and exported it through the harvesting processes. As a result of transporting Si out of the field, the seedfarm sites, which cultivated rice at a higher intensity, decreased available Si faster compared with non-seedfarms (Table 3). Ma and Takahashi (2002) stated that the rice husk accounts for approximately 20% of the weight of the rice grain and up to 20% of the husk consists of SiO₂. Assuming that the rice productivity in seedfarms and non-seedfarms was similar (approximately 5.5 Mg husked rice per hectare per cropping season), seedfarm sites that planted rice

Table 3 Effects of cultivation intensity at seedfarm and non-seedfarm sites on the changing rate of available silica (kg SiO2 ha−1) during the period 1970–2003 in Java sawah soil, Indonesia

	Seedfarm				Non-seedfarm
	0–20 cm		0–100 cm		0–20 cm		0–100 cm
	1970	2003	1970	2003	1970	2003	1970	2003
n	18	18	18	18	22	22	22	22
Mean ± SD	1646 ± 581	1283 ± 533	7853 ± 4187	6906 ± 4024	1440 ± 645	1202 ± 563	5710 ± 2700	5063 ± 2528
Change mean		−363		−947		−238		−647
% change		−22		−14		−17		−12
t-test		**		*		*		*
*P = 0.05;
**P = 0.001. SD, standard deviation.

three times a year lost Si in the form of SiO₂ at a rate of approximately 660 kg ha⁻¹ year⁻¹; which is much higher than the rate in non-seedfarms. Over the study period, seedfarms and non-seedfarms had transported approximately 21,780 kg SiO₂ and 14,520 kg SiO₂, respectively, out of sawah through the harvesting process. These values were much higher than the decreasing rate of available Si in soils. Table 3 showed that, during the period 1970–2003, the available Si content in the 0–20 cm soil layer decreased by 363 kg SiO₂ ha⁻¹ and 238 kg SiO₂ ha⁻¹ in seedfarms and non-seedfarms, respectively. The contribution of other natural Si resources, such as irrigation water, appears to play an important role in maintaining the available Si content in the soil. Kawaguchi and Kyuma (1977) found the average Si content in river waters, which are the dominant sources for irrigation in Java, to be 29.82 mg SiO₂ L⁻¹. Although the average Si content measured in the irrigation water was much lower than that recorded in river water (14.00 mg SiO₂ L⁻¹), the Si input from this resource could possibly retain the decreasing rate of available Si in sawah soil (Table 4).

Effects of topographical position on the changing trend in available Si

Table 5 shows the effects of topographical position difference on the changing rate of available Si in Java sawah soil. Within similar management systems, the topographical position difference among the sampling sites may have also contributed to the different changing rates of available Si in the soils. In seedfarms, although both upland and lowland positions decreased the available Si content at the same statistical level, upland sites lost slightly more Si compared with

Table 4 Average silica content in irrigation and river water from Java, Indonesia and other Asian countries

Water resources and location	SiO2 content (mg SiO2 L^−1)
Irrigation water in Java, Indonesia^†	14.00
River water in Java, Indonesia^‡	29.82
River water in Thailand^§	17.19
River water in West Malaysia^§	13.01
River water in Sri Lanka^§	13.07
River water in Japan^§	19.00
Irrigation water in Japan^¶	10.20
^†Water sample taken at 2003.
^‡Simple average of unsystematic measurement (Kawaguchi and Kyuma 1977).
^§Annual average (Kawaguchi and Kyuma 1977).
^¶Average content in 1996 (Kumagai et al. 2002).

lowland sites. Within the 0–20 cm soil layer, available Si decreased from 1,320 ± 349 kg SiO₂ ha⁻¹ to 1,005 ± 388 kg SiO₂ ha⁻¹ (−26%) upland; while in the lowland Si decreased from 1,806 ± 615 kg SiO₂ ha⁻¹ to 1,421 ± 557 kg SiO₂ ha⁻¹ (−21%) (Table 5). The effect of different topographical positions on the declining rate of available Si was stronger in non-seedfarms. In the upland position, available Si content significantly decreased at a higher statistical level compared with lowland sites. Non-seedfarms decreased available Si from 1,517 ± 443 kg SiO₂ ha⁻¹ to 1,226 ± 426 kg SiO₂ ha⁻¹ (−20%) in upland sites and from 1,386 ± 769 kg SiO₂ ha⁻¹ to 1,187 ± 658 kg SiO₂ ha⁻¹ (−14%) in lowland positions (Table 5). As limited irrigation water was a common problem in non-seedfarms (Syarifuddin 1982), the rain fed system at upland sites, which received no Si from the rain, decreased the available Si faster than irrigated sawah in lowland positions.

Table 5 Effects of topographical position on the changing pattern of available silica (kg SiO2 ha−1) during the period 1970–2003 in sawah soil in Java, Indonesia

	Upland (0–20 cm)		Lowland (0–20 cm)		Upland (0–100 cm)		Lowland (0–100 cm)
	1970	2003	1970	2003	1970	2003	1970	2003
Seedfarm
n	6	6	12	12	6	6	12	12
Mean ± SD	1320 ± 349	1005 ± 388	1806 ± 615	1421 ± 557	7770 ± 3176	6776 ± 3193	7896±4744	6973±4515
Change mean		−315		−385		−994		−923
% change	−12	−26		−21		−13		−12
t-test		**		**		*		*
Non-seedfarm
n	9	9	13	13	9	9	13	13
Mean ± SD	1517 ± 443	1226 ± 426	1386 ± 769	1187 ± 658	6096 ± 2018	5314 ± 1988	5445 ± 3139	4892 ± 2912
Change mean		−291		−199		−782		−553
% change	−11	−20		−14		−12
t-test		**		*		*		*
*P = 0.05;
**P = 0.001.

Although the changing rate of available Si in the 0–100 cm soil layer also decreased within the period of study, there were no significant differences found in the changing rate between upland and lowland positions, both in seedfarms and non-seedfarms. During the 33 year period of intensive rice cultivation, available Si in the 0–100 cm soil layer in seedfarms decreased by −13% and −12% for upland and lowland positions, respectively. Similar rates were also observed in non-seedfarms, where the decreasing rate was −12% and −11% for upland and lowland positions, respectively. Intensive rice cultivation with frequent puddling for the long-tem period in Java might have formed the massive clay horizon under the plow layer, which may have reduced the movement of water and nutrients, including Si, downward (Huke 1982).

Conclusion

Long-term intensive rice cultivation in Indonesia, particularly in Java, without artificial Si addition has mined Si from soils, and transported it away from the field, mainly through the harvesting process. This study showed that the decreasing rate of available Si was affected by the cultivation intensity of rice and topographical position. Seedfarm sites planted rice with higher intensity and lost more Si compared with non-seedfarm sites. The decreasing rate of available Si was much lower than the amount of Si lost through harvesting, because of the contribution of Si from the irrigation water. Using similar land management practices and cultivation intensity, sampling sites located in upland positions decreased in available Si more than lowland sites. Cultivated rice under a rain-fed system in some upland positions with no Si addition from rainwater could be the reason for this phenomenon. The decrease in available Si might be affected by rice production in Java and Indonesia as a whole. To increase rice production in the future, replenishment of Si through additional input is needed. Rice husk ash and coal fly ash are the two materials that have been applied as Si resources in some countries. As the availability of these two materials is abundant in Indonesia, their application could be one of the ways out of the Si problem in this country. As we lack information about Si status in Indonesian sawah soils and those materials itself, a detailed study to determine the doses for suitable locations is needed.

ACKNOWLEDGMENTS

The authors would like to express their deep gratitude to the Ministry of Education, Science, Sport and Culture of Japan for financial assistance for this study. The deepest appreciation goes to Dr Fahmuddin Agus from the Center of Soils and Agroclimate Research and Development (CSARD) Bogor, Indonesia, for providing the necessary support during soil sampling in 2003.