Crop, Livestock and Environment Division, Japan International Research Center for Agricultural Sciences (Tsukuba, Ibaraki , Japan) 2

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1 JARQ (3), (07) Soil silion nd ron under pddy in Mekong Delt Effets of the Continuous Applition of Rie Strw Compost nd Chemil Fertilizer on Soil Cron nd Aville Silion under Doule Rie Cropping System in the Mekong Delt, Vietnm Tkeshi WATANABE *, Hong Mn LUU nd Kzuyuki INUBUSHI * Crop, Livestok nd Environment Division, Jpn Interntionl Reserh Center for Agriulturl Sienes (Tsuku, Irki , Jpn) Cuu Long Delt Rie Reserh Institute (Thoili distrit, Cn Tho ity, Vietnm) 3 Grdute Shool of Hortiulture, Chi University (Mtsudo, Chi 7-8, Jpn) Astrt We onduted series of field experiments to study the effets of rie strw ompost (RSC) nd hemil fertilizer pplition on soil ron nd silion under doule rie ropping system for rops suessive in the Mekong Delt, Vietnm. The results showed tht the ontinuous pplition of RSC (6 Mg h - s fresh weight for eh rop) used higher yield nd more ville silion thn tretment without RSC. Aville silion in the surfe (0 - m from the ground surfe) soil ws 36.3 to 38.7 mg Si kg - in tretment with RSC, while it ws. to 30.0 mg Si kg - for tretment without RSC. Regrdless the mount of hemil fertilizer pplied, the rie yields for tretment without RSC were lower thn those for tretment with RSC nd entiled less hemil fertilizer. Moreover, RSC pplition used in omintion with moderte doses of hemil fertilizer inresed totl ron in the soil t rte of 36 to kg C h - yer -. Our results suggested tht Si vilility in pddy soil must e mintined or inresed in order to mintin rie yield, for whih the pplition of rie strw ompost offers n option. Our results lso showed tht the pplition of RSC inresed totl-c in the soil, suggesting tht pddy fields n e used effetively for ron sequestrtion. Disipline: Soils, fertilizers nd plnt nutrition Additionl key words: ron sequestrtion, soil fertility, sustinle lnd use Introdution The Mekong Delt is the lrgest rie-produing region in Vietnm, ontriuting more thn 0 million tonnes of rie nnully (Generl Sttistil Offie, Vietnm 0). The soil of the Mekong Delt ontins reltively high mount of orgni mtter in tropil Asi (Kyum 98, Ntionl Institute for Soils nd Fertilizers & Deprtment of Siene, Tehnology nd Produt Qulity 00). Rie strw n e used for mny purposes suh s roughge nd litter for niml husndry nd hortiulture. However, only prt of the rie strw is now eing used for suh purposes in the Mekong Delt. At present, mny frmers in the Mekong Delt simply urn rie strw in the field or inorporte it into the field fter hrvest, while some rie strw is shipped for mushroom ultivtion (Trn et l. 0). Annul rie strw prodution in the Mekong Delt is estimted to e nerly 0 million tonnes sed on hrvest index of out 0. for modern high yielding rie vrieties (Generl Sttistil Offie, Vietnm 0). This resoure is huge, even if some rie strw must e returned to pddy fields for sustinle rie prodution. When rie strw eomes prtil soure for io-fuel prodution, the Mekong Delt ould supply huge mounts of this iomss resoure for industry in the future. Thus, the est usge of rie strw should e explored. The ontinuous urning or removing of rie strw from the field my redue the ontent of soil orgni mtter. Moreover, intensified lnd use omined with redued supply of nutrient-lden sediments used y the improved ontrol of floodwter from the Mekong River my exerte the derese in soil fertility. Hene, there is need to develop mngement prties tht mintin *Corresponding uthor: e-mil wttke@ffr.go.jp Reeived Jnury 06; epted 0 Otoer

2 T. Wtne et. l or enhne soil fertility for longer term. For this reson, we hve studied the effets of the ontinuous pplition of rie strw ompost (RSC) on rie yield in lluvil soil in Vietnm s Mekong Delt (Wtne et l. 009, 03), nd our results showed tht RSC pplition with redued rtes of hemil fertilizer pplition n mintin rie produtivity for longer term thn under onventionl fertiliztion prties. Furthermore, our results suggested tht the ontinuous removl of rie strw my redue silion (Si) vilility in the soil, nd tht the defiieny of Si ppers fster thn tht of other nutrients (N, P, K, C, Mg, Fe, Zn, Mn, Cu) in the field, therey deresing rie produtivity. The ojetive of this study ws to evlute the effets of the ontinuous pplition of RSC on totl ron nd ville Si in the soil under the doule rie ropping system in the Mekong Delt, Vietnm. Mterils nd methods. Experimentl design This experiment ws onduted in n experimentl field t the Cuu Long Delt Rie Reserh Institute (CLRRI), Thoili distrit, Cn Tho ity, Vietnm ( 08 N, 3 E). The soil of the experimentl filed is Typi Humquept (U.S. Deprtment of Agriulture nd Ntionl Resoures Conservtion Servie 0). Tle lists ertin soil properties for omprison with other Asin pddy soils (Wtne et l. 009, 03). Long-term studies on the effets of rie strw ompost pplition were initited in the wet seson of 000. Rie rop hs trditionlly een ultivted twie yer in the wet nd dry sesons. The wet seson rop ws plnted in April or My, followed y the dry seson rop in Novemer or Deemer. This experiment ws onduted with the seven tretments desried elow rrnged in rndomized lok design with three replitions from the 009/0 dry seson to the 0/03 dry seson, s shown in Tle. Through the experimentl period, the vriety OM900 ws diretly seeded t the rte of 0 kg h - y using row seeder. Eh plot overed n re of 30 m ( 6 m). The F0C tretment ws sed on onventionl frming prties in the Mekong Delt region. RSC prepred solely from rie strw hrvested from the experimentl field nd phosphorus (P) fertilizer (single superphosphte) were pplied on the surfe of soil, nd then mixed into the soil down to depth of out m y using spdes efore sowing. N fertilizer (ure) ws split into three pplitions, with equivlent thirds rodsted t, 0 nd 30 dys fter sowing (DAS), respetively. The potssium (K) fertilizer (potssium hloride) ws split into two pplitions: hlf of whih ws rodsted t DAS nd the reminder t 30 DAS. F0C, F0C+, FC+, F60C+, nd F0C tretments were initited in the wet seson 000. Before the 009/0 dry seson, the FC nd F60C tretment plots were fertilized with RSC (6 Mg h - ) + NPK (6:6:6 for the wet seson, 0:6:6 for the dry seson N:P :K O kg h - ) nd RSC (6 Mg h - ) + NPK (6:: for the wet seson, 80:: for the dry seson N:P :K O kg h - ), respetively. At hrvest, the rie strw ws ut t out m ove ground level in ll plots, removed from the field, heped up nd then treted with wter. Every three to five dys, the hep ws tuned over nd wtered. The hep eme RSC fter two months. The verge dry mtter ontent of RSC ws 97 g kg - (nd rnged from 70 to, the sme hereinfter). The verge C, N, P, K nd Si onentrtions in dry mtter were 3 g kg - (30 to 33), 9.8 g kg - (7. to 3.0),. g kg - (.3 to.6),. g kg - (.3 to.9) nd 97.0 g kg - (96.9 to 97.0), respetively. Hene, 6 Mg h - (wet weight sis) of RSC Tle Properties of surfe soil (0 - m) of the experimentl field nd pddy soils in tropil Asi Soil of the experimentl field.3 Pddy soils in the tropil Asi 6. ph (H O) 0 Bulk density no-dt Totl-C (g kg - ) 3.. Totl-N (g kg - ) Aville-P (mg kg - ) 38 : Mesured in August 000 exept for ulk density. Wtne et l. 009, 03 : Kyum, 98 (n=) 3 : Mesured in Mrh 007. : Mesured y the Truog method. : Mesured y the Bry No. method. 3 JARQ (3) 07

3 Soil silion nd ron under pddy in Mekong Delt per seson ws equivlent to 37 kg h - s C, 3. kg h - s N, 6.6 kg h - s P (.9 kg h - s P),. s K O (7.6 kg h - s K) nd 0 kg h - s Si, respetively.. Mesurement of grin yield, rie strw prodution, totl ron nd ville Si in the soil We determined the grin yields nd rie strw prodution t hrvest for four rops from the 0 wet seson to the 03/0 dry seson. Grin yield ws sed on hrvested re of. m in eh replition. The rie strw ws smpled from four smpling res (eh m) within eh plot (n = 3). Aville Si ws determined for soil olleted t 0 - m nd - 0 m from the ground surfe fter hrvest of the 0/03 dry seson rop. The soil smples were ir dried, pssed through sieve with -mm mesh, then inuted under flooded ondition t C for one week. Wter-solule Si ws determined using olorimetri method (Tkhshi 986). Surfe soil (0 - m) smples from the experimentl plots were periodilly olleted following initition of the experiment in 000. Totl ron (totl-c) ws determined y NC nlyzer (Sumigrph NC-0F; Sumik Chemil Anlysis Servie, Osk, Jpn). As the tretment ws hnged to FC nd F60C from the 009/0 dry seson, oth tretments were exluded from the nlysis of totl-c. Sttistil nlysis of the dt ws onduted using softwre (JMP 9.0., SAS Institute In.). Differenes in rie yields nd ville Si in the soil mong the tretments were ssessed with Tukey s test. Signifine of the hnge in totl ron in the surfe soil (0 - m) ws ssessed y the t-test regrding whether the slopes of regression lines for totl ron onentrtions during the yers sine the strt of the experiment differed signifintly from zero. Results The rie yields for the FC+ nd F60C+ tretments were signifintly higher thn those for the other tretments in the four ropping sesons, exept for F60C in the 0/03 dry seson, s shown in Fig.. The verge yields for the FC+ tretment were 7.63, 7.36,.60 nd.6 Mg h - for the 0/0 dry seson, 0/03 dry seson, 0 wet seson, nd 03 wet seson, respetively. The verge yields for the F60C+ tretment were 7.8, 7.8,.7 nd.76 Mg h - for the 0/0 dry seson, 0/03 dry seson, 0 wet seson, nd 03 wet seson, respetively. The yields of the four rops under F0C+ tretment were signifintly higher thn under F0C tretment. The verge yields for the F0C+ tretment were 6.7,.83, 3.63 nd 3.6 Mg h - for the 0/0 dry seson, 0/03 dry seson, 0 wet seson, nd 03 wet seson, respetively. The verge yields for the F0C- tretment were.36,.,.36 nd.3 Mg h - for the 0/0 dry seson, 0/03 dry seson, 0 wet seson, nd 03 wet seson, respetively. In ddition, the yields for F0C+ were not signifintly different from those for FC, F60C nd F0C for the two wet sesons. The verge ville Si onentrtions in the Tle Rie strw ompost nd hemil fertilizer pplition rtes (kg h - ) sine the wet seson in 000 Seson F0C- F0C+ FC- FC+ F60C- F60C+ F0C- Rie strw ompost (RSC) Wet Dry Wet N Dry Chemil fertilizer P Wet Dry Wet K O Dry : Chemil fertilizer pplition rte for eh rop. N, P nd K O were pplied s ure, single superphosphte, nd potssium hloride, respetively. RSC nd single superphosphte were pplied efore sowing. Ure ws split into three pplitions nd equivlent thirds rodsted t, 0 nd 30 dys fter sowing (DAS). Potssium hloride ws split into two pplitions nd hlf ws rodsted nd 30 DAS. : Before the 009/0 dry seson, the FC nd F60C tretment plots were fertilized with RSC (6 Mg h - ) + NPK (6:6:6 for the wet seson, 0:6:6 for the dry seson N:P :K O kg h - ) nd RSC (6 Mg h - ) + NPK (6:: for the wet seson, 80:: for the dry seson N:P :K O kg h - ), respetively. 3

4 T. Wtne et. l surfe (0 - m) lyer of the plots with RSC (37.8,. nd 38. mg Si kg - for F0C+, FCL nd F60C+, respetively) were higher thn those without RSC (., 8., 3.3 nd 8. mg Si kg - for F0C-, FC-, F60C- nd F0C-, respetively), s shown in Fig.. The lowest nd seond lowest onentrtions were oserved in the F0C nd F0C tretments, respetively, where RSC hd not een pplied sine the wet seson in 000. No signifint differene ws found in ville Si onentrtions (.0 to 8. mg Si kg - in dry soil on verge) mong the tretments for su-surfe ( - 0 m) soil smples. Fig. 3 shows hnges in totl-c in the surfe (0 - m) soil. Bsed on the slopes of regression lines, totl-c in the surfe lyer ws found to e signifintly inresed in the FC+ nd F60C+ tretments (0. g kg - yer - nd 0.37 g kg - yer -, respetively), ut signifintly deresed in the F0C tretment (0.7 g kg - yer - ). No nnul signifint hnge in totl-c ws deteted in the F0C+ nd F0C tretments. Disussion Regrdless the mount of hemil fertilizer pplition, the yield dt indited tht s ompred to onventionl fertiliztion prties, the pplition of RSC long with redued mount of hemil fertilizer mintined higher rie yields (Fig. ). This result is in greement with tht reported erlier (Wtne et l. 03). Our previous report (Wtne et l. 03) showed tht the Si onentrtion of rie strw in the F0C tretment ws signifintly lower thn tht in the F0C+, FC+ nd F60C+ tretments; nd tht the Si onentrtions of rie strw in ll the tretments verged.9 g kg -, lower thn the proposed ritil Si onentrtion of 0 g kg - (Doermnn nd Firhurst 000, Bros-Filho et l. 00) or the typil onentrtions in rie strw olleted in Jpn (Fujii 00). Although there is room for disussion regrding whether rie Si onentrtion of less thn 0 g kg - ws relly ritil, rie plnts in the experimentl field might suffer from Si defiieny. Our previous report (Wtne et l. 03) suggested tht the defiieny of Si ppers Rie yields (Mg h - ) /0 Dry seson e d Rie yields (Mg h - ) 3 0 Wet seson F0 F F60 F0 F0 F F60 F0 Rie yields (Mg h - ) /03 Dry seson f e de d Rie yields (Mg h - ) 3 03 Wet seson F0 F F60 F0 F0 F F60 F0 Fig.. Yields of four rie rops under different fertilizer mngement tretments. Error rs re stndrd errors (n=3). Dt with the sme letter is not signifintly different (Tukey p < 0.0). nd RSC indite the pplition nd non-pplition of rie strw ompost, respetively. 36 JARQ (3) 07

5 Soil silion nd ron under pddy in Mekong Delt 0- m -0 m Aville Si (mg kg - ) 0 Aville Si (mg kg - ) 0 0 F0 F F60 F0 0 F0 F F60 F0 Fig.. Aville silion in the experimentl pddy field following sesons of rie ropping. Error rs re stndrd errors (n=3). Vertil xis indites ville Si (mg) in ir-dry soil (kg). Dt with the sme letter does not differ signifintly (Tukey p < 0.0) (0- m). There ws no signifint differene in ville Si in the tretments of su-surfe (-0 m) soil smples. nd indite the pplition nd non-pplition of rie strw ompost, respetively. fster thn tht of other nutrients (N, P, K, C, Mg, Fe, Zn, Mn, Cu) when rie strw is ontinuously removed from pddy fields in the Mekong Delt. Our dt on ville Si onentrtions in the surfe (0 - m) lyer (Fig. ) showed tht the ontinuous pplition of RSC inresed ville Si in the pddy field. Therefore, we onluded tht the ontinuous pplition of RSC mintined Totl-C g kg - Totl-C g kg Yers 3 FC+ F60C+ F0C- F0C- F0C Yers Y= 0.0X (F0C+) Y= 0.7* X (F0C-) Y= 0.**X+ 3.7 (FC+) Y=0.37***X (F60C+) Y=0.0X+ 36. (F0C-) Fig. 3. Chnges in totl ron (g kg - ) in the soil (0- m) in the experimentl pddy. X-xis indites the yer fter Mrh, 000 when the field experiment strted. to indite regression lines for F0C, F0C+, FC+, F60C+ nd F0C, respetively (see Tle ). *, ** nd *** indite tht the slope ws signifintly different from 0 (p < 0.0, 0.0 nd 0.00, respetively). ville Si in the pddy soil, whih ontriuted to higher yields thn those from plots tht did not reeive RSC. This onlusion does not exlude the possiility tht other unidentified mehnisms re involved in inresing rie yields when using RSC. Phytoliths hve een proposed s mjor soure of Si ville to plnts (M & Tkhshi 989, Wikrmsinghe & Rowell 006). A long-term tril onduted t Rothmsted Experimentl Sttion reveled tht nnul strw exports from whet fields redue the mount of phytolith input to the soil, whih dereses the io-ville Si in the soil (Guntzer et l. 0). Moreover, plnt-ville Si in the surfe soil with ontinuous rie strw pplition ws found to e. times higher thn tht without pplition in long-term tril with rie onduted t four experimentl sttions loted in northern Jpn (Koyshi 006). Our results gree with those of the previous reports. However, whether RSC hs ny differene s soure of Si for pddy rie ompred to fresh rie strw or rie strw sh remins unler. The mount of Si removed s rie strw from our field ws 36 to 337 kg h - for the dry sesons nd 03 to 30 kg h - for the wet sesons, s shown in Tle 3. These mounts were lulted from the Si onentrtions of rie strw in our previous report (Wtne et l. 03) nd in iomss (rie strw) prodution. Furthermore, some Si must e removed s grin, lthough we ould not lulte the mount due to lk of dt regrding Si onentrtions in the grin. However, only 0 kg h - of Si were returned to our experimentl field s RSC. Phutel nd Shni (03) showed tht fungi promote the soluiliztion of Si omponents in rie strw, whih 37

6 T. Wtne et. l suggests tht RSC my ontin more plnt-ville Si. Conversely, mking RSC entils onsiderle lour ost. It is neessry to lrify whether RSC pplition is etter thn the pplition of fresh strw or sh of rie strw to mintin plnt-ville Si in pddy fields. Totl-C in the surfe (0 - m) soil ws found to derese y ontinuously ultivting rie without RSC or hemil fertilizer, nd removing the rie strw. In the F0C- tretment, totl-c onentrtion in the surfe soil deresed y 0.7 g kg - nnully (Fig. 3). Bsed on the ulk density of the soil (0.96), the rte of derese ws lulted s 6 kg C h - yer - in the F0C tretment. Both RSC pplition (F0C+) nd hemil fertilizer pplition (F0C-) pprently worked to mintin totl-c in the soil. Totl-C inresed in the FC+ nd F60C+ tretments where RSC nd hemil fertilizer were pplied together (Fig. 3). The rtes of inrese for the FC+ nd F60C+ tretments were lulted s kg C h - per yer - nd 36 kg C h - per yer -, respetively. In studying wht used the differene in totl-c in the F0C nd F0C tretments, rie strw prodution in plots with the F0C tretment ws higher thn tht in the F0C tretment y. Mg h - nd.7 Mg h - for dry nd wet sesons, respetively (Tle 3). It is resonle to onsider tht stules (i.e., plnt residue out m from the ground surfe), roots nd root exudtes in the F0C plots were more extensive thn those in the F0C tretment in proportion to their rie strw iomss. This would explin the differene in totl-c etween the F0C nd F0C tretments, despite removl of the rie strw. We n explin the differene in totl-c etween the F0C+, FC+ nd F60C+ tretments in similr wy. As shown in the FC+ nd F60C+ tretments, the pplition of RSC with redued hemil fertilizer inresed totl-c in the soil. This suggests tht pddy fields in the Mekong Delt ould e used effetively for ron sequestrtion. Cssmn et l. (998) reported tht soil orgni ron inresed in tropil pddy soil despite the omplete removl of ll oveground rop residues. Pmpolino et l. (008) reported tht totl orgni C in the topsoil (0-0 m) inresed fter 7 to yers of ontinuous rie ultivtion in four pddy fields in the Philippines, long with the removl of ll oveground plnt iomss fter every ropping. As reported previously, we found tht RSC pplition lso deresed ulk density nd soil penetrtion resistne (Wtne et l. 009). However, these results do not neessrily indite the indispensility of RSC or the pplition of other orgni mtter for sustinle rie prodution. It my e possile to inrese Si vilility y using the sh of rie strw or rie husks, or silieous fertilizer. Inresing orgni C in the soil my not inrese rie yield in the Mekong Delt, given its generlly high onentrtion (Kyum 98, Ntionl Institute for Soils nd Fertilizers & Deprtment of Siene, Tehnology nd Produt Qulity 00). The high orgni C ontent suggests tht soil physil properties in the region re fvorle for rie prodution. As next step, we should determine how muh rie strw must e pplied to mintin rie produtivity. In onlusion, our study showed tht we need to mintin or inrese Si vilility in pddy soil to mintin rie yield, for whih RSC pplition offers n option. Our results lso showed tht the pplition of RSC with redued hemil fertilizer inresed totl-c in the soil, suggesting tht we n effetively use pddy fields in the Mekong Delt for ron sequestrtion. Aknowledgments Tle 3 Rie strw prodution nd mount of Si in rie strw Rie strw (Mg h - ) Si (kg h - ) DS WS DS WS F0C F0C FC FC F60C F60C F0C WS, wet seson; DS, dry seson. Averge of four rops from 009/0 DS to 0/03 DS. Averge of four rops from 0 WS to 03 WS. Averge of two rops of 009/0 nd 0/0 DS. Averge of two rops of 0 nd 0 WS. Si onentrtions used for this lultion were quoted from Wtne et l. 03. We wish to thnk Ms. Nguyen Ngo Hn for her devoted support of field mngement nd the olletion of dt. We re grteful to Dr. Osmu Ito for his invlule dvie nd support for ontinuing this study. We lso ppreite Dr. Kzunou Toriym nd Dr. Knwr Shrwt for their most useful dvie regrding this doument. Referenes Brros-Filho, M. P. et l. (00) Evlution of soil test proedures for determining rie-ville silion. Commun. Soil Si. Plnt Anl., Cssmn, K. G. et l. (998) Opportunities for inresed nitrogen-use effiieny from improved resoure mngement in irrigted rie systems. Field Crops Res. 6, 38 JARQ (3) 07

7 Soil silion nd ron under pddy in Mekong Delt Doermnn, A. nd Firhurst, T. (000) Nutrient disorders nd nutrient mngement, Interntionl Rie Reserh Institute, Potsh & Phosphte Institute nd Potsh & Phosphte Institute of Cnd, Mkti City, Philippines (distriuted in the form of CD-ROM). Fujii, H. (00) Effet of silite on growth, yield nd tste of rie, in Jpnese soiety of soil siene nd plnt nutrition. (eds.), Silion nd rop prodution. Hkuyu-sh, Tokyo, pp. [In Jpnese]. Generl Sttistis Offie, Vietnm (0) Sttistil Yerook 009. Sttistil pulishing house, Hnoi, pp.9. Guntzer, F. et l. (0) Long-term removl of whet strw dereses soil morphous sili t Brodlk Rothmsted. Plnt Soil, 3, Koyshi, N. (006) Effet of long-term pplition of rie strw on the plnt ville silion of pddy soil. Proeedings of the 8th World Congress of Soil Siene, July 9-, Phildelphi, Pennsylvni, USA. Kyum, K. (98) Fundmentl Chrteristis of Wetlnd Soils. Pro. of Wetlnd Soils Workshop, IRRI, M, J. F. nd Tkhshi, E. (989) Relese of silion from rie strw under flooded onditions. Soil Si. Plnt Nutr. 3, Ntionl Institute for Soils nd Fertilizers nd Deprtment of Siene, Tehnology nd Produt Qulity (00) The si informtion of min soil units of Vietnm. Thegioi pulishers, Hnoi, Pmpolino, M. F. et l. (008) Soil ron nd nitrogen hnges in long-term ontinuous lowlnd rie ropping. Soil Si. So. Am. J. 7, Phutel, U. G. nd Shni, N. (03) Mirosopi struturl hnges in pddy strw pretreted with Trioderm reesei MTCC 6 nd Coriolus versiolor MTCC 38. Indin J. Miroiol. 3, 7-3. Tkhshi, K. (986) Stndrd methods of soil nlysis. (eds.), Committee of stndrd methods of soil nlysis. Hkuyush, Tokyo 6-6 [In Jpnese]. Trn, S. N. et l. (0) Estimted quntity of rie strw nd its uses in some provines in the Mekong Delt region. Cn Tho University s Journl of Siene nd Tehnology, (3A), United Sttes Deprtment of Agriulture, Nturl Resoures Conservtion Servie (0) Key to soil txonomy, twelfth edition, 0. Wtne, T. et l. (009) Effets of ontinuous rie strw ompost pplition on rie yield nd soil properties in the Mekong Delt. Soil Si. Plnt Nutr.,, Wtne, T. et l. (03) Comined effets of ontinul pplition of omposted rie strw nd hemil fertilizer on rie yield under doule rie ropping system in the Mekong Delt, Vietnm, JARQ, 7, Wikrmsinghe, D. B. nd Rowell, D. L. (006) The relese of silion from morphous sili nd rie strw in Sri Lnkn soils, Biol. Fertil. Soils,,

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