Silicon sources for agriculture

Transcription

1 Silicn surces fr agriculture Chapter 2 Silicn surces fr agriculture 2.1. SILICON SUPPLY FOR PADDY RICE FROM NATURAL SOURCES Natural surces f Si fr rice are irrigatin water and sil. The amunt f Si supplied varies with the parent material and gelgy f river basin Irrigatin water Paddy sil is irrigated with an average f 14,000 tns f water per hectare during the grwth perid f rice. Therefre, Si in irrigatin water has an imprtant impact n rice prductin. Kbayashi (1954) cllected 116 samples f rice straw frm varius places in Japan and fund a psitive relatinship between Si cncentratin in rice straw and in irrigatin water (Figure 2.1). Q i ^ 10 1 c (U c CJ rs ^ I 1 I 1 1.,. - J Si02 cncentratin in irrigatin water ( ppm) Figure 2.1. The relatinship between Si cncentratin in irrigatin water and Si cntent in rice straw.

2 Chapter 2 Regins I II III IV V Hkkaid 16 prefectures 6 prefectures 16 prefectures 8 prefectures N. f rivers SiO., (ppm) Maximum Minimum Average Figure 2.2. Average SiO.^ cncentratin f river waters in five regins f Japan.

3 Silicn surces fr agriculture 7 Table 2.1 The effect f gelgy f basin n SiO.;> cncentratin in river water Gelgy f basin Number f rivers SiO.^ cncentratins (ppm) Palezic (aqueus rck) Meszic (aqueus rck) Granite Vlcanic ash He als cllected river water used fr irrigatin and measured its Si cncentratin by a clrimetric methd (Kbayashi, 1961). As shwn in Figure 2.2, amng 380 rivers investigated, the lwest cncentratin f SiO^ was 4.1ppm, and the highest was 61.5 ppm with an average f 21.6 ppm (fr details, refer t Appendix I). If rice is irrigated with 14,000 tns water per hectare, it is calculated that an average f 300 kg SiO^ per hectare is supplied t rice frm irrigatin water annually. In Figure 2.2, the rivers in I, II and V regins have a high Si cncentratin. This is related t existence f vlcanes in these areas. The Si cncentratin f river water als varies with gelgy f basin. River water riginating frm aqueus rck and granite usually has a lw Si cncentratin, while that frm vlcanic ash has a high Si cncentratin (Table 2.1) Sils The Si supply capacity f sils varies greatly with paddy field. In 1955, the Ministry f Agriculture, Frestry and Fisheries f Japan made a natinwide survey n Si cntent in the flag leaf, using the data frm 37,949 samples cllected frm varius paddy fields. The Si cntent in the flag leaf varied widely with the regin as shwn in Table 2.2 (fr details, refer t Appendix II). In abut 5% f samples examined, the Si cntent was less than 7.5%, while 9% f samples shwed the Si cntent higher than 23%. The Si cntent in the flag leaf reflects the Si supply pwer f paddy sils. In regins II and V, the Si cntent is high, while the Si cntent in regins III and IV is lw. This trend is cnsistent with that fr Si cncentratin in river waters reprted by Kbayashi (Figure 2.2). In the regins where the Si cntent in the leaf flag is lwer than 7.5% and where the Si cntent is between %, there is a pssibility f Si-deficiency in the sil. The applicatin f Si fertilizers wuld be effective in these regins, mainly in regins III and IV (Table 2.2).

4 8 Chapter 2 Table 2.2 Average SiO., cntent in flag leaf in the five regins f Japan in 1955 Regins N. f sampling sites Average SiO^ cntent (%) Percentage f samples with a SiO., cntent (%) f I II III IV V Whle land ,902 8,059 9,102 4,608 37, <7.5 % % Each sample (flag leaves) was cllected at different sites in each regin. Imaizumi and Yshida (1958) summarized the Si-suppl3dng capacity f sils with different parent materials, based n the field trial data frm natinwide agricultural experiment statins. As shwn in Table 2.3, vlcanic ash (Tchigi) and shale (Gifu)-riginating sils have a high Si-supplying capacity. Furthermre, it was fund that new vlcanic ash sil is rich in water-sluble Si, but the Si-supplying capacity f vlcanic ash sils decreases with aging because f desilicating prcess. Lwer Si cncentratin in vlcanic ash sils f Gifu and Table 2.3 The relatinship between parent material and SiO^ supplying capacity f sil Parent material Vlcanic ash Shale Quartz prphyry and granite Vlcanic ash (desilicated) Vlcanic ash (desilicated) Granite Granite Peat Lcality Tchigi Gifu Gifu Gifu Yamagata Nagan Yamagata Yamagata Sample size "Acetate buffer (ph 4.0) sluble SiO, SiO., cntent Range Rice straw (%, DW) Sil available Si; (mg/loog sil) ~ Average Rice straw (%, DW) Sil available si; (mg/loog sil)

5 Silicn surces fr agriculture Table 2.4 Balance f Si measured with a lysimeter charged with varius kinds f paddy sils (SiO.^ g m ^ Hkuriku Agricultural Experiment Statin, 1968) Sil texture Sandy sil Heavy clay sil Peat sil Supply frm irrigatin water (A) Lss in Perclating water (B) Uptake by Rice plant (C) Frm irrigatin water (A-B) Frm Sil (C-(A-B)) Yamagata prefectures than that f Tchigi is attributed t different age f vlcanic ash. Sils derived frm quartz prphyry and granite, and peat have lw capacity f Si supply. A balance sheet fr Si taken up by rice and fr Si supplied frm irrigatin water was calculated. The average amunt f Si uptake by rice was estimated t be 950 kg SiO^ per hectare, f which 260 kg SiO^ riginated frm irrigatin water. This fact suggests that 27% Si in rice is supplied frm irrigatin water, and the remaining Si is frm sil. Therefre, Si in sils is a majr natural Si surce fr rice. The results f a lysimeter experiment cnducted in Hkuriku Agricultural Experiment Statin (1968), shwed that except in sandy sil, 40% f Si in irrigatin water is taken up by rice, which cntributed t 10 t 20% f ttal Si f rice (Table 2.4) SILICON SUPPLY FROM ORGANIC AND INORGANC FERTILIZERS Cmpst Applicatin rate in the past and the present Befre the intrductin f silicate fertilizers, the main surce f Si supply was cmpst. Fr example, cmpst prductin was abut 20 millin tns in Because f fertilizer shrtage during Wrld War II, the use f self-supplied fertilizer was encuraged, and cmpst prductin reached 60 millin tns arund Assuming that cmpst cntained 5% SiO^, 1 millin (1927) and 3 millin (1945) tns f Si annually fr 3 millin hectares f paddy sil, wuld be equal t 330 and 1000 kg SiO^ per hectare, respectively.

6 10 Chapter > 4 t 3 " ^0"" 0 0 O 0 0 O O # Cmpst 0 Calcium silicate ^«'' ^ J j ;B^ i i 300 i 100 U Year Figure 2.3. Changes in the amunts f Si supplied t the paddy field frm cmpst and silicate fertilizer. In 1955, when slag was intrduced as silicate fertilizers, cmpst was applied at an average rate f 6.5 tns per hectare. Hwever, frm 1960, the rate f cmpst applicatin gradually decreased, resulted in 4.5 tns in 1970, and less than 2 tns in Silicn supply frm cmpst therefre decreased frm 325 kg SiO^ (1955) t 225 kg (1970), and t 100 kg (1990) per hectare. The decrease f Si supply frm cmpst was supplemented by silicate fertilizers (Figure 2.3). The average rate f silicate fertilizer applicatin per hectare increased frm 150 kg in 1960 t 200 kg in 1965, and t mre than 400 kg in Assuming that the Si cntent f silicate fertilizer is 25% SiO^, 38 kg, 50 kg, and 100 kg SiO.^ wuld have been supplemented in 1960, 1965 and 1968 respectively. Hwever, the rate f silicate fertilizer applicatin has been gradually decreasing thereafter (Figure 2.3). 2.2.I.2. Shrt-term availability f Si in rice straw fr rice plants Rice straw used fr preparatin f cmpst cntains up t 20% SiO^. Ma and Takahashi (1989b, 1991a) reprted the shrt-term availability f Si cntained in rice straw fr rice. They examined the availability as the slubility f Si in the rice straw. Rice straw cntaining 15.5% SiO^ was successively extracted with distilled water at 40''c. This extractin methd is the same as that fr measurement f sluble Si in sil prpsed by Takahashi as described in the next chapter. The cncentratin f Si disslved in water was determined every week. The first and secnd extractins gave a cncentratin f abut 100 ppm

7 Silicn surces fr agriculture Number f extractins 10 Figure 2.4. Slubility f Si in rice straw. SiO^ in the water (Figure 2.4). Hwever, the slubility f Si in the later extractins decreased t abut 40 ppm Si02 and remained at a similar level thereafter. The high cncentratin f Si during the first tw extractins might have resulted frm the presence f mnmeric Si and Si with lw plymerizatin in rice straw. They accunted fr 4.1 and 3.6% f ttal Si fr the first and secnd extractin, respectively. Mst f the Si in rice straw shwed slubility between that in silica gel and pal. 100 a, 80 ex c B 60 a c c h Cntrl - Si straw + Si straw Days after flding 60 Figure 2.5. Changes in SiO^ cncentratin in the perclating water.

8 12 Chapter 2 Table 2.5 Ttal amunt f silicn released during the experiment (mg SiO.^) Treatment Cntrl -Si straw -hsi straw 380(100) 448(118) 830(218) Nte: Figures in parentheses indicate the relative percentage f cntrl. The effect f rice straw n the slubility f sil Si was investigated under rice planting and nn-planting cnditins. As rganic matter may affect the slubility f sil Si, rice straw withut Si (-Si straw), prepared by cultivating rice hydrpnically in the absence f Si, was used as a cntrl. When rice plants were nt planted, the Si cncentratin in the water perclating thrugh the sil with +Si straw was fld higher than that in the treatment with -Si straw (Figure 2.5), suggesting that Si in the straw is released (Ma and Takahashi, 1989b). The amunt f Si released frm the sil with +Si straw during the experiment (60 days) was 450 mg larger than that released frm the sil withut rice straw (Table 2.5), but this accunts fr abut 10% f Si cntained in the +Si straw (4650 mg SiO^). Additin f -Si straw als caused the Si cncentratin in the perclating water t increase (Figure 2.5). This increase is attributed t the increased slubility f Si in sil, resulting frm the enhanced sil reductin. The effect f rice straw n the amunt f Si extracted frm the sil determined at the end f the experiment, varied with the methd f extractin. When acetate buffer was used t extract Si, n significant increase in the amunt f extractable Si was fund by the applicatin f rice straw (Table 2.6). Hwever, when Si was extracted by incubatin under a submerged cnditin, abut 40% increase in the amunt f extractable Si by the applicatin f +Si rice srtaw. In the field under rice cultivatin, the Si cncentratin in the perclating water decreased sharply cmpared with that in the perclating water withut rice cultivatin (Figures 2.5, 2.6), suggesting that rice rts absrb Si actively. Additin f +Si rice straw increased the Si cncentratin in the perclating water during the first tw weeks but nt thereafter (Figure 2.6). Table 2.6 Amunt f Si slubilized frm sil with r withut rice straw (mg SiO/100 g sil). Treatment Withut straw -Si straw +Si straw Methd f extractin Acetate buffer Incubatin under submerged cnditin

9 Silicn surces fr agriculture 13 OH 60 r Cntrl -Si straw +Si straw 30 O h Davs after flded Figure 2.6. Si02 cncentratin in the perclating water during grwth under different cnditins (first cultivatin). Applicatin f -Si straw did nt affect the cncentratin f Si in the perclating water. Applicatin f +Si straw increased Si uptake by rice (Table 2.7), while applicatin f -Si straw did nt affect Si uptake. The increased amunt f Si uptake due t +Si straw applicatin was mg SiO^ per pt, which accunted fr 8% f Si cntained in the +Si straw. Thus the availability f Si fr the rice plant was lw in the rice straw in the shrt-term experiments. 2.2,1.3. Lng-term availability f Si in cmpst fr rice plants One example f the lng-term effect f Si in cmpst is shwn in Table 2.8. Cmpst was applied t the field frm 1933 t 1973 (40 years) at Shiga Prefectural Agricultural Experiment Statin. The summer crp was rice and the winter crp was wheat. The trial was dne using 16 plts: 8 plts withut cmpst (chemical fertilizer nly) treatments, and 8 with cmpst (cmpst was Table 2.7 Uptake f Si by rice Withut straw -Si straw +Si straw frm sil with r withut rice straw Si uptake (SiO^ mg/pt) First cultivatin Secnd cultivatin Ttal

10 14 Chapter 2 Table 2.8 Cntents (%) f Si and ther elements in the straw f rice (first crp) and wheat (secnd crp) cultivated in the sils cmpsted fr 40 years and in nn-cmpsted sils (Reprt f Lng-term Field Experiment ( ) cnducted at Shiga Prefectural Agricultural Experiment Statin) Rice straw Wheat straw SiO, N P K Ca Cmpsted Series, A Nn-cmpsted Series, B A/BxlOO Cmpsted Series, A Nn-cmpsted Series, B A/BxIOO applied at a rate f 11.5 tns per hectare fr summer crp, and 9.9 tns fr the winter crp tgether with chemical fertilizers). In 1974, the year after the last cmpst applicatin, rice and wheat were cultivated similarly, and the accumulative effect f cmpst applied fr 40 years was investigated by mnitring yield, mineral cntent, physical and chemical prperties f sil (Nakada, 1980). Amng the minerals investigated, the Si cntent in rice and wheat that had been grwn in cmpsted sil was higher than that in nn-cmpsted sil (Table 2.8). Table 2.9 shws that the Si cntent in bth rice and wheat was higher in the treatments withut N and P fertilizer applicatin. The amunt f nutrients taken up by rice during the 40 years was estimated frm the mineral cntent and dry matter f bth straw and grain (Table 2.10). The amunt f Si uptake by rice was much higher than that f the ther minerals. It was 5-7, 16-23, 3-4, and 9-12 times as large as that f N, P, K and Ca, respectively. Furthermre, the uptake f nutrients, especially Si, was increased by cmpst applicatin. Cmpared with the nn-cmpsted plts, 16 tns mre SiO^ per hectare was taken during the 40 years in cmpsted plts. As 460 tns cmpst (estimated as 23 tns SiO^) was applied during the 40 years, the Si taken up by rice accunts fr 70% f the cmpst applied. If it was derived frm the cmpst, its use efficiency was quite high. The physic-chemical prperties f sil were als imprved by the applicatin f cmpst (Table 2.11). In additin t the depth f tp sil, ttal carbn, and the extractable Si in the sil (extracted with 0.2 N HCl) was significantly increased cmpared t riginal sil. This implies that Si in cmpst als increases the amunt f sluble Si in sil. By cntrast, in the nn-cmpsted plts, the available Si in sil decreased significantly because f the uptake by rice.

11 Silicn surces fr agriculture 15 Table 2.9 Effect f fertilizer treatments n Si cntents f the straw f rice and wheat harvested in 1974 Treatment -NPK N P K PK NK NP NPK Average Rice straw (SiO^ %) Cmpsted Nn-cmpsted Wheat straw Cmpsted (SiO,%) Nn-cmpsted The difference in the slubility f Si in the rice straw between the shrt-term and lng-term experiments may be attributed t the slw release f Si frm the rice straw (Figure 2.4). Althugh the effect f cmpst applicatin n the Si-supplying capacity f sil was bvius in the lng-term experiment as described abve, it takes many years t utilize all Si cntained in the rice straw. The slw release f Si frm rice straw gave birth t the fast-release inrganic silicate fertilizers. Table 2.10 Estimatin f ttal amunts f nutrients taken up by rice plant during 40 years f cultivatin Amunt f nutrient mineral taken up SiO, (Si) N P K Ca (tn ha^ ' 40yr. ^) Cmpsted, A (16.667) Nn-cmpsted, B (9.120) A/BxlOO

12 16 Chapter 2 Table2.11 Physic-chemical prperties f the sils at the start and the end f the 40-year experiment Cmpsted Nn-cmpsted Start f expt. Depth f tp sil (cm) Apparent density Ttal Carbn (%) Ttal Nitrgen (%) C/N rati 0.2 M HCl sluble P (ppm) K(ppm) Ca (ppm) SiO., (ppm) ^average f 8 plts. plts'" plts'" m Rice husk Rice husk accunts fr abut 20% f the weight f paddy (unhusked rice) and up t 20% cnsists f SiO^. Since the annual rice prductin in Japan is abut 10 millin tns as husked rice, 2 millin tns f rice husk cntaining nearly 0.4 millin tns f SiO^ is prduced annually. Rice husk is prduced lcally at the threshing spt. Therefre Si cntained in the husk culd be a ptential Si surce fr paddy fields. Table 2.12 Effect f carbnized rice husk applicatin n the Si cntent f rice seedlings* Treatment (SiO, g/plt) SiO, cntent (%) Cntrl (0) 5.04 Silica gel (160)** 5.68 Burnt rice husk ashes (160) 6.51 Carbnized rice husk *1 plt is 3.3 m' ** Silica gel was prepared by neutralizing Na^SiOg slutin with H^SO^ and thrughly washed with water

13 Silicn surces fr agriculture 17 Table 2.13 Effect f applicatin f carbnized rice husk n Si cntent f rice seedlings at the seed beds f farmers Carbnized rice husk Applied Nt applied SiO, cntent (%) Paddy seed bed 9.12 (13)* 7.40(11) *() indicates the number f seed beds tested. Upland seed bed 7.58 (2) 5.30(1) Ishibashi (1956) reprted that carbnized rice husk is a gd Si surce althugh unprcessed rice husk is nt. Carbnized rice husk was prepared by burning the rice husk slwly at as lw temperature as pssible. As shwn in Table 2.12, applicatin f carbnized rice husk increased greatly the Si cntent f rice seedlings. Si in carbnized rice husk was taken up better by rice seedlings than that in the ashes f rice husk burned accrding t the cmmn practice. Table 2.13 shws the amunt f Si in rice seedlings sampled frm the seed-beds f farmers. Applicatin f carbnized rice husk increased the Si cntent f yung rice significantly Silicate fertilizers Calcium silicate slags Slag was als initially applied as a liming material in Japan as in Germany (1937) and USA (1939). Sn after Wrld War II, althugh Japan was having a fd shrtage prblem, the amunt f nitrgen fertilizers fr crp prductin was nly ne tenth f that used in nrmal times. Phsphrus and ptassium fertilizers that entirely relied n imprt were cmpletely lacking. Hwever, lime resurces were abundant, and therefre, many trials using liming material were carried ut. One f them is slag, which was fund t have better effect in paddy sil than lime. On the ther hand, in the prcess f imprvement f degraded paddy sils that were widely distributed, it was fund that silicn applicatin is effective in additin t the applicatin f irn and base. As a surce f silicn, slag that mainly cnsists f calcium silicate was applied in agricultural experiment statins and universities all ver the cuntry (Ohta, 1964 and many ther papers and dcuments). Because the beneficial effect f slag was cnfirined, slag was apprved as a silicate fertilizer by the Ministry f Agriculture, Frestry and Fisheries f Japan in An fficial standard f slag as cmmercial silicate fertilizers was prvided. Slag is made by melting the re cntaining Fe, Mn, Ni, and Cr with limestne

14 18 Chapter 2 Table 2.14 Cmpsitin f slags used as calcium silicate fertilizer (%) CaO SiO, MgO MnO Pig irn slags Steel mill slags Stainless steel slags Ferrmanganese slags Silicmanganese slags Ferrnickel slags Nickel slags Ferrchrme slags Magnesium slags Dephsphrated slags ~ ~ A1,0, Fe,0, and ckes in a blast furnace r electric furnace, and then cling (by either air r water) material flated n the surface. Silicn cmpnents in the re react with limestne, leading t separatin f calcium silicate, and Fe and ther metals in the re are reduced and separated. After the metals needed are separated, slag remains as a by-prduct. The main cmpnents f slag are calcium silicate, Mg, Al, Fe and trace Mn, Ni, Cr, etc. are als included. As a silicate fertilizer, slag must have mre than 20% f 0.5 N HCl sluble Si02, mre than 35% f alkali cmpnent, and the cntent f txic cmpnent must be under permissible limit (0.4% Ni, 4% Cr, and 1.5% Ti). In 1987, slag having mre than 10% sluble SiO.^ was als recgnized as a silicate fertilizer. The cmpsitin f 10 slags used as calcium silicate fertilizers is shwn in Table Fused magnesium phsphate Fused magnesium phsphate fertilizer was manufactured in Japan frm It is made by melting phsphate rck with serpentinite and grund after quick cling. It cntains P, Mg, Ca and Si. Fused magnesium phsphate appeared as a furnace phsphate fertilizer but cntains 16-26% sluble SiO^. Fertilizers cntaining mre than 20% sluble SiO.^ are recgnized as silicate fertilizer Ptassium silicate fertilizer Ptassium silicate appeared as a slw-releasing ptassium fertilizer in Fly ash that is prduced frm cal pwer plant is used as silicate material. Fly ash is mixed with ptassium carbnate r ptassium hydrxide and magnesium

15 Silicn surces fr agriculture 19 hydrxide and calcined at abut 900''C. The main minerals are K^O (Al, Fe)03 SiO,and K.O MgO SiO^CAnd et al., 1985, 1990; Izumi et al., 1995). Accrding t the fficial standard f cmmercial fertilizer, the fertilizer must have mre than 20% K,0 f citrate-sluble ptassium, 25% SiO., f 0.5 M HCl-sluble silicate, 3.0% MgO f citrate-sluble magnesium, and less than 3% f nn-reactive water-sluble ptassium. Ptassium in the ptassium silicate fertilizer is slwly released, and it was fund that silicate in this fertilizer increases the resistance f rice t diseases and insect pests. At present (2000), the demand f this fertilizer is 50,000 tns, and mre than 90% is used fr rice. In 1986, a liquid ptassium silicate, which is guaranteed by 12% f water-sluble SiO.^ and 6.0% f water-sluble ptassium, appeared as a readily available silicate fertilizer. It was prduced by diluting ptassium silicate and ptassium carbnate in water Prus hydrate calcium silicate Prus hydrate calcium silicate (tbamlite) used fr light wall material in cnstructin is prduced frm quick lime, quartz and cement, which are reacted under 180 C, 10 atm pressure. Because f the strict standards fr wall material, a high percentage f nnstandardized prduct resulted. This waste material was used as a fertilizer. In 1993, "a light prus cement pwder fertilizer" was recgnized as a new silicate fertilizer and the standard fr this fertilizer was decided. This fertilizer must have mre than 15% f 0.5 N HCl sluble SiO^ and mre than 15% f base cmpnent Silica gel Rice seedlings need sufficient Si t be taken up during seedling grwth. Hwever, the present cmmercial silicate fertilizers are nt suitable fr use in nursery bed because they cntain alkali cmpnents that raise the ph and weaken the resistance t diseases. Silica gel is a pssible Si surce that des nt increase sil ph. Silica gel is usually used as a desiccating agent and is made by neutralizing water glass, gelling, and finally dehydrating. Fr ptential applicatin f silica gel in nursery beds f rice seedlings, in 1999, fficial standard fr this fertilizer was decided. Different frm ther silicate fertilizers, silica gel is nt disslved in hydrchlric acid, and must have mre than 80% f 0.5 N sdium hydrxide sluble SiO^ Estimatin f available Si in silicate fertilizers In an fficial methd fr quantitative analysis f slag, Si is extracted with 0.5 N HCl at a rati f 1:150 (slag pwder : 0.5 N HCl) with shaking. Hwever, Si extracted by this methd smetimes des nt crrelate with Si taken up by rice. Takahashi (1981) prepared 11 kinds f slag with different chemical

16 20 Chapter 2 cmpsitins and different cling methds, and cmpared the Si uptake f rice frm slag-appued sils in pt experiment. As shwn in Table 2.15, the percentage f Si taken up frm the slag varied widely with the kind f slag, ranging frm 73.2% (slag frm phsphrus prductin) t 25.8% (ferrnickel slag). The absrptin percentage f Si by rice is lwer in acidic slag with less than 1 mlar rati f CaO t SiO^ (e.g. silicmanganese slag and ferrnickel slag). Slag with carse particles shwed a lwer percentage f Si uptake than thse with fine particles and cled slwly in air (Takahashi, 1981). Since particle size and mlar rati f Ca t Si f the slag were nt taken int accunt, the fficial methd using 0.5 N HCl wuld nt reflect the availability f Si fr rice plants. Further investigatins revealed that extractin with 4% ammnium citrate at ph 4.5 is suitable fr evaluatin f the amunt f Si available in the slag fr rice plants. The amunt f Si extracted by this methd is relatively lw in an acidic slag and significantly decreased as the mlar rati f Ca t Si decreased. Furthermre, the slubility was larger in a slag with a fine particle size and was crrelated with Si uptake by rice. Hwever, the prblem is the slag frm the phsphrus prductin, which is highly available t rice, but has lw slubility by this methd. Table 2.15 The availability f 0.5 M HCl - sluble SiO, fr rice plant amng 11 kinds f slags Kinds f slags 0.5 M HCl sluble S-SiO, Availability f Average (cling methd)"" SiO, (%) CaO (%) MgO (%) /T-SiO, (%) S-SiO^ by rice plant (%)' particle size (pm) Slag frm phsphrus prductin (B) Cnverter slag (A) Slag frm pig irn manufacture (A)-l (A)-2 (B)-3 (B)-4 (B)-5 Silicmanganese slag (A)-l (C)-2 (B)-3 Ferrnickel slag (B) ''(A) slwly cled in air; (B) cled and crushed by water jet; (C) cled in water at the maturity stage

17 Silicn surces fr agriculture 21 At the early grwth stage Apphcatin f the slag Si disslutin Increase f Ca cncentratin Increase f sil frm the slag [^ t^e sil slutin slutin ph Increase f water sluble Si in the sil Decrease f slubility f the slag Increase f sil ability t absrb silicic acid Suppressin f the increase in Si cncentratin in the sil slutin At the later grwth stage Plant rt Leaching Uptake Supply f CO2 gas Decrease f Si cncentratin in the sil slutin Decrease f Ca cncentratin in the sil slutin Neutralizatin effect f CO2 gas Prmtin f Si disslutin frm sil slid phase Cntinuus disslutin f the slags Cntinuus supply f silicic acid int the sil slutin Figure 2.7. Scheme describing the pssible reactins in a paddy sil with slag applied.

18 22 Chapter 2 Recently, Kat and Owa (1996a) shwed that the slubiuty f the slag in acid slutin with a ph lwer than 6 was higher than that in a slutin with ph 6 r 7 and that this ph effect varied with the type f slag. These results suggest that a slutin with a ph range between 6 and 7 shuld be used fr extractin instead f an acid slutin such as 0.5 M HCl when estimating the slubility f the slag in paddy sils where the sil ph is nearly neutral. Kat and Owa (1996b, 1997a) investigated the disslutin prcess f slags in paddy field under rice cultivatin. They fund that the increase in the sil slutin ph caused by the disslutin f Ca and Mg frm the slag and/r the develpment f sil reductin, was suppressed by the neutralizing effect f CO^ gas released by the respiratin f the plant rts and the activity f the micrrganisms present in the rhizsphere, hence enhancing the slubility f the slag (Figure 2.7). These neutralizatin effects shuld be taken int accunt fr the estimatin f Si availability in the slag. Kat et al. (1997) tried t determine "active Si" in sil supplied with slag using stable istpe ^^Si. They fund that ^"Si-labelled silicic acid added t the sil was diluted nt nly by silicic acid in sil slutin, but als by silicic acid desrbed frm the sil slid phase. They calculated the Si amunt which takes part in istpic dilutin within 60 minutes based n ^"Si cncentratin in sil slutin. This amunt was named as Dg,,-value and cnsidered as a parameter f active Si. The cntent f active Si was higher in the sil that was supplied slag with a higher alkali cntent (Figure 2.8). There was a psitive crrelatin between Si uptake by rice and the cntent f active Si (Figure 2.9). Based n these findings n the disslutin prcess f the slag in paddy field, Kat and Owa (1997b) prpsed a new extractin methd fr the evaluatin f the availability f Si in the slag. They disslved the slag in water with the additin f a weakly acidic catin exchange resin (H^ frm), and examined the effects f the slag/water rati, the amunt f resin, and the temperature n the Si disslutin frm the slag t determine adequate extractin cnditins. The ph f the extractin was well cntrlled between 6 and 7 during the extractin, and the Si disslutin frm the slag was enhanced by the additin f resin. Figure 2.10 shws the relatinship between the percentage f Si taken up by the rice plants frm the 20 kinds f slag and the percentage f Si disslved in 0.5 M HCl r acetate buffer slutin. Mre than 70% f Si in the slag was disslved in 0.5 M HCl, while the percentage f Si taken up by rice plant varied widely frm 3.4 t 64.4%. The percentage f Si disslved in the acetate buffer slutin ranged frm 32.8 t 95.3%, which was higher than the range f the Si recvery rate. Therefre, it is difficult t estimate the amunt f Si available t the plant in the slags by these extractin methds.

19 Silicn surces fr agriculture 23 /u 60 ^ ^ _S 50 ^ 40 1^ ^ 30 :3 13 ^ 20 c? 10 - i O y r J ^^ #, withut slag. O, vv ith slag. n A/Si rati Figure 2.8. Relatinship between the A/Si rati f the slag and the amunt f Si in istpic dilutin within 1 h (D^^-value) # 300 g 250 #, withut slag. O, with slag. ^ 200 a, = 150 y=2.88x+178 r=0.770**(n=21) D^^-value(mgkg'^) Figure 2.9. Relatinship between the Si taken up by rice plant and the D^-value.

20 24 Chapter 2 On the ther hand, a better relatinship was bserved between the percentage f Si taken up by the rice plants, and that disslved in water frm the slag (Figure 2.11). The percentage f Si disslved either in the presence r absence f resin was higher in the magnesium slag and cnverter slag that shwed a high availability f Si. Even in the extract withut resin, the percentage f Si disslved was psitively crrelated with that taken up by the rice plants. Hwever, in this case, the percentage f Si disslved was much lwer than the latter. When the slag was disslved with resin at 25"C, the crrelatin cefficient and the regressin cefficient were slightly lwer than thse at 35 C, hwever, fr the cnvenience f temperature cntrl, extractin at 25 C was adpted. Thus, the new extractin methd using a catin exchange resin was established as fllws: Put 0.2 g slag and 0.5 g f a weakly acidic catin exchange resin in the H^ frm (Amberlite IRC-50) int a 500 ml vlume plastic bttle. After adding 400 ml f distilled water, immediately shake the bttle fr a while by hand and then with a reciprcal shaker (100 rpm) at 25 C fr 96 h. After filtratin, determine the Si cncentratin in the filtrate clrimetrically. The percentage f Si absrbed by rice plants frm slag fertilizers was reprted t be 20 t 50% in rice. Sumida and Ohyama (1991) investigated the percentage f Si absrbed frm rganic and inrganic Si fertilizers in cld area and fund that the percentage was abut 30% frm electric furnace slag, 6% 120 e 100 a fe 20 0-* kjk ^D A> ^ 0.5MHCL I X X X A 8 Acetate buffer Sirecvetybyriceplant (%) Sirecveryby rice plant (%) Figure Cmparisn between the Si recvery rate by rice plant and the slubility f the slags in the acid slutins. O, blast furnace slag; A, silic-manganese slag; x, ferrnickel slag; A, phsphrus slag; *, rdinary steel slag; O, stainless steel slag; D, ferrchrme slag;, magnesium slag;, cnverter slag. 100

21 Silicn surces fr agriculture O With resin (35 C) X XX L D 0 8>^ J A y=0.94x+0,ll r=0.68** With resin (25'C) X >/ ^ y^a y=0.92x-1.52 y ^ O r=0.65** y ^ & \ ^XX A,, U Withut resin (35"C) '% O J^C^^ 0^ \ 1 A 1 y=0.40x-2.50 r=0.61** I Si recvery b\ rice plant (%) Figure Cmparisn between the rate f Si absrbed by rice plants and the slubility f the slags in water with r withut the additin f resin. O, blast furnace slag; A, silic-manganese slag; X, ferrnickel slag; A, phsphrus slag; *, rdinary steel slag; O, stainless steel slag; D, ferrchrme slag;, magnesium slag;, cnverter slag. **/?<0.01. frm rice straw, and 3% frm rice straw cmpst. In additin, althugh Si supply frm slag was recgnized at the early stage f rice grwth, abut 70% f available Si was supplied during the perid frm panicle frmatin stage t full heading stage. Silicn supply after heading stage may nt be expected. Hwever, as the residual effect f cntinuus applicatin f slag was bserved fr several years after stpping the applicatin f slag, it is suggested that slag cntains readily released fractin and slwly released fractin f Si. The percentage f Si absrbed by the rice plants frm the slag was cnventinally calculated based n deductin. Hwever, Owa and Kat (1998)

22 26 Chapter 2 used calcium silicate labeled with the stable istpe ^"Si and fund that the percentage f Si absrbed by rice was 38% in mineral acid sil and 45% in gray lwland sil. They als fund that 40% f Si in rice straw in mineral acid sil and 30% in gray lwland sil resulted frm calcium silicate applied. Frm these results, it is clear that the percentage f Si absrbed frm calcium silicate differs with the sil. Gray lwland sil has a higher Si-supplying capacity and the applicatin f calcium silicate is mre effective in it than in mineral acid sil (Kat, 1998).