Potentiality of Indian rock phosphate as liming material in acid soil

Geoderma 263 (2016) 104–109

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Potentiality of Indian rock phosphate as liming material in acid soil B.B. Basak a,⁎, D.R. Biswas b a b

ICAR — Directorate of Medicinal and Aromatic Plants Research, Boriavi 387310, Anand, India Division of Soil Science and Agricultural Chemistry, Indian Agricultural Research Institute, New Delhi 110012, India

a r t i c l e

i n f o

Article history: Received 17 February 2015 Received in revised form 2 September 2015 Accepted 18 September 2015 Available online xxxx Keywords: Rock phosphate Lime potential Calcium carbonate equivalence Acid soil Soil incubation study

a b s t r a c t Most of the Indian rock phosphates (RPs) are not suitable for production of commercial phosphate fertilizer because of their low phosphorus (P) content (low-grade). They are suitable as source of P in acid soil, but not effective for direct use in neutral to alkaline soil. The main aim of this study was to evaluate the potentiality of these RPs as liming material and subsequently their effect on P availability in acid soil. Four Indian RPs namely, Udaipur, Mussoorie, Jhabua and Purulia were evaluated for their liming potential through theoretical calculations, laboratory titration and soil incubation studies. The incubation experiment was carried out for 90 days to quantify more accurate per cent calcium carbonate equivalence (%CCE) values of RPs as well as the changes in pH, exchangeable aluminum (Al) and P availability in an acid soil having pH 4.53. Results emanated from the theoretical calculation showed that the %CCE of Indian RPs varied from 59 to 62. While, laboratory studies on quantification of %CCE (AOAC method 955.01) of RPs by titration ranged between 39.9 and 53.7 which were lesser than the theoretical values. The %CCE values obtained in soil incubation study at lower rate of RP application followed the similar pattern as those values obtained in the theoretical calculation. Though RP was less effective in increasing soil pH as compared to CaCO3, there was an increase in soil pH due to application of RPs, which is sufficient enough to decrease the content of Al much below the toxic and safe limit for supplying available P for plant growth. Thus, application of low-grade RPs could be a potential option as liming material in acid soil in addition to a source of P for crop production. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Worldwide, approximately 30% of the total land area consists of acid soils and 50% of the potential arable lands are acidic (von Uexküll and Mutert, 1995). In India, acid soils occupy about 45 million hectare (Mha) which is 30% of the total cultivated area (Panda, 2007). Acid soils, in general, are deficient in nutrients, particularly phosphorus (P). It is reported that the performances of water soluble phosphate (WSP) like single superphosphate (SSP) or triple superphosphate (TSP) in acid soils are very low because of high P-fixation due to the presence of iron (Fe) and aluminum (Al) in these soils. It is estimated that about 260 million tonnes (Mt) of rock phosphate (RP) deposits are available in India (FAI, 2012), most of which are categorized as lowgrade because of their low P content and unsuitable for commercial production of P-fertilizer. Application of RP for direct use in acid soils offers a potential utilization of this mineral phosphate as source of P. From economic point of view, these low-grade RP is considered as a suitable alternative to costly WSP fertilizer in acid soils (Narayanasamy et al., 1981). However, the feasibility or the agronomic effectiveness of lowgrade RPs for direct use as source of P in neutral to alkaline soils is less effective than that of SSP or TSP (Narayanasamy and Biswas, 1998). ⁎ Corresponding author. E-mail address: [email protected] (B.B. Basak).

http://dx.doi.org/10.1016/j.geoderma.2015.09.016 0016-7061/© 2015 Elsevier B.V. All rights reserved.

Apart from supplying P in soil for crop production, RP also imparts liming action due to its chemical reactivity in soil and could be used as 2− amendment in acid soils. There are three types of anions (PO3− 4 , CO3 and F−) present in the carbonate apatite structure of RPs, which may cause an increase in pH after dissolution. As the pKa value for hydrofluoric acid (HF) is so low F− has hardly any effect on increasing pH. But the pKa values for two protons on H2PO− 4 and H2CO3 were and CO2− into soluhigh enough to increase the pH by releasing PO3− 4 3 tion. The chemical reactivity of RP determines its potentiality and thereby its suitability as a direct source of P-fertilizer. The chemical reactivity can be defined as the rate at which P in apatite is released under favorable soil condition. The dissolution of RP in soil solution using fluorapatite as an example occurs according to the following hypothetical reaction (Hammond et al., 1986), resulting in the release of H2PO− 4 and Ca2+ in solution: Ca10 ðPO4 Þ6 F2 þ 12 Hþ →10 Ca2þ þ 6 H2 PO4 − þ 2 F− :

ð1Þ

Thus, dissolution of RP in soil, according to law of mass action, could be favored under conditions of low (a) soil pH, (b) soil exchangeable Ca, and (c) P concentration in soil solution. The above reaction has led to the view that soil acidity or proton (H+) supply is the single most important factor influencing the dissolution of a RP in soil. Further, as a result of dissolution of RP at the expense of H+ and concomitant release

B.B. Basak, D.R. Biswas / Geoderma 263 (2016) 104–109 2+ of H2PO− in soil system, the latter being considered as the 4 and Ca reason behind the liming effect of use of RP. It was found in an earlier study that application of SSP over a period of 6-years decreased the soil pH by 0.16 units, while application of RP maintained the soil pH level in various soils with initial pH ranging from 5.5 to 6.0 (Sinclair et al., 1993). He et al. (1996) reported that application of North Carolina RP to a P-deficient acid loam soil resulted in significant increase in soil pH from 4.0 to 4.3, indicating its potential use as liming material. However, no information is available on the potential use of low-grade Indian RPs as liming material in acid soils. The main objectives of this study were to assess the liming ability of low-grade Indian RPs and subsequently their effect on P availability when applied to an acid soil through various laboratory methods.

2. Materials and methods 2.1. Experimental soil To evaluate the liming potential of Indian RPs, an incubation experiment was conducted with an acid soil. For this, bulk surface soil (0–15 cm depth) was collected from the rice growing field of Kuttanad region of Kerala, India. The study area located at a latitude of 9°25′30″ N and longitude of 76°27′50″ E. Kuttanad is a region in the state of Kerala, India, well known for its vast rice fields and geographical peculiarities. The region has the lowest altitude in India, and is one of the few places in the world where farming is carried around 1.2 to 3.0 m below sea level. The bulk soil was air-dried under shade, passed through a 2-mm sieve after crushing with pestle and mortar and analyzed for different parameters namely, pH (soil:water ratio 1:2), (Jackson, 1973), electrical conductivity (EC) (Richards, 1954), cation exchange capacity (CEC) (Jackson, 1973), organic carbon (OC) (Walkley and Black, 1934), available P (Mehlich, 1953) and 1 N KCl extractable Al (Jackson, 1973). The soil belongs to subgroup of Typic Sulfaquents (Soil Survey Staff, 2010). The experimental soil is clay loam in texture having sand, silt and clay content of 63.7, 10.3 and 24.9%, respectively. It had pH, 4.53; EC, 1.39 dS m−1; CEC, 28.6 cmol (p+) kg−1 soil; OC, 3.76 g kg−1; available P (Mehlich-I P) 2.85 mg P kg− 1 soil; and KCl extractable-Al, 126.7 mg kg−1 soil.

2.2. Rock phosphates and their composition Four Indian RPs namely, Udaipur (Udaipur RP) from Rajasthan State Mines and Minerals Ltd., Udaipur, Rajasthan; Mussoorie (Mussoorie RP) from Pyrites, Phosphate and Chemicals Ltd., Dehradun, Uttara Khand; Jhabua (Jhabua RP) from Madhya Pradesh State Mining Corporation Ltd., Meghnagar, Madhya Pradesh and Purulia (Purulia RP) from West Bengal Mineral Development and Trading Corporation Ltd., Purulia, West Bengal were collected for this study. These four RPs were used as liming materials along with standard lime (CaCO3) in the present laboratory and incubation experiments. The chemical composition of the RPs (100 mesh particle size) was determined as per the standard procedure. The calcium carbonate equivalence (expressed as %CCE) of the RPs as a liming material was determined as per the method outlined by AOAC 955.01 (Kane, 1995). The %CCE represents the alkalinity of the liming material based on the alkalinity present as CaCO3 in it. It is assumed that the theoretical %CCE of chemically pure CaCO3 is 100. To determine the %CCE of RPs, sample of each RP (1.0 g) was digested with 0.5 N HCl for 5 min and cooled to room temperature. The acid digest was then titrated with 0.25 N NaOH solution to a pH value of 5.0 and 7.0 for the samples of RP and lime (CaCO3), respectively in order to quantify the excess amount of acid not neutralized by the liming materials.

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2.3. Incubation experiment An incubation experiment was carried out under laboratory conditions to see the liming effect of different Indian RPs obtained from various deposits. The treatments consisted of four RPs (i.e. Udaipur, Mussoorie, Jhabua and Purulia) and one lime (CaCO3) material applied at four rates of liming viz., 0.5, 1.0, 5.0 and 10.0 g kg−1 of soil replicated three times in a completely randomized design. An absolute control (soil alone) was also run along with the above treatments to see the changes in different parameters occurring from soil during the incubation period. The changes in different parameters in soil under control were subtracted from the values obtained in each treatment to account for the changes occurred only due to treatment application. Air-dried soil sample (100 g, 2-mm sieve) was taken on a clean polythene sheet. Required quantities of RPs and lime (CaCO3) were added as per the treatments details and mixed thoroughly with the soil. Soil moisture content was maintained by adding distilled water to air-dried soil at field capacity (23%, w/w). After thorough mixing, the moist soil sample was placed in 250 ml conical flask fitted with screw cap. The screw cap was tightened in such a way that it maintained an open space but minimizes the rate of water loss due to evaporation. The conical flasks were then incubated in an incubator and maintained at 25 ± 1 °C temperature for 90 days. During the incubation period, water content was maintained by taking the weight of the sample including conical flask and the loss of water was adjusted, if any, by adding distilled water. Assuming the weight of 1 ha of furrow soil (0–15 cm depth), application of each liming material was calculated and presented in Table 1. After 90 days of incubation, all the conical flasks were taken out and the screw caps were removed. After mixing, the soil samples were allowed to air-dry for 3 days in the conical flasks at ambient temperature (25 °C), then removed from the conical flask, crushed with a wooden pestle and mortar, passed through a 2-mm sieve and analyzed. The pH was measured using a pH meter at 1:2 ratio of soil:water. For the determination of available Al, soil sub-sample (5 g) was extracted by shaking the sample with 50 ml of 1 N KCl solution (Jackson, 1973) on a rotary shaker for 30 min at 170 rpm at an ambient temperature followed by filteration through a 0.2 μm millipore filter. The Al content in the extract was determined by an atomic absorption spectrophotometer (AAS). Available P in the soil was analyzed after extracting a sub-sample (10 g) with Mehlich I extractant (Mehlich, 1953) and the P content in the extract was determined spectrophotometrically after developing phosphomolybdate blue color method using ascorbic acid as reductant (Watanabe and Olsen, 1965). The amount of P dissolved from a RP was measured by the method as outlined by Bolan and Hedley (1989). To determine the amount of total P dissolved except P in the RP, a sub-sample of soil (1.0 g) was extracted by shaking the sample with 100 ml of 0.5 N NaOH solution on a rotary shaker for 16 h and filtered it through a 0.2 μm millipore filter. The amount of P released from limed soil without RP application was considered as the base level of NaOH extractable P. The amount of P dissolution from a RP was calculated from the difference between the NaOH extractable P in the RP treated soils and base level of NaOH extractable P. Table 1 Rock phosphate application rate in soil incubation study and calculation for field application. Rock phosphate application rate (g per 100 g soil)

Rock phosphate application rate (g kg soil−1)

Rock phosphate application rate in field (t ha−1)a

Phosphorus application rate (kg ha−1)b

0 0.05 0.10 0.25 0.50 1.0

0 0.5 1.0 2.5 5.0 10.0

0 1.1 2.2 5.5 11 22

0 93.4 186.9 467.0 934.0 1869.0

a b

Assuming 1 ha of arable soil (furrow slice) having weight of 2200 tonnes. On an average P content in Indian rock phosphate is 8.5%.

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3. Results and discussion 3.1. Theoretical calculations Based on the information available regarding chemical composition and solution chemistry of RPs, theoretical calculation of per cent calcium carbonate equivalence (%CCE) was worked out. Carbonate apatite is the predominant mineral in RPs (McClellan and Gremillion, 1980) with structural formula represented by Ca10-a-bNaaMgb(PO4)6x(CO3)xF0.4x(F)2. The compositions of carbonate apatite in various RPs are highly correlated with a-axis spacing which is determined through X-ray diffraction analysis (XRD) of RP (Smith and Lehr, 1966; McClellan and Gremillion, 1980). Specifically, the a-axis spacing is related with molar ratio CO3:PO4 as follows:

3.2. Observed values in laboratory

CO3 : PO4 ¼ z ¼ x=ð6  xÞ ¼ 4:9 ð9:374  a  axisÞ: Molar quantity of Na (a) and Mg (b) are found to be related to z as a = 1.237z and b = 0.515z. So, by mathematical manipulation of the equation, x value can be computed from z as: x = 6z/(1 + z). The anions present in the structure of RPs that can cause an increase 2− − in pH are PO3− 4 , CO3 and F . The capacity of each of the anions that can cause an increase in pH depends on two factors. Firstly, the molar quantity of anion present in RP structure; and secondly, the ability of anions to associate with protons to remove acidity from solution (Narayanasamy et al., 1981). The pKa values for first, second and third are 12.4, 7.2 and 2.2, respectively proton associations related to PO3− 4 from RPs is associ(Lindsay, 1979). It is found that any release of PO3− 4 ated with 2 protons where H2PO− 4 is the predominant phosphate form in the pH range from 4.0 to 6.0. The third proton association would only be possible if pH is less than 2.2. The pKa values for first and second protons association related to CO2− 3 are 10.3 and 6.4, respectively. In the pH rage from 4.0 to 6.0, the CO2− 3 is released into solution leading to removal of 2 protons from solution and formation of H2CO3 as predominant carbonate species. The role of fluorine ion (F−) in increasing pH is negligible as the pKa value for one proton association of F− is very low as compared to other anions (Smith and Martell, 1976). The chemical compositions of Indian RPs under study and their range of a-axes found in carbonate apatite structures are presented in Table 2. It was observed that with the increase in carbonate substitution in apatite structure, solubility of the minerals also increased which might lead to higher agronomic effectiveness of RPs in acidic environment. In the present study, the moles of CO23 − in different RPs were found to vary from 5.69 in Mussoorie RP to 5.98 in Udaipur RP and PO3− in apatite was (Table 2). The %CCE contribution from CO2− 3 4 calculated as: %CCE from CO3 2− or PO4 3− ¼ moles anion=mwCA  2 protons=1 mol anions −1  100 g CaCO3 mole =2 protons  100

where, mwCA is the molecular weight of carbonate apatite; molecular weight of CaCO3 is 100; protons i.e. number of proton neutralized. Since there is negligible %CCE contribution from F−, the total %CCE values of RPs were calculated as the sum of the %CCE contribution from CO23 − and PO34 −. Based on the calculation, it was revealed that was much greater than PO3− (Table 2) %CCE contribution from CO2− 3 4 present in apatite as comdue to the higher amount of moles of CO2− 3 pared to PO3− 4 . Very little difference was observed in total %CCE contriband PO3− uted from both CO2− 3 4 . The standard deviation computed in 2− and total %CCE were 1.32, 1.40 and %CCE from CO3 , %CCE from PO3− 4 0.91, respectively. Small differences in total %CCE values were observed in different RPs which might be due to increase in carbonate substituin apatite structure. tion which leads to decrease in moles of PO3− 4

ð2Þ

Laboratory titration was conducted to determine the %CCE of different RPs as per the standard procedure outlined by AOAC method 955.01 for lime material (Kane, 1995). The usual method involves dissolving the liming material in an excess of HCl solution and then back titrating the residual HCl with standard NaOH to a pH value of 7.0. Here, the method was modified by dissolving the RPs in an excess HCl and residual (unreacted) HCl was back titrated with NaOH to a pH value of 5.0. The usual method works fine for carbonate minerals because in HCl solution CO2− 3 consumes two protons to form H2CO3, which in turn, dissociates to H2O and CO2 and emit out from the system. But in case of phosphate mineral, release of PO3− 4 in solution does not represent complete removal of acidity from solution. It is reported that during dissoluis released into solution to form H3PO4. tion of carbonate apatite, PO3− 4 For each mole of PO34 − released, three moles of H+ are neutralized. Unlike H2CO3, H3PO4 remains in solution and during back titration, along with excess HCl, the proton present in H3PO4 can be titrated and estimated as excess acidity. In the present study, titration was conducted to an end point of pH 5.0 for determination of %CCE in RPs to overcome this problem. At pH 5.0, only one proton of H3PO4 would be titrated to form H2PO− 4 which would be the most dominant phosphate species in soils from pH 4 to 6 where the liming effect of RPs would take place. It is evident that the %CCE values obtained from the present laboratory titration method were significantly lower than the theoretical values (Table 3). This might be due to formation of precipitate of dicalcium phosphate (CaHPO4) or dicalcium phosphate dehydrate [CaHPO4·2H2O] which would restrict the neutralization of 2 protons per mole PO34 − released from carbonate apatite. The formation of these secondary reaction products might have released proton from H2PO− 4 present in solution that would be computed as excess HCl. It is very difficult to remove this problem from the system because the dissolution of RPs creates a concentrated solution of calcium and phosphate ions which start to react as the pH is increased during titration. Visible precipitate was clearly found above pH 5.0 but below pH 5.0 precipitate was not visible. But still lower %CCE values was obtained as

Table 2 Theoretical calculation of CCE of Indian rock phosphates. Rock Phosphate (RP)

a-axis (A0)

mwa

Mole of CO2− 3

Mole of PO3− 4

Mole ratio 3− CO2− 3 :PO4

%CCE due to CO2− 3

%CCE due to PO3− 4

Total %CCE

Udaipur RP Mussoorie RP Jhabua RP Purulia RP

9.373 9.352 9.368 9.371

1009.3 989.1 1003.7 1008.0

5.98 5.69 5.91 5.92

0.02 0.30 0.01 0.08

0.003 0.050 0.002 0.010

61.76 58.77 60.03 61.14 Avg. = 60.4 SD = 1.3

0.21 3.10 0.10 0.83 1.06 1.4

61.97 61.87 60.13 61.87 61.49 0.905

and PO−3 were expressed as mole per 10 mol Ca for comparisons with accepted formula Ca10(PO4)6F2 (Smith and Lehr, 1966). Amount of CO−2 3 4 + %CCE due to CO−2 Total %CCE = %CCE due to PO−3 4 3 . and PO−3 Sources (a-axis, mw, mole of CO−2 3 4 ): Smith and Lehr, 1966; Kaleeswari and Subhranian, 2001. a mw = molar weight.

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Table 3 Measured % CCE in different RPsa and lime (CaCO3) using AOAC method 955.01. Liming materials

Measured %CCE

St.dev.

Theoretical %CCE

% difference

Lime Udaipur RP Mussoorie RP Jhabua RP Purulia RP

94.60 53.12 51.86 49.17 52.21

1.66 1.42 1.37 1.92 1.32

100 61.97 61.87 60.13 61.87

5.40 14.28 16.15 17.52 15.59

a

An endpoint of pH 5, rather than 7 was used for RPs.

compared to theoretical calculation based on the assumption that 3− from RPs. H2PO− 4 would just form at pH 5.0 after release of PO4 3.3. Soil incubation experiment It is evident that determination of %CCE for RPs in laboratory titration through AOAC method did not represent the actual condition when RPs is applied to soil. In the process of RP dissolution, it is hypothesized that several reactions are taken place, resulting formation of various reaction products in soil which would interfere in further dissolution of RPs. Therefore, to get a more precise analytical determination of %CCE in RPs, an incubation experiment was conducted with an acid soil (pH 4.53) from Kuttanad region of Kerala by applying four Indian RPs as liming materials along with standard liming material (CaCO3). Data emanated from the incubation study revealed significant increase in pH due to addition of liming materials. It was further noticed that much greater influence on increase in pH due to application of standard liming material (CaCO3) was found in soil where there was an increase in pH as compared to the RPs treated soil (Fig. 1), irrespective of different rates of application and days of incubation period. Among the sources, Udaipur RP was found to have greater effect in increasing pH than others. This might be due to greater carbonate substitution in Udaipur RP and thus more solubilization which increased soil pH to a greater extent. Similar trend was observed in case of 1 N KCl extractable Al in soil due to application of CaCO3 and RPs (Fig. 2). The liming material (CaCO3) was found to be most effective in reducing extractable Al. Among the RPs, Udaipur RP was found to be most effective in reducing extractable Al in this acid soil, whereas Mussoorie RP was found to be least effective. It was observed that initially the soil had very high Al content (126.7 mg kg−1) which decreased significantly due to application of RP at the highest level and brought down to a level below 15 mg Al kg−1 soil that may be considered as safe for plant growth (Brown and Johnston, 1982). Under acid soil environment P released from RP structure immediately react with Al ions present in soil solution and convert it into Al phosphate which ultimately reduce the Al toxicity. It was observed that although the RPs were less effective in increasing the pH of the acid soil as compared to the liming material but here acid soils need lime to increase pH just up to 5.5 where toxic

Fig. 1. Soil pH as affected by application of liming material (CaCO3) and various rock phosphates. Bar indicates the STDEV value (n = 3) (URP, Udaipur Rock phosphate; MRP, Mussoorie Rock phosphate; JRP, Jhabua Rock phosphate; PRP, Purulia RP).

Fig. 2. 1 N KCl extractable Al (mg kg−1) in soil as affected by soil application of liming material (CaCO3) and various rock phosphates. Bar indicates the STDEV value (n = 3) (URP, Udaipur Rock phosphate; MRP, Mussoorie Rock phosphate; JRP, Jhabua Rock phosphate; PRP, Purulia RP).

level of Al could be eliminated. It was found that though liming effect of RPs increased pH of acid soil to a few units, the reduction in available Al content in soil might have significant role in improving plant growth environment in highly acidic soils as observed in the present study. Similar results were observed by other workers where significant increase in soil pH and decrease in exchangeable Al content occurred in the soil incubated with different organic wastes like coconut husk compost, poultry manure and vermicompost (Wong et al., 1998; Swarnam and Velmurugan, 2014). Phosphorus content in soil was measured after extraction with NaOH in the incubation study. It was observed that significantly higher per cent of P dissolution was found at lower application rate of RPs. Application of only liming material (CaCO3) without any P addition resulted in substantial amounts of P extracted from the soil. These results might be attributed to decrease in phosphate adsorption on amphoteric soil surface as the pH increase with lime application. It was also observed that with increase in rates of application of liming material, P dissolution decreases due to increase in pH. This increase in pH might have decreased the efficiency of NaOH to extract P. Results also revealed that application of liming material showed very low values of Mehlich-I extractable P as compared to soil treated with RPs (Fig. 3) indicating that the acidic Mehlich I extractant is not efficient in extraction of soil P, particularly the P present as Al-phosphate. The increase in Mehlich I extractable P with increase in rate of RPs, application was due to contribution of P from RPs. It is reported that P remains in acid soil dominantly as Al phosphate. During incubation of RPs in acid soil, P present in RPs undergone solubilization due to the presence of H+, resulting release of P which, in turn, formed Al-phosphate with

Fig. 3. Available P (mg kg−1) in soil as affected by soil application of liming material (CaCO3) and various rock phosphates. Bar indicates the STDEV value (n = 3) (URP, Udaipur Rock phosphate; MRP, Mussoorie Rock phosphate; JRP, Jhabua Rock phosphate; PRP, Purulia RP).

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combines with 2 protons to form H2PO− 4 , the predominant P form in acid soil solution. The ideal situation does not exist in soil environment as it is a complex system having different organic and inorganic constituents. There is possibility of formation of initial reaction product (CaHPO4 or CaHPO4·2H2O) just after RP application that causes hindrance in proton neutralization which is not expected in ideal situation. The same chemical reaction was not likely to occur in soil because of different chemical environment such as dilute soil solutions and low soil pH values. However, AlPO4·2H2O and FePO4·2H2O precipitation may have occurred and reduced proton neutralization. On the other hand, sorption of H2PO− 4 onto clay surface would release hydroxyls and increase the alkalinity of the system (Sample et al., 1980).

Fig. 4. Phosphorus dissolution (%) as affected by application of various rock phosphates in soil. Bar indicates the STDEV value (n = 3) (URP, Udaipur Rock phosphate; MRP, Mussoorie Rock phosphate; JRP, Jhabua Rock phosphate; PRP, Purulia RP).

interaction of Al present in soil. It was also observed that Mehlich-I extractant efficiently extracted P from Al-phosphate and same is reflected in result. The %CCE was calculated for each of RP application rate by comparing the effect of liming material (CaCO3) on soil pH as standard curve. The lime equivalence (LE) of RPs applied was worked out via lime (CaCO3) application up to 1.0 g kg−1 soil based on the assumption of complete dissolution of materials. Here %CCE of each RP was calculated based on the effect of RP application on soil pH. The resultant function of RP dissolution versus soil pH curve was used in the calculation of lime equivalence. The lime equivalence (mmolc/kg soil) was determined for each application rate from the above resultant function. The %CCE of the material was determined with following equation: %CCE ¼ LE ð0:1 kg soilÞ  100 mg CaCO3  100=2 mmolc  RP wt ð3Þ

where, RP wt is the mg weight of RP applied to 0.1 kg of soil. The %P dissolved from different RPs and %CCE are presented in Figs. 4 and 5, respectively. The %P dissolved and %CCE followed the similar trend due to application of RPs in soil where higher %P dissolution was observed with RP having greater %CCE. The %CCE and %P dissolved followed the same order as the effect on soil pH with URP N JRP N MRP N PRP. The disparity between observed value and theoretical value of %CCE was noticed in the present study. Despite of having differences in theoretical and observed value of %CCE, the theoretical value did well in predicting qualitatively the %CCE of applied RPs into the acid soil but only in case of lower rate of RP application. The variation between observed and predicted %CCE was probably due to prevalence of differential chemical environments. The theoretical %CCE calculation is based on the assumption that PO34 − releases from RPs

Fig. 5. Per cent CCE as affected by application of various rock phosphates in soil. Bar indicates the STDEV value (n = 3) (URP, Udaipur Rock phosphate; MRP, Mussoorie Rock phosphate; JRP, Jhabua Rock phosphate; PRP, Purulia RP).

4. Conclusions It can be concluded from the series of three experiments that although theoretical calculation did not match with the experimental findings but %CCE of RPs in incubation study followed the similar pattern as those values obtained in the theoretical calculation. Thus, theoretical calculation validates well in predicting %CCE values of Indian RPs in acid soil. Results demonstrated that application of RPs in acid soil improved the P availability through increase in soil pH with substantial decrease in exchangeable Al content. Though RPs was less effective as liming material in increasing soil pH as compared to CaCO3, there was an increase in soil pH treated with the former which is sufficient enough to decrease the content of Al much below the toxic and safe limit for plant growth. The higher per cent P dissolution from the application of RP having more %CCE strongly suggests the liming ability of RPs. Thus, low-grade RPs could be a potential option as liming material in acid soil in addition to as a source of P for direct application. Further detailed study is needed to see the liming ability of RPs through geochemical model under field conditions at various locations which would provide better understanding for large scale application. Acknowledgments The senior author is thankful to the Director, ICAR-DMAPR, Anand for providing necessary facilities during the period of this investigation and Dr Geena Mathew, Scientist, ICAR-CPCRI for providing acid soil sample from Kerala. References Bolan, N.S., Hedley, M.J., 1989. Dissolution of phosphate rock in soils. 1. Evaluation of extraction methods for the measurement of phosphate rock dissolution. Fertil. Res. 19, 65–75. Brown, A.J., Johnston, J.A., 1982. Exchangeable aluminium in Victorian soils. Trace Element Review Papers, 1982. Agricultural Services Library, Department of Agriculture, Victoria. FAI, 2012. Fertiliser Statistics 2011–2012. The Fertiliser Association of India, New Delhi. Hammond, L.L., Chien, S.H., Mokwunye, A.U., 1986. Agronomic value of unacidulated and partially acidulated phosphate rocks indigenous to the tropics. Adv. Agron. 40, 89–140. He, Z.L., Baligar, V.C., Martens, D.C., Ritchey, K.D., Kemper, W.D., 1996. Factors affecting phosphate rock dissolution in acid soil amended with liming materials and cellulose. Soil Sci. Soc. Am. J. 60, 1596–1601. Jackson, M.L., 1973. Soil Chemical Analysis. Prentice Hall of India, New Delhi, India. Kaleeswari, R.K., Subhranian, S., 2001. Chemical reactivity of phosphate rocks — a review. Agric. Rev. 22, 121–126. Kane, P.F., 1995. Agricultural liming materials. In: Cunniff, P. (Ed.), Official Methods of Analysis of AOAC International. AOAC International, Arlington, VA. Lindsay, W.L., 1979. Chemical Equilibria in Soils. John Wiley and Sons, New York. McClellan, G.H., Gremillion, L.R., 1980. Evaluation of phosphatic raw materials. In: Khasawneh, F.E. (Ed.), The Role of Phosphorus in Agriculture. ASA, CSSA, SSSA, Madison, WI. Mehlich, A., 1953. Determinations of P, Ca, Mg, K, Na, and NH4, North Carolina Soil Testing Div. Mimeo. NC Dep. Agric., Raleigh. Narayanasamy, G., Biswas, D.R., 1998. Phosphate rocks of India—potentialities and constraints. Fertil. News 43, 21–32. Narayanasamy, G., Ghosh, S.K., Sarkar, M.C., 1981. Chemical and mineralogical composition of phosphate rock deposits occurring in India. Fertil. News 25, 3–9. Panda, N., 2007. Management of acid soils. In: Ratta, R.K. (Ed.), Nutrient Management in Acid Soils. Bull. Indian Soc. Soil Sci. 25, pp. 1–9.

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