Rock phosphate enriched compost: An approach to improve low-grade Indian rock phosphate

Bioresource Technology 97 (2006) 2243–2251

Rock phosphate enriched compost: An approach to improvelow-grade Indian rock phosphate

D.R. Biswas *, G. Narayanasamy

Division of Soil Science and Agricultural Chemistry, Indian Agricultural Research Institute, New Delhi 110 012, India

Received 29 November 2005; received in revised form 23 January 2006; accepted 5 February 2006Available online 20 March 2006

Abstract

In this study, rock phosphate enriched composts (RP-compost) were prepared by mixing four low-grade Indian rock phosphates withrice straw with and without Aspergillus awamori. RP-compost had higher total P, citrate soluble P (CSP), organic P (Org.P), acid andalkaline phosphatase activities, and lower water soluble P (WSP) and microbial biomass C (MBC) than normal compost. Inoculationwith A. awamori increased total P, WSP, CSP, Org.P, MBC and acid phosphatase activity. RP-compost recorded lower Olsen P atthe initial period of incubation study than diammonium phosphate (DAP), but improved significantly with the progress of time. RP-compost prepared at 4% charged rate resulted in higher Olsen P throughout the incubation period compared to 2% charged rate. Similartrend were obtained with those RP-composts prepared with A. awamori. Data on pot experiment revealed higher yield and P uptake bymungbean (Vigna radiata) due to addition of RP-composts over control. The effectiveness of RP-compost ranged from 61.4% (Mussoo-rieRP-compost) to 94.1% (PuruliaRP-compost) as that of DAP on dry matter yield and 48.8% (JhabuaRP-compost) to 83.7% (Puru-liaRP-compost) on total P uptake. Enriched compost prepared at 4% charged rate recorded 15.8% and 10.6% extra yield and Puptake, respectively by mungbean over 2% charged compost. Also RP-compost inoculated with A. awamori resulted in 13.0 and21.5% extra yield and P uptake than without A. awamori treated group. Thus, RP enriched compost could be an alternative and viabletechnology to utilize both low-grade RPs and rice straw efficiently.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Rock phosphate; Rice straw; Compost; Aspergillus awamori; Olsen P

1. Introduction

Phosphorus (P) is the second limiting nutrient afternitrogen in majority of soils for crop production. The costof applying conventional water soluble P fertilizer is high inIndia because their manufacture requires importing high-grade rock phosphate (RP) and sulphur. Thus, alternativeuse of indigenously available low-grade RP is gainingimportance in India. It is estimated that about 260 milliontonnes (Mt) of RP deposits are available in India (FAI,

0960-8524/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biortech.2006.02.004

* Corresponding author. Tel.: +91 11 25841921; fax: +91 11 25841529.E-mail address: [email protected] (D.R. Biswas).

2002) and only a fraction of it (about 5.27 Mt) meets thespecification of the fertilizer industry because of their lowP content (low-grade). Most of the RPs are reasonably suit-able for direct use in acid soils, but has not given satisfac-tory results in neutral to alkaline soils (Narayanasamy andBiswas, 1998). Some methods for improving the efficienciesof these materials are mixing with elemental sulphur(Basak et al., 1987), partial acidulation with nominalamount of acid (Hammond et al., 1986; Stephan and Con-dron, 1986; Biswas and Narayanasamy, 1998) and drycompaction with water soluble P fertilizers (Menon andChien, 1996; Begum et al., 2004). However, the feasibilityof commercial production for P fertilizers by partial acidu-lation and compaction are low due to the cost involved inthose methods.

mailto:[email protected]

Mg

(%)

S(%

)F

e(m

gk

g�1)

Mn

(mg

kg�

1)

Zn

(mg

kg�

1)

Cu

(mg

kg�

1)

203.

48±

0.13

0.40

±0.

0558

70±

2590

4±

521

3±

840

±3

505.

88±

0.23

0.52

±0.

0770

20±

4012

00±

2548

6±

1170

±4

205.

64±

0.14

0.31

±0.

0687

20±

5036

80±

2163

1±

1212

0±

530

5.64

±0.

140.

39±

0.03

6850

±35

2764

±16

261

±5

70±

4

P=

Pu

ruli

aro

ckp

ho

sph

ate;

Ud

aip

urR

P=

Ud

aip

ur

rock

ph

osp

hat

e.(i

i)W

SP

=w

ater

solu

ble

P;

%;

P=

0.03

%;

K=

0.95

%an

dC

/Nra

tio

=86

.2.

2244 D.R. Biswas, G. Narayanasamy / Bioresource Technology 97 (2006) 2243–2251

The loss of soil organic matter due to intensive agricul-ture is responsible for a decrease in soil fertility. The mostcommon practice to preserve and/or restore soil fertility isto add organic matter to these soils regularly. Therefore, inpresent day agriculture, the recycling of agricultural andindustrial wastes is of prime importance not only becauseit adds much needed organic matter that improves physicaland microbiological properties of soil but also supplementssufficient amount of nutrients to the soil. It is estimatedthat about 246 Mt of cereal straw is available annually inIndia (Ramaswamy, 1999). A substantial amount of which(mainly rice straw) is burnt in the field after the harvest ofcrop in order to clean the field for growing the next cropbecause of labour constraints with consequent losses ofnutrients and CO2 inputs to atmosphere.

Considering all these factors, there is a need to develop acost effective, eco-friendly and sustainable system where thesupply of P to plants can be ensured. In this respect prep-aration of rock phosphate enriched compost (RP-compost)using crop residue holds a lot of promise in developingcountries like India. The present study was therefore,undertaken to explore the possibility of increasing theavailability of P from low-grade RP incorporated duringdecomposition of rice straw along with and without phos-phate solubilizing microorganism (PSM) viz. Aspergillus

awamori, where potential biochemical transformations ofP could be expected and to evaluate the products througha soil incubation study and a greenhouse experiment.

stin

gw

ith

rice

stra

w

CIS

P(%

)K

(%)

Ca

(%)

.06

6.15

±0.

090.

13±

0.02

9.0

±1.

.04

7.06

±0.

090.

39±

0.01

12.0

±0.

.05

8.62

±0.

100.

36±

0.01

7.8

±0.

.06

7.36

±0.

090.

19±

0.02

6.4

±0.

P=

Mu

sso

ori

ero

ckp

ho

sph

ate;

Pu

ruli

aRic

est

raw

con

tain

edO

C=

43.1

%;

N=

0.5

2. Methods

2.1. Rice straw

Rice straw (Oriza sativa L.) was collected after harvestat the experimental farm of Indian Agricultural ResearchInstitute (IARI), New Delhi, India. It was air-dried andchopped into small pieces, about 5–6 cm in length. Ondry matter basis, it contained (%) 43.1 total C, 0.50 N,0.03 P, 0.95 K and the C/N ratio of 86.2.

Tab

le1

Ch

emic

alco

mp

osi

tio

no

fro

ckp

ho

sph

ates

use

dfo

rco

mp

o

Ro

ckp

ho

sph

ate

To

tal

P(%

)W

SP

(%)

CS

P(%

)

Jhab

uaR

P7.

25±

0.09

0.00

3±

001

1.10

±0

Mu

sso

ori

eRP

8.25

±0.

050.

001

±0.

01.

19±

0P

uru

liaR

P9.

87±

0.18

0.00

4±

0.00

11.

25±

0U

dai

pu

rRP

8.62

±0.

080.

002

±0.

001

1.26

±0

No

te:

(i)

Jhab

uaR

P=

Jhab

ua

rock

ph

osp

hat

e;M

uss

oo

rieR

CS

P=

citr

ate

solu

ble

P;

CIS

P=

citr

ate

inso

lub

leP

.(i

ii)

R

2.2. Rock phosphate

Four Indian RPs, namely, Jhabua (JhabuaRP) fromMadhya Pradesh State Mining Corporation Ltd., Meghna-gar, Madhya Pradesh; Mussoorie (MussoorieRP) fromPyrites, Phosphate and Chemicals Ltd., Dehradun, Uttar-anchal; Purulia (PuruliaRP) from West Bengal MineralDevelopment and Trading Corporation Ltd., Purulia, WestBengal and Udaipur (UdaipurRP) from Rajasthan StateMines and Minerals Ltd., Udaipur, Rajasthan were usedfor the preparation of RP-composts. All the rock phos-phates were of sedimentary origin and categorised as low-grade because of their low P content. The chemical constit-uents of the RPs (100-mesh size particle) were determinedas per the standard procedure and given in Table 1.

D.R. Biswas, G. Narayanasamy / Bioresource Technology 97 (2006) 2243–2251 2245

2.3. Preparation of rock phosphate enriched compost

Firstly various RP-composts were prepared by mixingground RP @ 2 and 4 kg P (elemental P basis) per100 kg of air-dried rice straw (corresponding to 2% and4% charged rate) with and without phosphate solubilizingmicroorganism (PSM) viz. A. awamori. There were alto-gether eighteen treatment combinations [(4 RP · 2charged rate + 1 rice straw alone) · 2 PSM]. Each treat-ment was replicated three times in a completely random-ised design. Rice straw was soaked in water for 24 h andspread on a polyethylene sheet and RPs were mixed thor-oughly with it. To enhance activity of the natural andadded microflora, a uniform dose of urea solution @0.25 kg N per 100 kg of rice straw was sprayed. Freshcow dung @ 5 kg per 100 kg of rice straw was made intoslurry and sprinkled to each treatment as natural inocu-lums. A uniform dose of Trichoderma viride was inocu-lated @ 50 g per 100 kg of rice straw (on fresh myceliaweight basis) to hasten the composting. The whole compo-sting mass was then mixed thoroughly and placed in 10 Lglazed pots. The surface of the composting mass was plas-tered with slurry prepared from soil and fresh cow dung.The pots were covered with polyethylene sheets to avoidexcessive wetting by rain. Turning was done at monthlyintervals to provide adequate aeration. Moisture content(50–60% of field capacity) was maintained throughoutthe composting period. A. awamori was introduced @50 g per 100 kg of rice straw (on fresh mycelia weightbasis) one month after the starting of composting in orderto avoid thermophilic condition (about 55–65 �C tempera-ture) prevailing during the first month. Composting wascontinued till the C:N ratio reached a level between 10:1to 15:1 (after 130 days of composting).

2.4. Chemical and biological properties

Samples of matured composts were analysed for total P,water soluble P (WSP), citrate soluble P (CSP), citrateinsoluble P (CISP), microbial biomass C and acid and alka-line phosphatase activities for characterization. Compostsamples were oven-dried at 70 �C and ground to passthrough a 20-mesh sieve size. Samples were digested as fol-low: one g compost was first digested in 25 mL of concen-trated HNO3 followed by 20 mL of 60% HClO4 and total Pcontent in the acid digest was determined by spectropho-tometer after developing the vanadomolybdo-phosphoricyellow colour complex in nitric acid medium (Jackson,1973). Water soluble P, CSP and CISP were determinedas per the procedure outlined by Fertiliser (Control) Order(1985). Organic P was determined by ignition method(Saunders and Williams, 1955). Microbial biomass C wasestimated by the chloroform fumigation incubationmethod as outlined by Jenkinson and Powlson (1976) usinga Kc value of 0.45 (Jenkinson and Ladd, 1981). Phosphatesactivities of compost samples were analysed as per theprocedure reported by Tabatabai and Bremner (1969).

2.5. Release of P from RP-compost in a soil

incubation study

An incubation study was carried out to monitor therelease of P from various RP-composts along with di-ammonium phosphate (DAP) as standard fertilizer. Bulksurface soil (0–15 cm depth) was collected from the experi-mental farm of IARI, New Delhi, India. The soil belongedto Mehrauli series, member of coarse loamy, non-acid,mixed hyperthermic family of Typic Haplustept. The phys-icochemical properties of the experimental soil as deter-mined by standard procedures were: texture sandy loam;pH (1:2, soil:water) 8.1; EC (1:2, soil:water) 1.1 dS m�1;organic C 4.3 g kg�1 soil; CEC 7.5 cmol (p+) kg�1 soiland available N, P and K 102, 3.5 and 94 mg kg�1 soil,respectively. To this soil the various RP-composts andDAP were added to provide 25, 50 and 75 mg P kg�1 soil.Thus, there were a total of 58 treatments [{(4 RP · 2charged rate + 1 rice straw alone) · 2 PSM + 1 DAP} · 3levels of P + 1 no-P control]. The incubation experimentwas laid out in a completely randomised design with threereplications.

Processed soil (500 g, 2 mm) was mixed thoroughly withthe compost and fertilizer materials on a polyethylenesheet. Water was added to each mixture so as to bringthe moisture content to 50% of the water holding capacity.These moist samples were then placed in plastic bottlesfitted with screw caps and incubated at 35 ± 1 �C. Moistsoil samples were drawn from each bottle after 15, 30, 45,60, 75, 90 and 120 days of incubation and analysed for0.5 M NaHCO3 (pH 8.5) extractable P (Olsen et al.,1954). Phosphorus in the extract was determined spectro-photometrically using ascorbic acid as reductant (Watan-abe and Olsen, 1965). Water content in the moist sampleswas determined by drawing a sub-sample at each sampling,which facilitated to maintain the loss of soil moisture dueto evaporation by periodic addition of water and to expressthe results on the basis of weight of dry matter.

2.6. Greenhouse study

The effectiveness of various RP-compost as a source of Pwas evaluated through a pot experiment using mungbean(Vigna radiata) as the test crop. The mungbean wasselected as the test crop because it is a legume and knownto be better utilizers of P from RP than non-legumesthrough accelerated dissolution by the roots. The soil wasthat of the previous incubation experiment and phosphatewas applied @ 50 and 100 mg P kg�1 soil. There were 39treatments [{(4 RP · 2 charged rate + 1 rice strawalone) · 2 PSM + 1 DAP} · 2 levels of P + 1 no-P con-trol]. The pot experiment was carried out in triplicate ina completely randomised design. Processed soil (<5 mm)was filled in polyethylene lined earthen pots having 5 kgsoil capacity. Entire quantities of composts and fertilizermaterials were applied as basal and mixed thoroughly withthe soil. Nitrogen was applied as urea (25 mg N kg�1 soil)


and potassium (50 mg K kg�1 soil) as potassium chloride,both as in solution form and mixed thoroughly with soil.Water was added to each mixture to bring the moisturecontent 50% of the water holding capacity. Ten seeds ofmungbean were sown in each pot, which after germinationwas thinned to five plants per pot. The pots were keptweed-free and maintained in an optimum soil moistureregime throughout the experiment. At maturity, the cropwas harvested and the dry matter as well as pod and stoveryields were recorded after drying at 65 ± 1 �C. Concentra-tion of P in plant samples was determined according to themethod described by Jackson (1973). The agronomic effi-ciencies of the various RP-composts were computed takingDAP as a standard by the relationship given below:

Agronomic efficiency ¼ ðYield or P uptake in treated pot� yield or P uptake in control potÞðYield or P uptake in DAP treated pot� yield or P uptake in control potÞ � 100

2.7. Statistical analysis

The data obtained from the composting, incubation andpot experiments were subjected to the analysis of varianceappropriate to the experimental design. F-test was carriedout to test the significance of the treatment differencesand compared with the critical difference (CD) at 5% levelof probability by the Duncan’s test.

3. Results and discussion

3.1. Chemical and biological properties of enriched

compost

Rock phosphate enriched compost had significantlyhigher content of total P (2.20% P) compared to strawcompost (0.37% P) where no RP was added. However,RP-compost resulted in significant reduction in WSP(2.1% of total P) than straw compost (15.8% of total P).The reduction of WSP content in compost due to additionof RP could be explained as dilution effect of it in a rela-tively larger mass of the compost material. The result con-firmed the findings of Bangar et al. (1985). The decreasein WSP on addition of RP could also be due to reactionof soluble P with other components (e.g. CaCO3) of RP(Singh, 1985). The RP-compost, on the other hand, hadsignificantly higher CSP (0.72% P) compared to straw com-post (0.10% P). This was obvious because of production oforganic acids like citric, oxalic, tartaric, etc. during compo-sting of organic matter, which in turn, enhanced the disso-lution of P from RP. Further, lots of CO2 evolved duringthe process of decomposition of organic manures andresulted in the formation of weak carbonic acid, which dis-solved RP and rendered the availability of P, thus increasesthe efficiency of RP (Chien, 1979). The results were in closeconformity of the work done by others (Mathur et al.,1980; Bangar et al., 1989). Similarly, significantly higher

amount of organic P as well as available P(WSP + CSP + Org.P) was also observed in RP-compostthan straw compost. In earlier study, Biswas et al. (1996)reported that P from low-grade RP could be mobilizedby using fresh cow dung and pyrite. However, RP-composthad lower microbial biomass C than straw compost possi-bly because of dilution of C in relatively larger mass ofcomposting material. Significantly higher amount of acidand alkaline phosphatase activities were also observed inRP-compost than in ordinary compost.

Out of four RP used in this study, PuruliaRP-compostrecorded higher total P (2.28% P) as well as CSP (0.89%P) and organic P (0.15% P) than the other RP-composts.UdaipurRP-compost yielded maximum WSP (0.06% P)

than other enriched composts. PuruliaRP-compost hadthe highest (1.09% P) available P (WSP + CSP + Org.P)followed by UdaipurRP (0.92% P) = MussoorieRP(0.88% P) > JhabuaRP (0.65% P). The highest microbialbiomass C was observed in MussoorieRP-compost thanothers probably because of presence of higher amount ofCaCO3 in raw RP. Both acid and alkaline phosphataseactivities were also highest in MussoorieRP-compost. Sig-nificantly higher total P and WSP were found in compostsprepared with 4% charged rate compared to 2% chargedrate. However, in terms of per cent of total P, addition ofRP @ 4% charged rate had lower WSP compared to 2%charged rate. This is because of dilution of WSP in a rela-tively larger mass of the substrate. Composts prepared athigher charged rate had significantly higher CSP (0.84%)over lower charged rate (0.61%). Increase in charged ratealso increased the organic P from 0.11% to 0.13% P. How-ever, the extent of increase of organic P in terms of total Pwas lower at 4% charged rate (4.7% of total P) compared to2% charged rate (6.1% of total P). Singh et al. (1982)reported that both organic and CSP increased significantlywith addition of RP but the higher level of RP reduced thecontent of both forms drastically. The carbonic acid andorganic acids produced during the decomposition oforganic matter solubilized insoluble phosphate in the RP,resulting in the release of phosphate and calcium into thesolution (Chien, 1979). Thus, preparation of RP enrichedcompost is based on the concept of solubilization of insol-uble RP into plant available form (water and citrate solubleforms) during the process of composting. Most of thesereleased phosphates are taken up by microflora and the restis re-fixed due to the abundance of calcium in the system(Singh et al., 1982). Significant increase in microbial bio-mass C as well as acid phosphatase activity was obtainedwhen charged rate increased from 2% to 4%. However,alkaline phosphatase activity due to 2% and 4% chargedrate was at par.

0

2

4

6

8

10

12

14

16

18

20

15 30 45 60 75 90 120

Period of Incubation (Days)

Ols

en P

in s

oil

(mg

/kg

)JhabuaRPMussoorieRPPuruliaRPUdaipurRP

CD (P=0.05)= 1.37

Note: ‘Ι’ indicates +/- variation

Fig. 2. Changes in 0.5 M NaHCO3 extractable P as affected by various


Inoculation with A. awamori into the composting massincreased the content of total P (2.35%) as well as WSP(0.05% P) and CSP (0.85% P) significantly. Introductionof A. awamori increased the CSP possibly due to releaseof organic acids in greater extent through microbial medi-ated processes during composting. Increased dissolution ofdirectly applied RP in soil incubated with crop residues andPSM had earlier reported by Sushama et al. (1993). Also,significant increase in organic P was observed due to inoc-ulation with A. awamori (5.5% of total P) as compared tothe control (4.9% of total P). The results indicated thatinoculation of A. awamori helped in converting more ofinsoluble P into organic P. On the other hand, introductionof A. awamori decreased the per cent CISP compared tothose not inoculated with it, possibly due to larger conver-sion of P to either WSP or CSP. Kapoor et al. (1983) andMishra et al. (1984) also reported that the same fungustransformed the P in RP from insoluble to soluble form.Microbial biomass C and acid phosphatase activityincreased significantly when RP-compost was inoculatedwith A. awamori. The result was reverse in case of alkalinephosphatase activity. This might be attributed to the factthat A. awamori releases various acids during the processof composting, which in turn, increased acid phosphataseactivity and decreased alkaline phosphatase activity.

3.2. Incubation experiments

The results obtained from incubation experimentshowed that addition of RP-compost resulted in elevated0.5 M NaHCO3 (pH 8.5) extractable P in soil throughoutthe incubation period of 120 days (Fig. 1). Further, OlsenP status in soil treated with DAP was markedly highercompared to soil treated with RP-compost. The latterhad lower Olsen P during the initial stages, but improvedsignificantly with the progress of time, indicating thatRP-composts released P for a longer period. In general,

0

5

10

15

20

25

30

15 30 45 60 75 90 120


Ols

en P

in s

oil

(mg

/kg

)

Straw compostRP-Charged compostDAP

CD (P=0.05)= 1.31


Fig. 1. Changes in 0.5 M NaHCO3 extractable P as affected by RP-charged composts.

the Olsen P in soil treated with any of the P sourcesdeclined for up to 45 days of incubation. This may bedue to fixation of available P into unavailable forms. Thisresult corroborates the findings of other workers (Bangaret al., 1985; Mishra and Bangar, 1986). There was a generaltrend of increase in Olsen P in soil after 45 days of incuba-tion. This indicates the slow conversion of insoluble phos-phate into available forms of P, which can be used by theplants at the later stages of crop growth.

Amongst the sources, UdaipurRP and PuruliaRP-composts recorded significantly higher Olsen P in soil overMussoorieRP and JhabuaRP-composts up to 45 days ofincubation (Fig. 2). Thereafter, no significant differenceswere observed amongst the sources of RPs. Compost pre-pared at 4% charged rate had significantly higher amountof Olsen P in soil compared to compost prepared at 2%charged rate (Fig. 3). This was obvious because of higheravailable P (WSP + CSP + Org.P) in the former group ofproducts. Further, RP-compost inoculated with A. awamori

0

2

4

6

8

10

12

14

16

18

15 30 45 60 75 90 120


Ols

en P

in s

oil

(mg

/kg

)

CD (P=0.05)= 0.98 2% Charged rate4% Charged rate


Fig. 3. Changes in 0.5 M NaHCO3 extractable P as affected by chargedrate.

sources of rock phosphates.

Fig. 6. Yield (g pot�1) of mungbean as affected by various RP-composts.

Without AspergillusWith Aspergillus

0

2

4

6

8

10

12

14

16

18

15 30 45 60 75 90 120


Ols

en P

in s

oil

(mg

/kg

)

CD (P=0.05)=0.85

Note: ‘ Ι ’ indicates +/- variation

Fig. 4. Changes in 0.5 M NaHCO3 extractable P as affected by addition ofA. awamori.

0

2

4

6

8

10

12

14

16

18

20

15 30 45 60 75 90 120


Ols

en P

in s

oil

(mg

/kg

)

25 mg P50 mg P75 mg P

CD (P=0.05)=1.08


Fig. 5. Changes in 0.5 M NaHCO3 extractable P as affected by levels of Papplication.



had a significantly higher amount of 0.5 M NaHCO3 (pH8.5) extractable P in soil compared to compost preparedwithout it during the whole period of incubation (Fig. 4).Significantly higher 0.5 M NaHCO3 (pH 8.5) extractableP in soil was noticed with increasing levels of P application,which is obvious in P-deficient soils like the present one(Fig. 5).

3.3. Greenhouse experiments

Significant differences in the yield of mungbean podwere observed between no-P (control) versus rest as wellas DAP versus RP-compost (Fig. 6(a)). Stover yield alsoincreased significantly with addition of phosphate com-pared to no-P (control) treatment. Also, significantlyhigher stover yield was observed in DAP treated pots(8.55 g pot�1) compared to RP-compost treated pots(7.53 g pot�1). Amongst the sources, PuruliaRP-compost

Fig. 7. Phosphorus uptake (mg P pot�1) by mun

had maximum pod as well as stover yield and was at parwith UdaipurRP-compost and both of them recorded sig-nificantly higher yield of pod and stover compared to Jha-buaRP and MussoorieRP-compost (Fig. 6(b)). As far ascharging of RP is concerned, significantly higher yield wereobtained with 4% charged compost than 2% charged com-post (Fig. 6(c)). This may be attributed to higher amount oftotal P present in the former group of materials. The treat-ment receiving RP-composts along with A. awamori hadsignificantly higher yield than those without it (Fig. 6(d)).The effective utilization of different RPs in combinationwith A. awamori was obvious because these microorganismsecrete organic acids like citric, malic, tartaric, etc. andenzymes like acid and alkaline phosphatase, which helpedin bio-transformation of insoluble P to available form. Thiswas also evidenced from the higher amount of available P(WSP + CSP + Org.P) in the former group of compostmaterials. Significantly higher yield of pod and stover were

gbean as affected by various RP-composts.


also recorded by increasing rates of P application from50 mg to 100 mg P kg�1 soil. This result supports the find-ings of the incubation experiment where significantlyhigher amount of 0.5 M NaHCO3 (pH 8.5) extractable Pwas obtained with higher dose of P application.

Uptake of P by mungbean pod, stover and the total Puptake increased significantly with the application of phos-phatic materials compared to no-P (control) (Fig. 7(a)). Thiswas expected because of little available P in soil used for theexperiment. The DAP treated pots showed significantlyhigher P uptake by mungbean pod (20.12 mg P pot�1) andstover (14.07 mg P pot�1) in comparison to RP-composttreated pots. Amongst the RPs, the total P uptake by them(Fig. 7(b)) were in the order: PuruliaRP (30.80 mgP pot�1) = UdaipurRP (30.36 mg P pot�1) > MussoorieRP(24.58 mg P pot�1) = JhabuaRP (23.56 mg P pot�1). Inc-rease in charged rate from 2% to 4% also enhanced P uptakeby mungbean pod and stover (Fig. 7(c)). Significantincreases in P uptake by mungbean pod, stover as well asin the total P uptake were recorded for A. awamori treatedpots (Fig. 7(d)). Higher level of P application also resultedin higher P uptake by mungbean.

Amongst the sources of RP the effectiveness rangedfrom 67.0% (MussoorieRP) to 89.2% (PuruliaRP) as com-pared to DAP (100%) in case of pod yield and 56.0% (Mus-soorieRP) to 97.6% (PuruliaRP) in case of stover yield.These values due to 2% charged rate were 66.9% and66.4% as effective as that of DAP in case of pod and stoveryield, respectively which further increased to 92.8% and89.9% with 4% charged rate. The efficiencies due to inocu-lation with A. awamori increased from 72.3% (withoutPSM) to 92.5% (with PSM) as that of DAP in case ofpod yield and 70.3% (without PSM) to 91.2% (withPSM) in case of stover yield. The better efficiencies of com-posts inoculated with A. awamori appeared to be due togreater availability of phosphate because of more dissolu-tion of P from RP during composting. This result corro-borates the findings of Singh et al. (1982). Most of thissolubilized P was immobilized in the microbial cells as evi-denced by higher WSP and CSP and the increase in organicP content of compostable material during composting. Thismicrobial (organic) P acts as a slow release fertilizer due toits slow rate of decomposition and provides available P tothe plants for a longer period instead of fixation and/orprecipitation in soil minerals as in case of conventionalwater soluble P-fertilizers.

Similar trends in relative effectiveness in terms of Puptake by mungbean pod, stover and total uptake wereobserved due to RP, charged rate as well as inoculationwith A. awamori. The highest relative effectiveness on totalP uptake by mungbean was observed in PuruliaRP-com-post (83.7%) followed by UdaipurRP-compost (81.5%),MussoorieRP-compost (53.7%) and least in JhabuaRP-compost (48.8%). With increase in charged rate from 2%to 4% the relative effectiveness increased from 60.4% to73.7% and with inoculation of A. awamori it increased from59.4% to 86.0%.

4. Conclusions

The study revealed that addition of low-grade Indianrock phosphate along with A. awamori to crop residue dur-ing composting helped to enhance the mobilization ofunavailable P in rock phosphate to available forms of P,which in turn helped in supplying P to mungbean. Thus,RP-compost could be an alternative and viable technologyto utilize both low-grade rock phosphates and rice strawefficiently and could be used successfully as a cheapersource of P-fertilizer in place of costly water soluble P likediammonium phosphate in crop production.

Acknowledgements

The authors would like to express their gratitude to theIndian Council of Agricultural Research (ICAR), NewDelhi, India for providing financial help for this research.

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Rock phosphate enriched compost: An approach to improve low-grade Indian rock phosphate