Composting of an Igneous Rock Phosphate

INTRODUCTION

In the soil literature it is generally accepted that igneous phosphate rocks ( PR ) , metamorphosed igneous PR , metamorphosed sedimentary PR , and sedimentary PR with low degree of anionic substitution of PO4 by CO3 , are all of very limited fertilizer value when applied directly to most soils ( 4 , 6 , 10 , 14 , 15 , 22 , 31 , 32 , 45 ) . The crystallinity , physical size , and chemical nature of the calcium phosphate and its substitution products , all influence their reactivity , and thus solubility in soil and availability to plants ( 4 , 6 , 10 , 15 , 32 , 17 , 44 ) . The availability of the P in PR to plants is also influenced by plant and soil properties . Plants with higher capacities to generate H ions ( 12 ) , and absorb or adsorb PO4 and Ca ions ( PR ) .

The importance of the plant-nutrient contents or quality of manures , and of their maturation by proper decomposition , to soil fertility , were recognized more fully during evolution of the scientific principles of composting by Sir Albert Howard and others in India , about 50 years ago ( 16 , 35 ) .

Compost is distinct from a pile of rotting refuse in that it is a designed environment where the plant nutrients are conserved while the carbonaceous bulk is decomposed to a relatively biostable humus-rich residue in a manner that destroys weed seeds , phytoinhibitory substances , and pathogens , without creating malodors that emanate from or remain in the product . These objectives are met by mixing various materials into a formulation where the C:N ratio , water:air ratio , macro- and micropores , are adequate for the desired decomposition ( 16 , 35 , 38 ) .

It was established that aliphatic organic acids ( e.g. citric , lactic , succinic ) , phenols , phenyl carboxylic acids , amino acids ; and complex humic and fulvic acids , and mineral acid ions ( NO3 , SO4 , and CO3 ) are produced during the decomposition of organic matter in composts . In cold or anaerobic composts , the mineral acids are produced in lower amounts but the organic acids persist longer than in hot composts . Also , more heat is generated in aerobic ( hot ) composts than in the anaerobic composts . In both composts , phosphorus is usually a limiting factor for optimally rapid decomposition . In the same time , any sparingly soluble or insoluble calcium phosphates added into the composts would be in intimate contact with warm or hot solutions of organic and mineral acids that can solubilize the P source ( 16 ) . As the chelating organic acids remove the Ca from the active phase the released inorganic PO4 ions are ( a ) utilized by microorganisms , to produce bioavailable compounds ; or ( b ) loosely held by humic compounds with or without bridges of metal ions ( 7 , 19 ) . Even if some of the dissolved Ca and phosphate ions are reprecipitated , the products should be reactive amorphous compounds , such as α-tri-calcium phosphate ( TCP ) , but not as crystalline unreactive compounds , such as β-TCP ( Whitlockite ) ( 33 , 34 ) . Also it is likely that due to the presence of CO2 such reprecipitation would occur in a CO3:HCO3 buffer so that some of the PO4 ions would be substituted by CO3 ions . It can be said that reactive sedimentary PR were originally laid down in CO3:HCO3 buffers of animal body fluids , oceans and inland seas . It has been shown that the reactivity of sedimentary PR increases with increasing substitution of PO4 by CO3 ( 4 , 6 ) . When more than 1 in 6 of the PO4 ions are substituted by CO3 the structure breaks down .

It should be emphasized that the overall pH of the compost does not have to be acidic to dissolve PR , as sufficient microenvironments exist in the compost around microbial colonies where the pH is acidic enough to help dissolve the PR .

Initial studies with phosphates in composts were to improve the quality of the product and hasten the process ( 2 , 46 , 16 ) . It was also found that the P in the phospho-composts was more available to plants than even the comparable P from superphosphate , particularly in soils with high P fixing capacity ( 46 ) . This effect of the compost can be attributed to : ( a ) the coating of P-fixing sites by organic compounds , ( b ) the continued production of carbonic and other acids by the slow decomposition of the composts in the soil ( 1 , 2 ) , and ( c ) the organic P in the phospho-composts may be taken by plants through hydrolysis by enzymes exuded by them into the soil around 130 their roots , without going through the general soil matrix ( 3 , 13 , 47 ) .

Indeed much of the recent work on phospho-composts in India relates to the composting of a sedimentary PR to produce a material that is a suitable source of P for crops grown in calcareous soils of high P-fixing capacity ( 23 , 24 , 29 , 30 , 41 , 42 ) . The PR used most in studies reported recently from India is from Mussoorie . In this sedimentary PR , some of the Ca has been displaced by Na and Mg , and some of the PO4 ions by CO3 . It is therefore a reactive PR , easy to dissolve , quite unlike an igneous PR . Under conditions where igneous PR are produced , or when magma heat a sedimentary PR , the CO3 ions in carbonato-phosphates are expelled . The same occurs when bones are burnt so that the Whitlockite-containing bone ash is a much poorer source of fertilizer P than the bone meal containing mostly non crystalline hydroxy and carbonato-phosphates . Therefore , carbonate-phosphates were naturally not present in the igneous rock phosphate used in this study . The PR came from Cargill , Ontario , Canada . ( 9 , 39 ) . The karst-type residuum near the surface , under glacial till and lacustrine clay , shows signs of weathering up to the stage where much of the calcite , dolomite and FeS have been leached , and some Al released from degrading clay produced a layer of crandallite .

The beneficiated ore was found to be of little value as fertilizer when applied directly to a neutral sand , a calcareous clay or an acidic peat of low biological activity . In that sense the results were similar to those obtained by workers in Burundi ( 5 , 18 ) who found that Matongo-Bandaga PR of igneous origin was only 1 to 5% as effective as superphosphate-P in 4 soils , an Oxisol , an Ultisol , a Vertisol and an Entisol . However , in a soil with high humus content - a Gleyic Oxisol or a Humic Oxisol - the corresponding value was 12% - thus suggesting that even an igneous PR is subject to the solubilizing effect of humus , and decomposing organic matter . Many workers have reported beneficial effects of certain manuring treatments on the direct use of PR in soils . As we felt that such effects of decomposing organic matter , and of the humus and CO2 produced by it , can be intensified during composting , and in recognition of the need to utilize PR which is otherwise unsuitable for agronomic use , we tested the feasibility of composting an igneous rock phosphate from Cargill , Ontario , Canada .

MATERIALS AND METHODS

The 'hard' igneous phosphate rock ( PR ) from the Cargill deposit in northern Ontario , Canada , used in this study , is mostly macrocrystalline apatite with some crandallite , calcite and dolomite . The PR has 16% total P , 0.8% P soluble in citric acid , and 0.03% P soluble in ammonium citrate . About 44% of the beneficiated ore is >1 mm in size and 88% is >0.25 mm . Consequently , the crystallinity , chemical nature and physical state all render the PR unsuitable for direct application to soils , particularly to those rich in active Ca or Fe .

Aerobic ( Hot ) Composting
The compost heaps were generally 1 m high , trapezoidal in cross section , with base and top planes of 2 m × 1.0 m and 2 m × 0.3 m , respectively , according to a method developed in Canada ( 25 , 26 , 27 , 36 ) . The composting was done in July to October 1985 on a rough cement floor , under a roof , in an open shed , in Kapuskasing , northern Ontario , about 50 km from the Cargill deposit .

To build a compost heap , a basal 10 cm thick layer of peat or other fluffy material or mixtures used in the formulations ( tables 2A and 2B ) was laid loosely on the ground . Two 30 cm long ABS soil pipes of 10 cm diameter with two rows of perforation ( 1.2 cm diameter ) were placed lengthwise on the basal layer , about 0.3 m from the margins . The two rows of perforations were both on the top side of the pipes . The rest of the material was then heaped on the pipes and shaped to fit the design . A light covering layer of peat , about 5 cm thick , was used in the end to curtail loss of ammonia , emanation of odours , the attraction for , and access of the waste to , flies .

In the composts where liquid manure was used ( Compost Nos. 11 to 14 ) a third ventilation pipe was placed about 0.5 m from the floor in the middle of the heap .

Anaerobic ( Cold ) Composts
These compost heaps were made according to the method of preparing heap of grass silage using 135 , 4m thick silage grade black polyethylene plastic sheeting obtained from Chemical Industries Limited Canada . Other details on the process , handling of the materials and chemical analyses will be reported elsewhere .

RESULTS AND DISCUSSION
It is understandable that the finished products of the composting with the PR contained higher percent mineral matter than the composts without the rock phosphate ( table 3 ) . The "mobile" humus content of the phospho-composts was generally lower than that of the control composts , perhaps partly due to the lower organic contents of the phospho-composts .

In five of the eight pairs of composts , those with PR had slightly lower pH than the controls . This was not obviously related to the type of composting ( aerobic vs anaerobic ) . It appeared that the Ca released from the PR may have been preferentially complexed with humus ( truncated )

The various P fractions in the phospho-composts and the extent of PR solubilization in the different systems are given in tables 4 to 11 . The H2O-P fraction was lower in P-enriched composts : this is probably due to the reprecipitation of P by the Ca present in the system while the compost sample were suspended and shaken in water . The 0.1 N HCl extractible P is associated with reliable forms of plant available P in organic soils ( peats and mucks ) which , among soils , resemble the composts most closely . The organic P fraction is obtained through determination of total P with and without ignition . This is a very important fraction , particularly for Ca or Fe-rich soils , as indicated in the introduction . The 2% citric acid ( CA ) extraction is a standard method for comparing the P-fertilizer value of PR with superphosphate which has a solubility of 80% in 2% CA . The Al+P fraction ( P extractible in 0.1 N NaOH ) is included because the Cargill PR contains some crandallite .

Upon completion in 2 to 4 months , the extent of the total P added by the igneous PR that was solubilized into 'plant available' forms in the various composts was : 37.1% in LH + WW + peat ( Ae ) ; 40.2% in FYM + peat ( Ae ) ; 47.3% in FYM + S ( An ) ; 47.7% in FYM + peat ( An ) ; 52.8% in FYM + WW + Bl + peat ( Ae ) ; 62.8% in LH + peat ( An ) ; 63.9% in FYM + peat ( Ae ) ; and 74.4% in LM + peat ( Ae ) ( tables 12A and 12B ) . The average solubilization of the PR by the composting was 53.2% . In all the composts some of the PR can also be substantially solubilized by composting with various wastes .

The extent of solubilization was not dependent on the P content of the composts nor the type of composting , but perhaps on the intensity of the process which in turn was influenced by the properties of the materials used . The details of these aspects will not be discussed here due to the constraints of time and space , as the focus of this presentation is on the feasibility of solubilization of an igneous PR through composting .

With respect to processes responsible for PR solubilization , it is worth mentioning that the aliphatic organic acids ( e.g. citric , lactic , succinic ) , phenols , phenyl carboxylic acids , amino acids , and complex humic and fulvic acids , which all chelate Ca and other metals ; mineral acid ions ( NO3 , SO4 , CO3 ) ; and heat , all naturally produced during decomposition in immediate vicinity of the PR granules in phospho-composts , dissolve the PR mainly as by partial acidulation treatments . Such solubilization of igneous PR should occur during decomposition of various types of organic materials during properly designed composting .

The peat was of special value as an absorbent for liquid manure and as an insulator for maintaining heat in areas where the night temperature in mid-summer could be below 10°C . Although useful at all times , the peat is not necessary to solubilize PR when dealing with solid wastes . Acidic peat can help curtail loss of N as NH3 from protein-rich composting materials . However , the same can be achieved by using acidic muck , and wastes from banana , mango and citrus fruits , and , perhaps , some aquatic weeds .

Where commercial composting operations are desirable and feasible , one should look for materials of institutional origins such as urban and municipal garbage , and wastes from barracks , hotels , hospitals , offices , food processing plants , slaughter houses , breweries , forestry and other wood-related operations .

CONCLUSION
This study has shown that composting can substantially solubilize igneous PR into 'plant-available-P' forms , and thus increase the fertilizer value of the PR many times , especially for soils where the efficiency of P fertilization is low .