Phosphates in agriculture

Phosphate rich organic manure



The world today consumes around 150 million tonnes of high grade phosphate mineral, ninety percent of which goes into the production of chemical phosphatic fertilizers. Organic manure enriched with high grade rock phosphate mineral in fine size, is being proved to be as efficient as chemical phosphatic fertilisers.

Phosphorous is an important nutrient element for plants.  The element P is a constituent of  DNA and RNA molecules as well as ADP and ATP, the molelcules that transfer energy, facilitating biochemical processes in the plants. Plants exude1 organic acids (e.g. malic,  and oxalic acids) through their roots that dissolve phosphates naturally present in the soil, into their water soluble forms viz. H2PO4, HPO42‑, PO43-, which in turn  are taken up by the plants. Single Super Phsosphate (SSP) that contains P in water soluble form was produced2 by the scientists of Rothamsted Experimental Station (England) in 1840 by reacting rock phosphate and sulfuric acid, with the idea of providing readily available P to the plants. This is followed by the development of more complex phosphatic fertilizers  such as Di Ammonium Phosphate (D A P), Mono Ammonium Phosphate (M A P), Triple Super Phosphate and  other NPK complexes.


Certain types of rock phosphates are applied  to acidic soils as  fertilizer directly. Phosphate rocks  that contain high percent of P soluble  (expressed as P2O5) in 2% citric acid are recommended for direct application . Phosphate rocks of sedimentary origin, that have PO43- partly replaced by CO32- isomorphically (carbonate apatites), show3 high content of P soluble in 2% citric acid. On the other hand phosphate rocks of igneous origin have P less soluble in 2% citric acid and hence they are not considered for direct application.

 The Problems

 Unfortunately 60% to 70% of the P  applied to the soils, in water soluble forms are unavailable to the plant4 as the applied phosphorous is fixed by Fe, Al, Mn ions in acidic soils and by Ca, Mg ions in alkaline soils into complexes that plants cannot take up. Phosphorous availability to the plants is maximum in the narrow soil pH range between 5.5 and 7. Excessive and  indiscriminate application of chemical fertilizers show adverse impact on the soils in that soil micro flora and fauna (which impart natural properties to the soils) are destroyed thereby resulting into decreased agricultural production after years of application.

Organic Manure


Often organic manures are assessed in terms of their content of nutrient elements such as N, P, K, etc. Microorganism that decay organic matter produce a variety of useful compounds4  such as gibberlins, auxins, vitamins,  fulvic and humic acids that are vital for the plant growth, therefore fertilizers and minerals  cannot replace manure in agriculture. Peat or lignite can partly replace organic manures or composts for they also contain fulvic and humic acids. Organic matter in the soil (i) greatly enhances the water holding capacity of the soil (ii) keeps the soil loose allowing aeration.


Phosphate Rich Organic Manure


It is observed5, 6 that farm yard manure (FYM) enriched with high grade (+34% P2O5) rock phosphate in fine size ( d80 at 23 microns) shows better agronomic efficiency than di ammonium phosphate when applied on equal P2O5 basis. Some initial results are shown6 in Table 1. The technique of making and applying Phosphate Rich Organic Manure (PROM) is studied7,8 and improved by several scientists. Few companies in India are producing and marketing PROM on commercial scale. The advantage with PROM is that it shows equal residual effect, that is it works for two consequent crops. The dissolution of P from PROM is slow and is due to the organic acids released by the plant roots and microorganism hosted (naturally present or advertently added) by the soil and particularly the soil organic matter. Further organic matter (that matrixed rock phosphate particles) complexes soil cations thereby preventing fixation of P.


Phosphate Rock Characteristics


We examine here four types of phosphate rock samples for their characterization and use efficiency as source of P  in PROM. Phosphate concentrate produced by beneficiating low-grade phosphate ore of Jhamarkotra is extensively studied in PROM. This material (1st type or 1T) analyses 34% total P2O5, 3.5% P2O5 soluble in 2% citric acid and 80% particles are finer than 23 microns (i.e. d80 = 23 microns). Directly mined high grade phosphate ore (2nd type, say 2T, upgraded by natural  geological processes from low grade ore), phosphate concentrate from Phalaborwa, South Africa (3rd type,  or 3T, beneficiated from low grade ore of igneous origin) and directly mined high grade ore from Egypt (type 4 or 4T)  which is of sedimentary origin.


Chemical analysis of the four types of rock phosphates are shown in Table 2. These samples are studied9 ­ to note the increase in P2O5 content soluble in 2% citric acid as the d80 decreases. As it can be seen (figure 1, the particle size in microns expressed as d80 on x-axis and P2O5 soluble in 2% citric acid as percent in total P2O5) that each type of rock phosphate shows a characteristic curve. More interesting is the fact that citric acid (2%) soluble P2O5 content of even the rock phosphate of igneous origin (Phalaborwa, South Africa) goes upto 7.9% (which works out to 21.47% of the total P2O5) at a d80 of 10.95 microns.


Studying  these four types of phosphate minerals in PROM, Pareek etal report8 comparable  yield of Vigna unguiculata (L) walp, to that  of DAP on equal P2O5 basis. A comparison of the performance of the four types  of phosphate rock minerals in PROM with DAP shows the following order :


3T(1.125) >2T(1.115)>DAP(1.05) 1T(1.025) 4T(1.002) >Control (0.465)


The  seed output per plant in grams is shown in parenthesis. Control is without application of phosphate in any form.  Earlier study on PROM using Jhamarkotra phosphate concentrate (1T) showed (Table 1) agronomic output equal to that of DAP in the first crop. Egyptian rock phosphate of sedimentary origin that contained higher percent of citric acid (2%) soluble P2O5 when used in PROM showed slightly lower performance compared to Phalaborwa rock phosphate of igneous origin. It may be true that carbonate apatites show high 2% citric acid soluble P2O5 which are qualified as direct application fertilizer for acidic soils. Carbonate present in the phosphate rocks either as isomorphic replacement or as carbonate  gangue mineral (such as calcite or dolomite) when applied to acidic soils are expected to adjust soil pH to a favorable range (5.7 to 7) at least locally facilitating   the uptake of P by plants. If so, carbonate content (assessed as loss on ignition) is more important characteristic of a phosphate rock than citric acid solubility, for use in acidic soils. On the other hand carbonate content in phosphate rocks may not be favorable for use in PROM, as  acidity generated by the  organic matter will preferentially be consumed by carbonates leaving phosphates undissolved. The slightly lower performance of the Egyptian rock used in PROM in fact corroborates this presumption.


Resource Utilization


While ensuring food security for the present generation is a big task, conserving resources for the future generations is equally or rather important. The fact that rock phosphate is a non-renewable resource, warrants efficient and judicious use of the mineral or the fertilizers based on this mineral.


Using rock phosphate mineral matrixed in organic manure, for application in alkaline soils can reduce its consumption just to half of what it is today due to the high residual effect, without reducing the agricultural output as may be noted from Table 1. This opens up the possibility of increasing the life of phosphate mineral deposits. Efficient recycling of manures obtained from farm wastes (e.g. straw of paddy or wheat), industrial organic wastes (such as press mud from sugar industry)  and farm yard manure will reduce the requirement of nutrient addition to the soils.


Rock phosphate mineral  is similar to bones in that both contain tri calcium phosphate. German farmers in fact used animal bones in agriculture during the period 1827-1840. An ancient Indian text “Vrikshayurveda” authored by Surapala describes the preparation of  ‘Kanupa’, a manure produced using the bones of horned animals. Bones are renewable resource although the available quantities may be limited.


Energy and Environment


Many of the chemical phosphatic fertilizers are produced using phosphoric acid as the raw material. Phosphoric acid is produced, in the wet process by reacting  high-grade, ground rock phosphate mineral with sulfuric acid. Di ammonium phosphate is produced by reacting phosphoric acid with ammonia. At every stage, say mining and production of rock phosphate, production of sulfuric acid, ammonia and finally di ammonium phosphate energy is consumed. Each unit of energy produced and consumed invariably disturbs the environment. Each tonne of phosphoric acid produced generates 4.5 tonnes of  phospho gypsum which is mostly unutilized and a waste.  Millions of tones of phospho gypsum is produced and stacked every year.




Phosphate rich organic manure (PROM) is a very simple product that has all the potential to replace complex phosphatic fertilizers such as di ammonium or mono ammonium phosphates. The production of PROM requires less energy and the product has no negative impact on the environment. In fact composting of organic matter via anaerobic route produces biogas, a renewable fuel. PROM is soil friendly and improves the natural properties of the soil. Finally, PROM overcomes the problem of phosphate fixation by soils and hence results into less consumption of rock phosphate mineral. An interesting fact is that phosphate rock of igneous origin (Phalaborwa) when used in PROM, showed better agronomic efficiency compared to DAP. Phalaborwa, igneous rock phosphate has heavy metals at p p m level which are micronutrients. It is also evident that size of the phosphate mineral particles is more important than the 2% citric acid soluble P2O5 content, for use in PROM.


The current research on phosphatic fertilization in no uncertain terms shows that the chemical phosphatic fertilizer technology is outdated. Phosphate minerals irrespective of their geological origin  and irrespective of citric acid (2%)  soluble P2O5 content can be used in making phosphate rich organic manure provided the mineral is high (+34% P2O5) in grade and fine (d80 between 20 to 30 microns) in size.



1.      Yagodin B.A, Agricultural Chemistry, Mir  Publishers, Moscow, 1984.

2.      Bhattacharya, P. and Jain, R.K,  Fertiliser News, Vol. 45 (10), Oct. 2000.

3.      Narayanasamy, G. and  Biswas, D.R,   Fertiliser News, Vol. 43(10), Oct. 1998.

4.      Brady, N.C,  Nature and Properties of Soils, Collier Macmillan, London, 1984.

5.      Sekhar, D.M.R. and  Aery, N.C,  Current Science, Vol. 80 No. 9, May 2001.


6.      Sekhar, D.M.R., Aery, N. C. and Gupta D. K, Indian Chemical Engineer, Vol. 44 No. 3,  July-Sept 2002.


7.      Shekhawat, M.S, etal, Editors, Phosphate Rich Organic Manure, Himanshu Publications, Udaipur, 2004.


8.      Aery, N.C, Rathore, N.S, Katewa, M.K, and Masih, M.R, Editors, PROM Vol. 1, Himanshu Publications, Udaipur, 2006.


9.      Sekhar, D.M.R., Prabhulingaiah, G.,  Gupta, D.K. and Katewa M.K,  in  the pre-prints, PROM Review-2005, an international workshop organized on  28th December 2005, Udaipur.


10.  Pareek, D.K, Masih, M.R, Banani Singh and Ashok Chowdary, in   PROM Review-2005.




Table I 


Effect of PROM and DAP on the Output of Cyamopsis tetragonoloba (Linn.)


Treatment No.


Seed Output per Plant (g)

Seed Output per Plant (g)

(residual effect )


PR(34/23-d80) @40 kg P205 ha-1


8.63 (+25.43)


Control (Soil)




PR(34/23-d80)  @40 kg P205 ha-1    + Urea @ 18 kg N2 ha-1


7.69 (+11.77)


DAP @ 40 Kg P205 ha-1


7.61 (+10.61)


PR(34/23-d80)  @ 40 kg P205 ha-1+ FYM @ 0.5ton ha-1


7.92 (+15.11)


PR(34/23-d80)  @ 40 kg P205 ha-1+ FYM @ 1ton ha-1


8.58 (+24.70)


PR(34/23-d80)  @ 40 kg P205 ha-1+ FYM @ 2 ton ha-1


8.60 (+25.00)


PR(34/23-d80)  @ 40 kg P205 ha-1+ FYM @ 4 tons ha-1


10.75 (+56.25)


DAP @ 40 kg P205 ha-1+ FYM @ 4 tons ha-1

7.59 (+64.28)

9.76 (+ 41.86)


Table 2

Chemical Analysis of Some Rock Phosphates


S. No.
Source & type
Percent by weight
Place of origin/detail
D80 in micro-ns
Total P2O5
P2O5 soluble in 2% citric acid
Loss on ignition
Jhamarkotra concentrate, 1T
Jhamarkotra High-Grade Ore, 2T
Phalaborwa (SA) Concentrate, 3T
Egyptian High-Grade Ore, 4T


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