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Chapter 8
Secondary nutrients, micronutrients, liming effect and
hazardous elements associated with phosphate rock use

Phosphate rock (PR) is recommended for application to acidsoils where phosphorus (P) is an important limiting nutrient on plant growth.The past 50 years have seen the accumulation of considerable knowledge regardingthe factors affecting the agronomic effectiveness of PR for direct application.However, much less information is available on other effects associated with PRuse, i.e. secondary nutrients, micronutrients, liming effect, and hazardouselements. This chapter presents a review of the information available inliterature that is relevant to these other effects.

Secondary nutrients in phosphaterock

Among the significant chemical and nutritional constraints oncrop growth on acid soils are deficiencies of calcium (Ca) and magnesium (Mg)nutrients. As the apatite mineral in PR is Ca-P, there is a potential to provideCa nutrient if there are favourable conditions for apatite dissolution.Furthermore, many sources of PR contain free carbonates, such as calcite(CaCO₃) and dolomite (CaMg(CO₃)₂), that canalso provide Ca and Mg in acid soils. However, if dissolution of free carbonatesraises pH and exchangeable Ca around PR particles significantly, it can hinderapatite dissolution and thus reduce P availability of PR (Chien and Menon,1995b). For example, Chien (1977) found that Huila PR (Colombia), whichcontained about 10 percent CaCO₃, increased soil solution pH from 4.8to 6.2 in one week compared with other PRs that increased pH to 5.1.Consequently, the maximum soil solution P concentration obtained with Huila PRwas lower than that obtained with central Florida PR (Figure 25), even thoughthe two PR sources had approximately the same degree of isomorphic substitutionof CO₃ for PO₄ in apatite structure.

Hellumset al.(1989) reported on the potentialagronomic value of Ca in some PRs from South America and West Africa. Theirstudy applied adequate P as KH₂PO₄ to an acid sandy loam(pH 4.5) with low exchangeable Ca to isolate the Ca from the P effect. Theresults showed that Ca uptake by maize with various PR sources followed theorder of the reactivity of the PRs except Capinota PR (Bolivia), which had about10 percent CaCO₃ (Figure 26). The relative agronomic effectiveness(RAE) of various PR sources with respect to CaCO₃ (100 percent) interms of increasing dry-matter yield and Ca uptake ranged from 28 to 89 percentand from 8 to 58 percent, respectively (Table 26). The results showed that PRsof medium and high reactivity have potential Ca value, in addition to their useas a P source, when applied directly to acid soils with low exchangeableCa.

In a three-year field trial conducted in central China, Huet al.(1997) reported that exchangeable Ca increased from 1 194 mg/kgwith the control to 1 300-2 100 mg/kg with PR treatments. The correspondingexchangeable Mg levels were 330 mg/kg with the control and 350-400 mg/kg withthe PR treatments. Because the content of apatite-bound Mg is very small (unlikeapatite-bound Ca), it is expected that PR will not increase soil exchangeable Mgsignificantly unless the PR contains a significant amount of dolomite. Moreresearch is needed to obtain information on the agronomic value of Ca and Mg(especially the latter).

FIGURE 25
Relationship between maximum Pconcentration in soil solution and mole ratio of CO₃:PO₄in the apatite structure

Source: Chien, 1977a.

FIGURE 26
Relationship between Ca uptake by maizeand citrate solubility of various PR sources

Source: Hellumset al., 1989.

TABLE 26
Relative agronomic effectiveness ofvarious PRs with respect to CaCO₃ as a Ca source formaize

Source: Hellumset al., 1989.

Some PR sources may contain a significant amount of sulphur(S) bearing accessory minerals, e.g. gypsum (CaSO₄) in Israeli PR(Axelrod and Gredinger, 1979) and pyrite (FeS₂) and pyrrhotite (FeS)in Mussoorie PR, India (PPCL, 1983). However, little information is available onS availability to plants from these PR sources.

Micronutrients in phosphaterock

Some PRs contain accessory minerals that may providemicronutrients to aid plant growth. However, limited information is available onthis potential additional benefit of using PR for direct application.

Work by Hammondet al.(1986b) on an Oxisol in Colombiasuggested that indigenous Huila PR, which contains 136 mg of zinc (Zn) perkilogram, produced a higher grain yield of one rice variety (Cica-8) than didtriple superphosphate (TSP) because of its Zn content (Figure 27). However,available Zn from Huila PR alone was not sufficient to provide adequate Zn fortwo rice varieties. When Zn was applied to the soil, both Huila PR and TSP wereequally effective in increasing rice grain yield.

In New Zealand, Sinclairet al.(1990) found thatSechura PR (Peru), which contains 43 mg of molybdenum (Mo) per kilogram,increased dry-matter yields of pasture herbage more than TSP did at sites wherethe PR increased Mo levels in clover significantly (Figure 28). More informationis needed on the micronutrient contents of PRs that have potential forincreasing crop production on acid soils.

FIGURE 27
Response of rainfed rice to huila PRand TSP on an Oxisol

Source: Hammondet al., 1986b.

FIGURE 28
Effect of Sechura PR and TSP on Moconcentration in clover

Source: Sinclairet al., 1990.

Liming effect associated with PRuse

Low pH coupled with toxic levels of aluminium (Al) andmanganese (Mn) frequently contributes to poor soil fertility for plant growth onacid tropical and subtropical soils in developing countries. Although lime iseffective in alleviating soil acidity and Al toxicity, it is often eitherunavailable or expensive to transport. Screening crop species and varieties toidentify those that are tolerant of soil acidity would reduce lime requirements(Sanchez and Salinas, 1981; Goedert, 1983).

The dissolution of apatite in PR consumes H⁺ ionsand, thus, it can increase soil pH, depending on PR reactivity. If a PR containsa significant amount of free carbonates, it can further increase soil pH.However, although an increase in soil pH may reduce the Al saturation level, itcan also reduce apatite dissolution at the same time. The optimum conditionwould call for a soil pH that is high enough to reduce the Al saturation levelbut still low enough for apatite dissolution to release P.

Research by the International Fertilizer Development Center(IFDC) has shown that the application of medium to highly reactive PRs with lowfree-carbonate contents can result in significant liming effects on acid soils.Although the increase in pH is generally less than 0.5 units, the decrease inexchangeable Al can be significant where the soil pH is less than 5.5 (Chien andFriesen, 2000) as the exchangeable Al level would be almost zero at this soil pHin Oxisols and Ultisols (Pearson, 1975). For example, exchangeable Al wasreduced from 2.0 to 0.4 meq/100 g when a Colombian Oxisol was treated withSechura PR in a soil incubation study (Figure 29). The soil pH increasedcorrespondingly from 4.6 to 5.0, and the exchangeable Ca rose from 0.2 to 1.5meq/100 g. Consequently, the Al saturation level also declined from about 80 to20 percent. Thus, the better performance of highly reactive PRs, e.g. Sechuraand North Carolina, compare with TSP in plant-growth response on the soil mayhave been related to the alleviation of Al toxicity (Figure 30). In a five-yearfield trial conducted in an Oxisol fertilized with various PR sources, Chienet al.(1987b) reported that the pH increased from 4.1 with the controlto 4.7-5.0 with the PR treatments. The corresponding increase in exchangeable Cawas from 0.17 cmol/kg with the control to 0.31-0.56 cmol/kg with the PRtreatments. However, no significant effect on exchangeable Al was observed. Intheir study with the red soil of China, Huet al. (1997) reported thatthe soil pH increased from 4.8 with the control to 4.9-5.3 with the PRtreatments. A reduction in exchangeable Al of up to 70 percent with respect tothe control was also observed with the PR treatments. Thus, the studies suggestthat the application of PR to acid soils can also improve soil properties aswell as the supply of available P for crop production.

FIGURE 29
Exchangeable Al and Ca in an Oxisoltreated with PRs and TSP at 200 mg P/kg during incubation

Source: Chien, 1982.

FIGURE 30
Effect of P sources onPanicummaximumdry-matter yield (sum of three cuts) on anOxisol

Source: Chien, 1982.

Sikora (2002) conducted a theoretical and experimental studyto calculate and quantify the liming potential of PRs by laboratory titrationand soil incubation. Of the three anions (PO₄^-3,CO₃^-2 and F^-) present in the carbonate apatitestructure of PR, CO₃^-2 and PO₄^-3 canconsume H⁺ and cause an increase in pH. Because of the greater molarquantity of PO₄^-3 compared withCO₃^-2, PO₄^-3 exerts a greater effecton the liming potential of PR. The results for the titration of two PRs (highlyreactive North Carolina and low-reactive Idaho) showed the ranges of calciumcarbonate equivalence (CCE) were from 39.9 to 53.7 percent, which were less thanthe theoretical values (59.5 to 62.0 percent). The experimental model obtainedfrom the soil incubation study showed qualitative agreement with theory as itshowed increased liming ability with increased dissolved P from the PRs.However, the model showed lower percentage CCEs than theoretical calculationswhen the P dissolved ranged from 20 to 60 percent. Further research is needed tocompare actual percentage CCE models across a variety of soil types in order toassess the potential liming effect associated with PR use.

Hazardous elements in phosphaterock

All PRs contain hazardous elements including heavy metals,e.g. cadmium (Cd), chromium (Cr), mercury (Hg) and lead (Pb), and radioactiveelements, e.g. uranium (U), that are considered to be toxic to human and animalhealth (Mortvedt and Sikora, 1992; Kpomblekou and Tabatabai, 1994b). The amountsof these hazardous elements vary widely among PR sources and even in the samedeposit. Table 27 shows the results of a chemical analysis of potentiallyhazardous elements in some sedimentary PR samples (Van Kauwenbergh,1997).

Among the hazardous heavy metals in P fertilizers, Cd isprobably the most researched element. This is because of its potentially hightoxicity to human health from consuming foods that are derived from cropsfertilized with P fertilizers containing a significant amount of Cd. Most of thestudies on Cd uptake by crops have used water-soluble P fertilizers such as TSP,single superphosphate (SSP), di-ammonium phosphate and mono-ammonium phosphate.However, the Cd reaction with soil treated with PR differs significantly fromthat with water-soluble P fertilizers because apatite-bound Cd in PR is waterinsoluble. Iretskayaet al. (1998) reported highly reactive NorthCarolina PR containing 47 mg of Cd per kilogram was as effective as SSP producedfrom the same PR in increasing grain yield of upland rice, but that the Cdconcentration in rice grain with the PR was only about half of that with SSP.Thus, the information on Cd availability from water-soluble P sources cannot beimplied directly to PR application.

TABLE 27
Chemical analysis of potentiallyhazardous elements in sedimentary phosphate rocks

Source: Van Kauwenbergh, 1997.

The reactivity of the PR influences the availability of Cd tothe plant because Cd is bound with P in the apatite structure (Sery and Greaves,1996). To separate the P effect on Cd availability from PR, Iretskayaet al.(1998) pretreated two acid soils with 200 mg of P per kilogram asKH₂PO₄ so that no P response from PR would be expected inincreasing grain yield of upland rice. They found that total Cd uptake by ricefrom the low-reactive Togo PR was 80 percent of that from the highly reactiveNorth Carolina PR in the soil with a pH of 5.0, and 52 percent in the soil witha pH of 5.6 when the soils were treated with 400 µg of Cd per kilogram fromthe two PRs. McLaughlinet al.(1997) found that Cd concentrations inclover grown on the soil treated with a lower reactive Hamrawein PR (Egypt)containing 5.3 mg of Cd per kilogram were lower than that treated with highlyreactive North Carolina PR containing 40.3 mg of Cd per kilogram at the same Prates. Thus, a PR source with a higher reactivity and Cd content can releasemore Cd than a PR with a lower reactivity and/or low Cd content for plantuptake. In addition to PR reactivity and Cd content, plant uptake of Cd alsodepends on soil properties, especially soil pH, and crop species (Iretskaya andChien, 1999). More research is needed to investigate their interactions andintegrate these factors on Cd availability associated with PR use.

Some PR sources may contain a significant amount ofradioactive elements compared with other PR sources, e.g. 390 mg of U perkilogram in Minjingu PR (the United Republic of Tanzania) versus 12 mg of U perkilogram in Gafsa PR (Tunisia) (Table 27). As Minjingu PR is highly reactive andagronomically and economically suitable for direct application to acid soils forcrop production (Jamaet al., 1997; Weil, 2000), there has been concernover the safety of using this PR. Samples of soil and plant tissue associatedwith the use of this PR were collected by the International Centre for Researchin Agroforestry and sent to the International Atomic Energy Agency forradioactivity testing. The results showed that the radioactivity of the soil andplant samples was about the same as the background levels. However, thepotential safety problem remains a concern for workers during miningoperations.

Most PRs also have high concentrations of fluorine (F) inapatite minerals, often exceeding 3 percent by weight (250 g of F per kilogramof P). Excessive F absorption has been implicated in causing injury to grazingstock through fluorosis. McLaughlinet al.(1997) reported no significantdifferences between F in herbage from plots fertilized with either SSPcontaining 1.7 percent F or North Carolina PR containing 3.5 percent F, orbetween sites that had received both fertilizers. Concentrations of F in herbagewere generally less than 10 mg of F per kilogram and often near the detectionlimit for the analysis technique (1 mg of F per kilogram). They concluded thatplant uptake of F is unlikely to lead to problems for grazing animals in mostsoils. However, they cautioned that ingestion of soil by animals or ingestion offertilizer material remaining on herbage after heavy topdressing could affectanimal health, depending on soil and fertilizer F concentrations. Thus, there isa need to manage F in PRs in long-term applications to acid soils.