The article below was published in Better Crops, Volume 94. (2010, No. 4), page 21-23.
It is re-produced here with the permission of Dr. S. H. Chien and we hope our readers will be able to apply this knowledge in improving soil fertility and crop production.
Agronomic Use of Phosphate Rock for Direct Application
By S. H. (Norman) Chien, Luis I. Prochnow, and Robert Mikkelsen
Phosphorus is critically needed to improve soil fertility and crop production in many areas of the world. Direct application of phosphate rock (PR) has been shown to be a valuable source of nutrients in some conditions. This article reviews the relative agronomic effectiveness of PR with respect to water-soluble phosphate fertilizer.
In many acid soils in the world, especially in the tropics, soil fertility limitations constrain successful crop production. These soils usually are low in plant-available P and often have a high P-fixing capacity that results in low efficiency of water-soluble P (WSP) fertilizers such as triple superphosphate (TSP) or diammonium phosphate (DAP) by crops.
Application of unprocessed PR to soil can be an attractive alternative to WSP fertilizers in such cases.
Source of Phosphate Rock
The best predictor of the agronomic performance of PR is solubility, which is normally measured in the laboratory with neutral ammonium citrate (NAC), 2% citric acid (CA), or 2% formic acid.
The solubility of PR reflects the chemical and mineralogical characteristics of the specific P minerals. The principal mineral in most PR sources is apatite, but it varies widely in physical, chemical and crystallographic properties.
The chemical formula of apatite in some representative PR is shown in Table 1. In general, the NAC solubility increases as CO32- substitution for PO43- in the apatite structure increases. The solubility of PR is known to correlate well with crop response.
Figure 1 shows that crop response to finely ground PR depends on the source and the solubility. The solubility of PR generally increases with smaller particle size. However, the agronomic effectiveness of ground and unground highly reactive PR sources does not strictly follow the solubility pattern.
For example, the solubility of unground PR (-35 mesh; 0.5mm) from North Carolina (USA) is less than that of the ground PR (-100 mesh; 0.15mm), but their agronomic effectiveness is similar under field conditions (Chien and Friesen,1992) and green house conditions. (See photos)
It is not sufficient to compare the solubility and the agronomic effectiveness of various PR sources based only on particle-size distribution.
A solubility database of many PR sources around the world has been compiled by Smalberger et al. (2006).
Soil Properties pH
Among the soil properties, pH has the greatest influence on the agronomic effectiveness of PR. Chien (2003) reported that the relative agronomic effectiveness (RAE) of a highly reactive Gafsa PR (Tunisia) compared to TSP (RAE = 100%) increases as soil pH dropped in 15 soils with widely varying properties.
However, soil pH alone was able to explain only 56% of variability of RAE in this study (Equation 1). By also considering the clay content (related to soil pH buffering capacity and cation ion exchange capacity), it is possible to explain 74% of variability of RAE (Equation 2).
Since pH is a logarithmic scale of acidity, the agronomic effectiveness of PR sharply decreases as soil pH increases above 5.5. Therefore, the agronomic value of PR diminishes above this pH unless with an effective crop species.
Equation 1) RAE, % = 181.4 – 21.1 pH (R2 = 0.56)
Equation 2) RAE, % = 163.4 – 20.6 pH + 0.78 clay (R2 = 0.74)
Soil P-fixing capacity
The release of P from PR generally increases with a greater P-fixing capacity of the soil. Adsorption and precipitation of soluble P provide a sink that favors PR dissolution.
However, as the soil P-fixing capacity increases, the concentration of soluble P released from PR may initially decrease more rapidly than that from WSP sources, despite the fact that the dissolution of PR increases with an increase of soil P-fixing capacity.
The negative effect of soil P- fixing capacity on RAE of PR may be most significant for short-term crops, such as some vegetables. For long-term crops or residual short-term crops, RAE of PR compared to WSP tends to increase with increasing soil P-fixing capacity .
Figure 2 shows that the RAE of multiple PR sources varying in reactivty increases from the first bean crop to the third crop grown on soil has a high P-fixing capacity (Chien, 2003) . This is due to the (1) the residual effect of TSP decreases rapidly in soils with high P-fixing capacity, and (2) slow dissolution of PR in the soil with time.
Presence of calcium and organic matter
Since dissolution of PR also releases Ca, soils with high initial Ca content typically have slower PR dissolution, according to the mass action law. For many tropical acid soils, exchangeable Ca is low and thus provides favorable conditions for PR dissolution.
The positive influence of soil organic matter on increasing the agronomic effectiveness of PR has also been reported (Chien, 2003). Enhanced dissolution of PR due to formation of a chemical complex between soil organic matter and Ca2+ ions is proposed to be the mechanism.
The most effective way to apply PR is to broadcast it onto the soil, followed by incorporation with tillage. This technique maximizes the reaction of PR with the soil and minimizes interaction between PR particles.
Band application of PR is not recommended because it limits the contact of PR particles with the soil, resulting in reduced dissolution.
The effectiveness of PR is also reduced by granulation of fine particles (Chien, 2003).
Management of PR application for flooded rice requires special attention because soil pH generally increases upon flooding. The agronomic effectiveness of reactive PR can be drastically reduced when it is applied at or after flooding, whereas the PR can perform well when applied to the soil at least 2 weeks before flooding (Chien, 2003).
Adding limestone to acid soils is a common practice to raise soil pH and decrease Al toxicity. However, the increased pH and additional Ca from the lime are both detrimental to PR dissolution.
Therefore, liming practices should balance the need to alleviate the Al toxicity with reducing PR dissolution (Chien and Friesen, 1992). It is recommended that liming to increase soil pH be limited to arange of pH 5.2 to 5.5 in order to optimize the agronomic effectiveness of PR.
The usefulness of PR as a nutrient source varies with the crop species. In general, the effectiveness of PR is higher for long-term or perennial crops than for short-term or annual crops. PR has been used extensively for many tree crops in Asia, including rubber, oil palm and tea.
Use of PR for perennial pastures has been successful too. Acidification in the plant rhizosphere accounts for some of the differences among crop species to utilize PR. In a study using six plant species, Van Ray and Van Diest (1979) found that Gafsa PR (Tunisia) was equivalent to TSP for buckwheat, which produced much lower rhizosphere pH than did other plant species.
Among the crop species, rape (canola) is known to be efficient in utilizing PR. Root exudation of organic acids is thought to contribute to PR dissolution. Habib et al. (1999) reported that rape was able to utilize a medium-reactive Ain Layloun PR (Syria), even in calcareous soils. Subsequently, Chien et al. (2003) found that the RAE of nine PR sources for rape grown on an alkaline soil (pH7.8) increased from 0% to 88% as the 2% citric acid (CA) solubility of PR increased from 2.1% to 13.1% P2O5 (Table 2).
Use of Phosphate Rock for Organic Farming
PR is sometimes used for direct soil application in organic farming systems. The success of PR for organic crop nutrition largely depends on its reactivity in the soil. The total P2O5 content provided on the package label is irrelevant to PR reactivity in the soil.
In fact, most igneous PR sources are high in P2O5 content (>34%) but very low in reactivity due to little CO3/PO4 substitution in apatite mineral structure, and therefore not suitable for direct application in organic farming (Chien et al., 2009).
However, details regarding the reactivity of PR are rarely provided for organic growers. Factors affecting the effectiveness of PR for organic farming should be considered more or less the same way as for conventional farming.
One exception is when PR is added during composting, where conditions may result in an alkaline rather than acidic environments (Chien et al., 2009) and the chelation of organic matter with Ca ions derived from apatite maybe important to dissolve PR.
Phosphate Rock Decision Support System (PRDSS)
Many global agronomic trials with PR have been integrated into a single tool to predict its agronomic effectiveness in specific situations.
IFDC (An International Center for Soil Fertility and Agricultural Development), in collaboration with FAO/IAEA, developed and published a PRDSS model for PR sources (Smalberger et al., 2006);
The PRDSS can be used in making decisions between use of WSP fertilizers and PR to meet crop nutrition needs. The PRDSS also provides assistance to determine conditions where the use of PR is more economical than WSP as a source of plant nutrients.
The agronomic and economic effectiveness of PR can be equivalent to or better than WSP fertilizers in some circumstances. Unlike WSP fertilizers, which can be widely used, there are specific factors – including the reactivity of PR sources, soil properties, management practices, and crop species–that must be taken into account in order to maximize the utilization of PR.
Use of the PRDSS model is an effective means to predict the best use of this nutrient resource.
Dr. Chien (firstname.lastname@example.org) was Principal Scientist – Soil Chemistry (retired), formerly with IFDC, Muscle Shoals, Alabama. Dr. Prochnow (email@example.com) is Director, IPNI Brazil Program, Piracicaba, SP, Brazil. Dr. Mikkelsen (firstname.lastname@example.org) is Director, IPNI Western North America Region, located in Merced, California.
Chien, S.H. 2003. Factors Affecting the Agronomic Effectiveness of Phosphate Rock for Direct Application. In Direct Application of Phosphate Rock and Related Technology: Latest Development and Practical Experiences, pp. 50-62, (S.S.S. Rajan and S. H. Chien, ed.). Special Publications IFDC-SP-37, IFDC, Muscle Shoals, Alabama.
Chien, S. H., and D. K. Friesen. 1992. Phosphate Rock for Direct Application. In Workshop on Future Directions for Agricultural Phosphorus Research, pp.47-52, TVA Bull. Y-224. Tennessee Valley Authority, Muscle Shoals, Alabama, U.S.A.
Chien, S. H., G. Carmona, J. Henao, and L.I. Prochnow. 2003. Commun. Soil Sci. Plant Anal. 34:1825-1835.
Chien, S. H., L.I. Prochnow, and H. Cantarella. 2009. Adv.Agron.102:261-316.
Habib, L., S. H. Chien, G. Carmona, and J. Henao. 1999. Commun. Soil Sci. Plant Anal. 30:449-456.
Smalberger, S.A., U. Singh, S. H. Chien, J. Henao, and P. W. Wilkens.2006. Agron.J.98:471-483.
Van Ray, B. and A. Van Diest. 1979. Plant Soil 51:577-589.