Igneous rock phosphate (PR), along with certain metamorphic and sedimentary PRs, is characterized by low reactivity and solubility in soils, making it unsuitable for direct use in most agricultural contexts. Factors such as PR's crystallinity, chemical composition, and physical size, as well as soil properties like pH, base saturation, and biological activity, influence its effectiveness. Composting provides a promising alternative for transforming this material into a more plant-available form.
Key Concepts and Background:
Direct application of PR to soils often yields poor results, as its P content remains locked in a form inaccessible to plants. The availability of P depends significantly on the plant’s ability to exude H+ ions and interact with soil factors, such as acidity and the presence of humus. Traditionally, the solubilization of PR has been achieved through chemical treatments using acids like sulfuric acid. However, composting offers a natural process where organic acids and microbial activity work synergistically to enhance PR solubility.
Historically, mixing PR with organic materials like manure and acidic substrates (e.g., peats) has shown potential, though economic and practical challenges have limited its widespread adoption. This study investigates whether composting—a controlled decomposition process—can effectively solubilize igneous PR by exploiting the acidic microenvironments created during organic matter breakdown.
Materials and Methods:
The study used a hard igneous PR from Cargill, Ontario, Canada, containing 16% total phosphorus, but with very low citric acid and ammonium citrate solubility, making it unsuitable for direct soil application. Various compost formulations were prepared, incorporating materials like farmyard manure (FYM), liquid manure, peat, wood waste, straw, and blood. Both aerobic (hot) and anaerobic (cold) composting methods were employed.
Aerobic composting involved the creation of ventilated heaps to facilitate oxygen flow and heat generation, which are critical for microbial activity and decomposition. Anaerobic composting used plastic sheeting to exclude air and simulate silage preparation. The composting duration ranged from 2 to 4 months, during which the PR's transformation into plant-available P forms was monitored.
Findings:
- Phosphorus Solubilization: Composting increased PR solubilization to an average of 53.2%, with the highest solubilization (74.4%) observed in aerobic compost containing liquid manure and peat. The extent of solubilization varied based on the compost formulation and intensity of the process rather than the composting method itself.
- Mechanisms of Solubilization: Organic acids such as citric, lactic, and humic acids, along with mineral acids (e.g., sulfuric acid), produced during composting, chelated calcium from the PR, facilitating the release of phosphate ions. Heat generated in aerobic composting further enhanced the reactivity of PR.
- Role of Compost Ingredients: Materials like peat contributed to nutrient retention and thermal insulation, while acidic components in composts reduced nitrogen loss and helped dissolve PR. Organic substances in the compost coated soil P-fixing sites, reducing immobilization and improving the bioavailability of P.
- Impact on Compost Quality: Phospho-composts (those containing PR) had higher mineral matter content but slightly lower organic matter compared to control composts. The acidic microenvironments around decomposing organic materials were sufficient for PR dissolution without requiring the overall compost pH to be acidic.
Implications for Agriculture:
The study underscores the potential of composting as an effective method to transform low-reactivity igneous PR into a valuable phosphorus source. This approach is particularly beneficial for soils with high phosphorus fixation capacity (e.g., calcareous or acidic soils), where traditional fertilizers are less effective. Moreover, composting offers a sustainable means to recycle organic waste and enhance soil fertility.
In practical terms, this method could be integrated into agricultural practices in regions where igneous PR deposits exist but are underutilized due to their poor direct fertilizer performance. Additionally, the process aligns with principles of sustainable agriculture by leveraging natural decomposition to improve soil health and reduce dependency on synthetic fertilizers.
Conclusion:
This study demonstrates that composting can substantially increase the fertilizer value of igneous PR, converting it into plant-available forms. The findings highlight the potential for integrating composting of PR into agricultural systems, especially in resource-limited settings where cost-effective and eco-friendly solutions are essential. By optimizing compost formulations and leveraging available organic materials, farmers can enhance crop yields while addressing environmental and economic challenges associated with conventional phosphorus fertilizers.
