Introduction
As farmers, we constantly strive to optimise crop yield and quality. In this quest, one element stands out as a critical factor for success: Phosphorus (P).
Phosphorus
Phosphorus plays a fundamental role in plant growth and development, as well as in maintaining soil fertility. However, the indiscriminate use of synthetic phosphorus fertilisers can have detrimental effects on soil health and the environment. With the growing focus on using fertilisers that only have positive effect across all aspects of soil, without detriment to biology or the physiology, understanding the importance of phosphorus and its management is paramount.
Phosphorus is an essential macronutrient required by plants for various biological processes. It is a fundamental component of adenosine triphosphate (ATP), the energy currency of cells. Moreover, phosphorus is involved in photosynthesis, respiration, cell division, and the transfer of genetic information. Adequate phosphorus levels in plants results in vigorous growth, improved root development, enhanced flowering, and increased crop yields. In soil, phosphorus availability influences nutrient cycling, microbial activity, and overall soil health.
Phosphorus exists in various forms in the soil, including organic and inorganic forms. Organic phosphorus is bound to organic matter and requires microbial activity for release, while inorganic phosphorus is readily available to plants. Synthetic phosphorus fertilisers, such as monoammonium phosphate (MAP) and diammonium phosphate (DAP), provide an immediate supply of inorganic phosphorus to plants, however, are only soluble for a short period of time. The overuse of phosphorus can negatively impact soil microorganisms and long-term soil fertility. The accumulation of non-labile forms due to its high-affinity chemical reactions and its occlusion to soil minerals and organic matter, can lead to the prevalence of “legacy P.” The accumulation of legacy P in soils and potential transfer to water bodies ultimately leads to environmental concerns, not to mention the disruption to the natural phosphorus cycle. Therefore, the development of methods to improve the bioavailability of immobilised phosphorus in agricultural soils are critical to mitigate the continued application of phosphorus beyond that which is required by plants.
Regenerative agriculture offers the holistic approach to soil health and nutrient management with practices that use cover crops playing a crucial role in unlocking soil phosphorus. Cover crops scavenge and take phosphorus from deeper soil layers, making it available for subsequent cash crops. Additionally, cover crops promote biological activity in the soil, fostering the growth of beneficial microorganisms that can solubilise phosphorus and enhance nutrient availability. This regenerative approach supports long-term soil health and minimises reliance on synthetic phosphorus fertilisers.
It is well documented that a thriving soil microbiome will improve phosphorus availability. Beneficial microorganisms, such as vesicular-arbuscular mycorrhizae (VAM) fungi, form symbiotic relationships with plant roots. VAM fungi enhance phosphorus uptake by extending the root system’s reach and releasing enzymes that breakdown organic phosphates into plant-available forms. Additionally, certain bacteria possess the ability to solubilise and make phosphorus more accessible to plants. These P-solubilising bacteria (PSB) release organic acids that breakdown phosphorus compounds, converting them into a plant-available form.
The inoculation of biology is different between VAM and bacteria. It is best to apply VAM to the seed or roots of plants to ensure the root exudates successfully from their symbiotic relationship. However, PSB are best inoculated through liquid application to soil before planting or during the cover crop phase. This gives growers a number of biological applications, known to enhance phosphorus availability.
Another intriguing approach to unlock soil phosphorus is the use of fish and worm juice. These natural inputs are rich in beneficial microorganisms that possess the ability to solubilise phosphorus. When applied to the soil, fish and worm juice feed and signal the desired native PSB, promoting nutrient availability. This practice can be implemented to encourage the PSB that have been present in that soil for millennia, or feed the PSB that were inoculated earlier in the season. Whichever is stimulated, it utilises nature’s resources to improve soil health and reduce reliance on synthetic fertilisers.
Soil tests have been a successful way of testing phosphorus levels in the soil, however, grid mapping has allowed growers to isolate their low phosphorus level areas from their high areas. This allows variable rate equipment to make applications of phosphorus, only to the areas that are lacking in phosphorus, reducing the occurrence of excess application in areas that were unwarranted.
When phosphorus levels are low, organic forms of compost and rock minerals offer valuable sources of phosphorus for long-term soil fertility. Composting organic materials, such as plant residues and animal manures, not only recycle nutrients but also enriches the soil with organic phosphorus. Organic compost and manures offer around 0.5% P which equates to 5kg P/ha per tonne. Rock minerals, such as phosphate rock or rock phosphate can contain from 2%-13%P (this is 2-13kg/ha P per 100kg of product applied). Natural rock phosphorus compounds are typically bound with calcium and require PSB to enhance availability. It is well documented and recommended to combine rock phosphate with compost before application with greater phosphorus response in soil and plant sap analysis, as compared with an application of straight rock phosphate.
Conclusion
Phosphorus plays a vital role in agricultural productivity and soil health. While synthetic phosphorus fertilisers have been widely used, their detrimental effects on soil and the environment are becoming increasingly evident. Embracing regenerative practices, such as cover cropping, harnessing P-solubilising bacteria, and utilising fish and worm juice, can unlock soil phosphorus and promote sustainable agriculture. Also of benefit is the use of technology to map soil phosphorus and variable rate to apply organic phosphorus sources like compost and rock minerals. By adopting these practices, farmers can enhance crop yields, improve soil health, and reduce the reliance on synthetic phosphorus fertilisers, thus safeguarding the future of agriculture.
References:
Kempf, John. (2019). Building Available Phosphorus without soluble fertilizers https://johnkempf.com/tag/phosphorus/
Khangura, R.; Ferris, D.; Wagg, C.; Bowyer, J. Regenerative Agriculture—A Literature Review on the Practices and Mechanisms Used to Improve Soil Health. Sustainability 2023, 15, 2338. https://doi.org/10.3390/su15032338
Cordell, D.; White, S. Sustainable Phosphorus Measures: Strategies and Technologies for Achieving Phosphorus Security. Agronomy 2013, 3, 86-116. https://doi.org/10.3390/agronomy3010086
Ditta, A., Muhammad, J., Imtiaz, M. et al. Application of rock phosphate enriched composts increases nodulation, growth and yield of chickpea. Int J Recycl Org Waste Agricult 7, 33–40 (2018). https://doi.org/10.1007/s40093-017-0187-1
