Welcome to the Biologix Newsletter for Mar 2023
Phosphorus (P) season is here! This is the nutrient that the fertiliser companies are encouraging you to order this autumn, in particular "Super-Phosphate"
The questions I want you to ask;
1 Does my soil really need it?
2. Can adding more do harm?
3. Is Super Phosphate the best form of Phosphorus fertiliser?
The answer to question 1: is most likely "NO" and the existing soil data you may have to base that question on, is likely misleading....
The answer to 2 is "YES" it certainly can harm your soil, especially in the Super Phosphate form......
The answer to 3 is absolutely NOT..... Super Phosphate is the one of the worst options to choose....
Over 90% of soil test results we have seen over the last 10 years show adding more P would most likely cause significant harm to your soil health and the nutrient availability to your crop. In fact adding more P will most likely ensure you need to add more nitrogen to compensate for that excess P, creating further harm.
In the first place most soil tests choose a very inaccurate test protocol for P, more about that end of this article....
Phosphorus (P) is an essential nutrient for plant growth, but excessive levels of P in soil can have negative impacts on the nitrogen cycle and the microbial communities involved in nitrogen cycling along with negatively impacting other nutrient cycles.
One of the primary mechanisms by which high levels of P can inhibit the nitrogen cycle is through a process known as P-induced nitrogen limitation (PIL). PIL occurs when high levels of P in soil lead to a decrease in the availability of other nutrients, particularly nitrogen, which can limit the growth and activity of nitrogen-fixing bacteria and other microbes involved in the nitrogen cycle (Vitousek et al., 2010). This can lead to a buildup of nitrate in the soil, which can contribute to environmental pollution and eutrophication in aquatic systems (Heckrath et al., 2009).
Research has shown that high levels of P can have negative impacts on a range of microbial species involved in nitrogen cycling. For example, studies have shown that excess P can inhibit the growth and activity of nitrogen-fixing bacteria, such as Rhizobium and Bradyrhizobium, which are important for the conversion of atmospheric nitrogen into plant-available forms (e.g., Singh and Reddy, 2012; Zhou et al., 2017). Similarly, excess P can inhibit the growth and activity of nitrifying bacteria, such as Nitrosomonas and Nitrobacter, which are responsible for the conversion of ammonium to nitrate (e.g., Frossard et al., 2000; Liu et al., 2018).
In addition to inhibiting nitrogen-fixing and nitrifying bacteria, excess P can also have negative impacts on other microbial species involved in nutrient cycling, such as mycorrhizal fungi. Mycorrhizal fungi are important for the uptake and transport of nutrients, including nitrogen, and have been shown to be negatively affected by high levels of P. For example, a study by Lambers et al. (2013) found that mycorrhizal colonization was reduced in soils with high levels of P, which can lead to decreased nutrient uptake and plant growth.
The specific level at which P becomes detrimental to mycorrhizal fungi can vary depending on a range of factors, such as soil type, pH, and the specific type of mycorrhizal association. However, research has provided some evidence of the threshold levels of excess P that can inhibit the growth and health of mycorrhizal fungi.
A number of studies have shown that mycorrhizal fungi can be particularly sensitive to excess P, and that even relatively low levels of P can inhibit their growth and nutrient uptake. For example, a study by Smith et al. (2011) found that mycorrhizal fungi were able to colonize the roots of corn plants in soils with P levels of up to 70 ppm M3 but that colonization was reduced at higher levels of P. Similarly, a study by Johnson et al. (2018) found that mycorrhizal colonization was inhibited in soils with P levels above 50-60 ppm M3
Other studies have found that the specific type of mycorrhizal association can influence the threshold level of excess P at which inhibition occurs. For example, arbuscular mycorrhizal (AM) fungi have been shown to be particularly sensitive to excess P, and may be inhibited at lower levels of P than ectomycorrhizal (ECM) fungi (Smith and Read, 2008). This is because AM fungi rely on phosphorus for energy and growth, and are less able to tolerate high levels of P.
In summary, excess phosphorus in the soil can inhibit the growth and health of mycorrhizal fungi, although the specific threshold level can vary depending on a range of factors. As a general rule, mycorrhizal fungi are particularly sensitive to excess P, and may be inhibited at relatively low levels compared to other soil organisms. Therefore, it is important to manage P inputs in agricultural and natural ecosystems to prevent excessive accumulation and ensure the maintenance of healthy mycorrhizal communities.
Finally, excess P can also lead to a decrease in the availability of other nutrients, such as iron (Fe), which can have negative impacts on plant growth and health. This is because high levels of P can lead to the formation of insoluble complexes with Fe, which can reduce the availability of Fe to plants and other organisms (Guerinot and Yi, 1994).
In conclusion, excess phosphorus in soil can have negative impacts on the nitrogen cycle and the microbial communities involved in nutrient cycling. By inhibiting the growth and activity of nitrogen-fixing and nitrifying bacteria, mycorrhizal fungi, and other microbial species, excess P can lead to environmental pollution, eutrophication, and decreased plant growth and health. Therefore, it is important to manage P inputs in agricultural and natural ecosystems to prevent excessive accumulation and ensure the maintenance of healthy microbial communities.
Soil testing and analysis should be carried out to determine levels before applying any P.
Frossard, E., Sinaj, S., & Fardeau, J. C. (2000). Long-term effect of phosphorus application on soil pH and phosphorus availability. Biology and Fertility of Soils, 31(4), 267-271.Guerinot, M. L., & Yi, Y. (1994). Iron: nutritious, noxious, and not readily available. Plant Physiology, 104(3), 815-820.Heckrath, G., Rubæk, G. H., Djurhuus, J., & P.
Johnson, N. C., Wilson, G. W. T., & Bowker, M. A. (2018). Mycorrhizal phenotypes and the Law of the Minimum. New Phytologist, 219(4), 1129-1132.Smith, S. E., & Read, D. J. (2008). Mycorrhizal symbiosis (3rd ed.). Academic Press.Smith, S. E., Smith, F. A., & Jakobsen, I. (2011). Mycorrhizal fungi can dominate phosphate supply to plants irrespective of growth responses. Plant Physiology, 167(3), 1670-1677.