March 23 Newsletter, Phosphorus Month

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.

References:

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.

Testing phosphorus accurately requires a better understanding of P and the testing protocol options and their reliability. Using an agronomist that understands these protocols and Phosphorus impacts on the soil. Choosing the right test protocol is the first step.

This image above: is of the same soil tested with both the Olsen and Mehlich III P tests. You can see that the Olsen P result is indicating it is a very P deficient soil and the Mehlich III P is actually showing it to be excessive in P already. The other issue that can confuse the results is the medium ranges can often differ between labs and years and soil types, that creates even more extreme result disparities. None of this is helpful to the farmer trying to decide if their soil needs more P so they mostly rely on the fertiliser companies that sell them the P to advise them. This has lead NZ farmers and growers to often massively over supply P and then require large amounts of N as a result, due to the harm done to the soil health.

This miss management of P can be taken to extremes as this graph result below of a horticultural soil shows. This has truly excessive P levels and yet the recommendation for the base planting blend from one of the biggest horticulture supply companies, also providing technical advice, was still telling them to add more Super Phosphate. One other issue with the recommendation was no lime was recommended even though the soil result is clearly deficient in Calcium and the pH was 5.8.

Poor quality advice is a common occurrence with fertiliser inputs as it is often based on poor quality information or lack of understanding of that information.

Both the Mehlich III and Olsen tests are commonly used to measure the amount of plant-available phosphorus (P) in soil. While both tests aim to determine P levels, there are some differences in how they work and the benefits they offer. Here are some benefits of using the Mehlich III protocol over the Olsen test:

More accurate results: The Mehlich III protocol is considered to be more accurate than the Olsen test, especially for soils with high levels of calcium, magnesium, and iron (typical of many NZ soils today). This is because Mehlich III uses a stronger acidic extractant solution that can extract more P from the soil, providing a more representative measurement of the available P in the soil.

Detects other nutrients: The Mehlich III protocol also extracts other nutrients like potassium, sulfur, zinc, copper, iron, and manganese. This makes it an effective tool for assessing overall soil fertility.

Broader soil types: Mehlich III can be used on a broader range of soil types, including sandy, loamy, and clay soils. In contrast, the Olsen test may not be as effective on soils with high clay content or high pH levels (also typical of many NZ soils today).

Greater precision: Mehlich III is also more precise than the Olsen test, allowing for smaller differences in P levels to be detected. This can be particularly useful in situations where small changes in P levels can have a significant impact on crop yield and quality.

Recommended by official bodies: The Mehlich III protocol is recommended by several official bodies, including the American Society of Agronomy and the Soil Science Society of America, which recognize its effectiveness in measuring P levels in soil. This also means that most often the scientific research results are shown as Mehlich III numbers and these can not be converted or directly compared to other results using different test protocols.

Overall, the Mehlich III protocol is generally considered to be more accurate and precise, making it a better option for soil testing in most situations.

Biologix can provide you with independent and accurate analysis of your soils nutrient levels and likely microbial health. We only advise on what is required that will improve it, even if that it shows your soils need far less or no inputs at all.

What if you do need P?Phosphorus Solutions

Paul Stamets, one of the worlds leading expert of fungi" also found that high levels of phosphorus in soil can negatively affect the colonization of mycorrhizal fungi on plant roots.

However Stamets also discovered that the use of mycorrhizal inoculants can help mitigate the negative effects of excess phosphorus on mycorrhizal fungi colonisation. Inoculating soil with mycorrhizal fungi can increase the production of hyphae and improve nutrient uptake in plants, even in soils with high levels of phosphorus.

Nutri-Tech Solutions sell the best Mycorrhizal Innoculant product that we have ever tested.

If the rare situation that calls for Phosphorus fertiliser, then the best known option is

Soft Rock Phosphate

Soft rock phosphate is a natural source of phosphorus that is derived from ancient marine deposits, and is mined from sedimentary rock formations.

Soft rock phosphate is rich in various minerals such as calcium, iron, magnesium, and other trace elements that are essential for plant growth.

Unlike chemical fertilizers, soft rock phosphate is a slow-release fertilizer, which means that it slowly releases nutrients over a longer period, thereby providing a sustained source of nutrition for plants.

Soft rock phosphate improves soil structure and fertility by increasing microbial activity, improving soil water holding capacity, and reducing soil compaction.

Soft rock phosphate is eco-friendly as it is a natural source of nutrients and does not contain harmful chemicals or synthetic substances that can pollute the environment.

Soft rock phosphate can be used for all types of plants, including vegetables, fruits, flowers, trees, and shrubs.

Soft rock phosphate can increase crop yields and improve the overall health and quality of plants, leading to higher profits for farmers.

Soft rock phosphate is relatively low cost compared to chemical fertilizers, and its application is relatively easy and straightforward.

Soft rock phosphate has a long shelf life and does not lose its nutrient content over time, making it an ideal choice for long-term fertilizer applications.