Technical “
Minimum, but it also references the fact that an abundance of mineral nutrients can also lead to significant issues, not least via total plant loss - this is commonly seen where excess application, or spillages of Nitrogen occur - the Law of Minimum makes no reference to such issues. Physical soil properties can also suffer from
the effects of too little or too much. For the purposes of this article, however, my focus in respect of the Law of Tolerance is the biological effects of excess phosphorus applications, which brings me on to my next point.
The Science(s):
I think it is fair to say that managing the soil biologically has experienced something of a resurgence in recent years. Utilising biological means for stimulating plant growth isn’t a new concept, far from it - in the first of this series of articles, I referenced the fact that Inca colonies utilised human excreta which was collected in pits, to be composted, before being applied to their food crops - the success of this system was fundamentally biological in nature. Successive generations also utilised this practice to great effect. Yes, the success was perhaps more a case of ‘luck rather than judgment’, but the fact still remains - for centuries, food supplies were, by and large, dependent on the use of human, and other sources of excreta. Despite the sustained success of the use of
excreta, it was not without issue. In 1854, John Snow, a leading English physician, traced the source of a Cholera outbreak to a water pump, which had been installed 3 feet from a leaking cesspit in Soho, London - this discovery led to significant changes in the manner in which waste was dealt with.
The wider scientific community is slowly acknowledging the importance of biological processes within the soil; something that Liebig's declaration that chemistry would rule agriculture had failed to identify
At a similar time to John Snow’s discovery,
Justus von Liebig was gaining momentum in the world of plant nutrition - in his book Die organische Chemie in ihrer Anwendung auf Agricultur und Physiologie (1840), he boldly stated that “chemistry would revolutionise” agriculture" - a claim that would ultimately go on to define our industry. Liebig went on to identify nitrogen, phosphorus and potassium as essential elements for plant growth, and this also stimulated Liebig’s popularisation of the Law of Minimum, as depicted in the ‘barrel’ image.
Once again, however, history repeats itself - the above theory of plant fertilisation is not without challenge, just as the use of excreta was brought to question in the 19th century. The wider scientific community is slowly acknowledging the importance of biological processes within the soil; something that Liebig’s declaration that chemistry would rule agriculture had failed to identify. With the slow recognition of the importance of biological activity within the soil ecosystem, I would suggest that something of a divide is forming within our industry - two opposing ends of a divisive spectrum, adjoined by a plethora of uncertainty, commercialisation and capitalisation is how I would describe it. On the one hand, there is still a chemical reliance for a large part of the industry, and would still be considered the dominant factor where plant nutrition is concerned; however, some forward thinking individuals and companies have grasped the biological angle, and applied it with aplomb - there are demonstrated cases of these methods being used to great effect - so, what is one to do?
I am not in a position, nor is it my aim, to advise on this issue. It is my aim, however, to
present information to you, gained from many hours of research, in this series of articles. The fundamental benefits of the traditional
N-P-K system have long been acknowledged by all, and most are well versed in the understanding of this method. I will, therefore be focusing on the biological effects of this chemical fertilisation or, more to the point, phosphorus use on the soil fauna. In terms of plant nutrition, phosphorus
requirements are commonly asserted via the use of soil testing. There are a number of extractants of differing strengths used for this process, along with various benchmark figures for what would be considered ‘Low, Medium or High’ values - this lack of clarity is, I believe, the first in a series of flaws within the current system. It has been suggested that a number of
extractants, used in soil testing to determine plant available P, only measure inorganic labile P, therefore completely disregarding the organic pool. Whilst, in certain circumstances, I can see the benefits of testing (identification of limiting nutrients), without a standardised measuring methodology, accurate benchmark figures, and field calibration data, the current soil test protocols, in my opinion, serve as little more than a tool to stimulate fertiliser sales in many cases. In respect of biological phosphorus acquisition in soils, there are several mechanisms that are employed:
- Various species of Arbscular Mycorrhizal fungi (AMF) have been shown to hydrolyze organic phosphorus and transport it to plant roots, where it is taken up as inorganic phosphorus
- AMF have the ability to modify both the supply and demand relationship for phosphorus acquisition - a technique used particularly where P conditions are limiting, and the demand outstrips potential supply
- Bacterial species, such as Pseudomonas, Azotobacter, Burkholderia, Bacillus and Rhizobium, Actinomycete and Serratia, have been shown to demonstrate variable ability to solubilise P in soils via exudation of citrate, malate, formiate, lactate and succinate, and possibly other dissolution promoting organic substances
- The earthworm (Eisenia fetida) enhanced the P solubilising ability of the fungi Aspergillus awamori, resulting in increases in both soluble organic and inorganic P, and further tests demonstrate that earthworms promote growth and phosphatase production in the P solubilising bacteria Bacillus megaterium, subsequently enhancing inorganic P availability in soil
Given the above, it would seem wise to
foster management techniques that preserve the soil ecosystem as nature intended. It is
PC APRIL/MAY 2015 I 131
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