Technical
Phosphorus II
Breaking the law?
In the second of a series of three, Mike Atherton, Head Groundsman at King William’s College, Isle of Man, discusses how Liebig’s Law of Minimum might be inappropriate and that the Law of Tolerance may be a better reference point where the effects of phosphorus use are concerned
minimum I 130 I PC APRIL/MAY 2015
n the first of this series of articles, my aim was twofold - to give the history of phosphorus use, from its humble beginnings through to the significant global industry that it is today, and to
demonstrate the vast array of functions which phosphorus undertakes within plant and soil systems. Given the growth of the phosphorus industry, and the sheer volume of scientific literature which supports phosphorus use, one thing is abundantly clear - we cannot do without it. That being said, I believe there to be a number of significant, pressing issues, from sourcing, through the use of the end product and, ultimately, to the environmental fate of the product.
The Law(s):
I’m sure that many of you will be familiar with Liebig’s Law of Minimum - this ‘law’ states that growth is not
controlled by the total available nutrient pool, but by the scarcest nutrient. As a basic principle, I like Liebig’s Law of Minimum. In field
situations, a crop’s failure to respond to fertiliser inputs usually points to a limiting nutrient, as per the ‘Law of Minimum’ - a full spectrum soil test should identify the limiting resource - once this has been identified, a solution can be implemented and healthy growth can recommence. Despite my liking of the law, I believe it to be
severely limited, if not outright inappropriate in certain situations. We work with a very complex system, which compromises not only chemical elements, but also numerous biological processes and physical factors - the Law of Minimum is, in effect, only one dimension of a three dimensional system - it would be erroneous to view it in isolation. With that being said, there is another law; one that I have never seen referred to in our industry, even though I believe it to be far more applicable; ‘Shelford’s Law of Tolerance’, which states: “the abundance or distribution of an organism can be controlled by certain factors (e.g., climate, topographic and biological requirements of plants and animals) where levels of these exceed the maximum or minimum limits of tolerance of that organism”. I believe that the Law of Tolerance applies to
the three dimensions of the system in which we work far better than the Law of Minimum. In terms of its application in respect of the mineral constituents required, the Law of Tolerance not only effectively incorporates Liebig’s Law of
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