Technical
Agrico waste stack near Mulberry, like a scene out of Jules Verne’s Journey to the Centre of the Earth. The sinkhole released 20.8 million pounds of liquid phosphoric acid into the ground below. The company was able to clean up the spill before it harmed the drinking water supply, regulators say
“ 132 I PC APRIL/MAY 2015
Given the sheer quantity of
Phosphogypsum that is generated, it poses something of a problem for the fertiliser industry - what is it to do with such a mass of a by-product which carries such environmental risks?
disappointing to note, however, that many practices that we undertake can disrupt the natural cycles within soils. A number of studies have demonstrated that hyphal growth and hyphal branching induction of mycorrhizal fungi is depressed at high soil P levels. Further studies have also demonstrated this effect - under very high levels of soil P (>50ppm), the plant is often able to acquire a sufficient amount of P so that the mycorrhizal demand is redundant. Mycorrhizal development in roots is often reduced in such circumstances. Conversely, under what would normally be
considered low P levels (5-20ppm), P acquisition by AMF can result in a net growth benefit to the plant. These studies would all confirm that, whilst P is required for plant growth, excessive applications can suppress microbial acquisition. My issue with excess P applications is not
limited to suppression of microbially acquired P mechanisms however, so I ask, if Mycorrhizal acquisition of P is suppressed, would it be safe to assume that the AMF become redundant? If this is the case, what happens to the remainder of the processes that AMF undertake, such as disease suppression, soil aggregation, enhanced water uptake etc? In addition to the above, research also
suggests that, on average, 30-65 percent of the total soil P is in organic forms - these compounds are mainly inositol phosphates, nucleic acids and phospholipids. This leads me to pose the following questions - given that a number of extractants only measure the inorganic phosphorus portion, if a soil test result demonstrates a ‘low’ reading, is the soil actually low in phosphorus, or could it be that the organic pool is in plentiful supply and, by responding to a low reading via the application of further fertiliser to correct levels, are we acting counter productively by further suppressing the various microbial acquisition mechanisms? I think that the mantra “don’t view things in
isolation” warrants a level of consideration here. If the soil test states low, but no visible signs of deficiency exist, I would suggest that the plant-soil relationship may well be operating at the point of Shelford’s Law of Tolerance - a careful balance to maintain, but a worthwhile one in my opinion. When Liebig proclaimed that chemistry
would revolutionise agriculture, I’m sure he envisaged only positives. With the benefit of
hindsight, however, having noted one scientific paper after another report on the world’s declining soil quality, I would have to question Liebig’s claims. I would summarise my own feelings in the words of Lady Eve Balfour, from her seminal book; The Living Soil, 1943: “When water borne sewerage was
introduced to our cities, the capital of the soil - its fertility - which is removed from it year by year in the form of crops and livestock - no longer found its way back to the land in the form of waste products of the community, but was poured into the sea, or otherwise destroyed”.
The environmental(s):
Environmental concerns run abound when discussing the subject of phosphorus - certainly too many to discuss in one article. For this reason, I will focus on what I consider to be the two main issues: Phosphogypsum - a by-product of the wet
process production of phosphoric acid. For every metric tonne of phosphoric acid produced, approximately five tonnes of Phosphogypsum on a dry weight basis is generated. It is important to note however, that this by-product is not the gypsum which many of us use as a soil conditioner. Whilst many characteristics are similar, Phosphogypsum is known to contain a number of potentially toxic elements. It primarily consists of calcium sulphate dihydrate, with small amounts of silica and phosphate rock. The toxic elements contained include, but are not limited to, arsenic, cadmium, chromium, mercury and fluoride. In addition to the aforementioned elements, appreciable quantities of radioactive materials, in the form of radon, thoron and uranium, are also present. Given the sheer quantity of Phosphogypsum
that is generated, it poses something of a problem for the fertiliser industry - put simply, what is it to do with such a mass of a by- product which carries such environmental risks? Re-use is a difficult proposition, given strict regulations regarding its use - there are limited uses in agriculture and road construction, where the radiation levels are proven to be low; however, in reality, this only represents a very small portion of the product. Current surveys of global Phosphogypsum suggest that 14% is re-used, 58% is stored in gyp-stacks, and the remaining 28% is discharged to inland watercourses and the sea.
Florida State regulators’ “worst nightmare” happened in June 1994 when a cavernous hole, 106ft. wide by 185ft. deep, opened in the centre of an IMC-
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