Technical


The focus has shifted back to the use of animal excreta, and recycled plant matter - the forms are common in today’s ‘organic’ fertiliser blends


”


degradation, and resulting famines ensued in Europe and, during this time, England commenced importation of bones from several countries to supplement the use of animal and human excreta. During the mid 19th century, the focus of phosphorus sourcing shifted to mining bat and bird guano, of which large deposits had been located on islands off the Peruvian coast and, latterly, on the islands of the South Pacific. More recently, the focus has shifted back to the use of animal excreta, and recycled plant matter - the forms are common in today’s ‘organic’ fertiliser blends. Mineral: Elemental phosphorus was


relatively easy to identify ‘in the field’. Visual signs of deficiency initially manifest as an uncharacteristic darkening of the sward, particularly on older leaves, and in the absence of any other factor which could induce ‘darkening’ of the turf, such as applications of Iron or Nitrogen. As the level of deficiency progresses, the accumulation of anthocyanin within the older leaves of the plant induces reddening of the leaf tips. At this point, a reduction in leaf growth should be noted; however, the plant can still produce starch, therefore root growth is still possible in all but the most severe cases of phosphorus deficiency. A low soil pH (<5.5) can lead to phosphorus


fixation with iron, manganese and aluminium oxides, thereby rendering phosphorus unavailable to the plant. A high soil pH can also lead to unavailability to the plant via fixation


with calcium. Whilst phosphorus toxicity is unlikely, excess application can cause negative interactions with other ions. Where excessive phosphorus applications are made, it is common to see iron deficiencies occur within the plant. In respect of the forms of phosphorus that


we apply, there are, in effect, two forms added to soils and/or plants for the purposes of increasing plant biomass: Organic: Historically, phosphorus application


for crop production was limited to manure, bone ash and human excreta - evidence exists of human excreta use as far back as the Inca colonies. It is also noted that the Chinese used human excreta as a means of fertilisation during the early stages of Chinese civilisation - this pattern was followed in Japan from the 12th century. During the 17th and 18th centuries, soil


discovered by the German alchemist Hennig Brand in 1669. Brand’s process required 1100 litres of human urine to yield 60 grams of elemental phosphorus; however, it took until the 1840s for the process of producing Superphosphate to be patented by John Bennett Lawes, at Rothamsted Manor (now known as Rothamsted Research Station). By the late 19th century, production of Superphosphate in Britain reached upwards of 15,000 tonnes. Today’s production of elemental phosphorus is borne from mining phosphate rock (Apatite). Phosphate rock itself is relatively inefficient as a fertiliser source (5-17% plant availability); therefore, it must undergo treatment to convert it to a more plant available source, typically using one of two processes: Wet Process: Rock phosphate is acidulated with sulphuric acid to produce phosphoric acid. The phosphoric acid can be further concentrated via evaporation of water which forms superphosphoric acid. During these reactions, much of the plant unavailable phosphorus is converted to a plant available source. Ordinary Superphosphate typically contains 20% P20S, whereas Triple Superphosphate contains a higher concentration of P20S, typically 46%. Further processing can take place by mixing phosphoric acid with anhydrous ammonia to create monoammonium phosphate (MAP) or diamonnium phosphate (DAP). The aforementioned forms of phosphate are what make up the inorganic phosphorus component in most commercially available agricultural and horticultural fertiliser blends. Dry Process: The dry process involves up to


four stages of thermal treatment in electric furnaces:


1. Water removal (drying) 2. Removal or organic matter (calcination) 3. Disassociation of carbonates (calcination) 4. Removal of fluorine (deflourination)


PC FEBRUARY/MARCH 2015 I 125


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