CHAPTER 2 LITERATURE REVIEW
2.3. AGRICULTURE AND THE PHOSPHORUS CHALLENGE
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Figure 2.4. Plant phosphorus uptake from struvite in comparison monosodium phosphate As these fertilisers have agronomic potential, a strategy may be devised to enhance their adoption in agriculture. Marketing strategies and technologies to decrease production costs may also be employed to effectively to enhance their adoption. The HEDMs must be able to release nutrients and make them available at critical stages of crop growth and development.
Awareness campaigns have to be conducted to remove the potential stigma and negative perceptions related to the reuse of human waste in agriculture. This will also help in attracting more consumers of foods produced with HEDMs. If consumers are willing to purchase output produced with these HEDMs, their growth in the fertiliser industry will be accelerated.
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Production of phosphorus-containing fertilisers from phosphate rock begins with addition of sulphuric acid to the mined rocks to produce phosphoric acid. Addition of ammonia to the phosphoric acid produces the final products such as di-ammonium phosphate, mono- ammonium phosphate, single superphosphate, triple superphosphate and various other phosphates (DAFF, 2015).
The world generally has low amounts of phosphorus rock deposits which are being depleted unsustainably, i.e., the rate at which they naturally form is slower than the rate at which they are mined. Globally, about nineteen million tonnes of phosphorus are produced annually. Most of the globally extracted phosphorus (about 85%) is reserved for agronomic purposes to produce edibles, either for human or animal consumption, with the remaining being used for other industrial purposes and other needs (Neset and Cordell, 2012). All this phosphorus is, however, lost in different ways with about 18.5 million metric tonnes being lost as solid wastes in the soil and through soil erosion and about 1.32 million tonnes being discharged in the air and water (Villalba et al., 2008). Phosphorus is also lost in human and animal wastes after excreting phosphorus-rich foods or feeds (Meyer et al., 2014).
Measures of sustainability have now been called for as a result of the anticipated phosphorus scarcity that the world is about to face which will lead to a scarcity of phosphorus-based plant nutrient sources. Phosphorus depletion is a major concern, as it has no alternative sources, as compared to nitrogen, which can be directly fixed in the soil by nitrogen fixing bacteria, lightning or other means as it is abundant in the atmosphere. Phosphorus has no other means of getting into the soil except through physical human application, which makes it a limiting nutrient. Whereas the rate of recycling of nitrogen can be measured in years or centuries, the phosphorus recycling rate is very slow and can be measured per millennia (Dawson and Hilton, 2011).
It is predicted that most of the remaining world’s phosphorus reserves will be depleted in the 21st century (Figure 2.5). The rate at which rock phosphates are harvested is on the rise and it is predicted to reach peak around the mid-2030s (Cordell et al., 2009). Over the second half of the 21st century, the level of phosphate rock mining, which is currently the main agricultural phosphorus source, will continue to decline until phosphorus is depleted in around 2130. The rising levels of extraction may also imply that the level of demand from agriculture and other industries is on the rise and the peak level of mining will have to be maintained so as to be able to supply phosphorus without negatively affecting the productivity of agriculture and the other
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industries. If the level of phosphorus supply declines after the mid-2030s, agricultural productivity may also decline, leading to food (and nutrition) insecurity. Low supply of phosphorus with a continuous high demand may also result in a rise of the phosphorus product prices (Villalba et al., 2008).
Source: Cordell et al., (2009)
Figure 2.5. Predicted global phosphorus mining and production curve over the next 80 years This means that crop production will require alternative phosphorus sources, as it is a basic crop nutrient that plants cannot grow without. Other sources of phosphorus, besides rock phosphate include crop residues or alternatively excreted animal materials, which sometimes may be directly applied to crops or processed to retrieve necessary plant nutrients, including phosphorus (Kern et al., 2008). Human waste is a rich source of phosphorus, because most of the phosphorus in agricultural products consumed finally gets disposed of as waste. Mined phosphorus gets lost back to the environment and in certain instances, also contaminates the environment (Neset and Cordell, 2012).
Researchers have discovered human urine as an alternative phosphorus source, it is more concentrated in urine than in any other excreted wastes from humans (Meyer et al., 2014).
Urine is currently being developed into potentially effective plant nutrient sources in South
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Africa to supply phosphorus and other nutrients. Struvite (a urine derivative) is a solid compound and a very good phosphorus source (12.6% P), produced by the precipitation of urine after adding to it a magnesium dose (MgO) (Grau et al., 2013). It has the potential to become a dominant agricultural phosphorus source, if there is effective urine collection, treatment and processing into this valuable product.
Nitrified Urine Concentrate (NUC) is another urine product which is a good source of nitrogen for agricultural purposes and also contains phosphorus, though in low amounts. Urine-derived plant nutrient sources can provide nutrients in high concentrations that compare to those provided by commercial fertilisers, which makes urine potentially an input of great value and importance (Dawson and Hilton, 2011). Latrine Dehydrated and Pasteurised (LaDePa) pellets are also a potentially good soil amendment product with low phosphorus concentrations (1.5%
P) (Harrison and Wilson, 2012).