Alternative Fertilisers

Phosphites and phosphonates Plants contain no enzymes which will convert phosphites into phosphates, and only the latter are practical fertilisers. Some phosphites will tackle fungal infections in plants however. Phosphonates are only of rare occurrence in plant metabolism and their utility as fertilisers has yet to be established. 

Nitric phosphate is derived from phosphate rock using nitric acid instead of sulphuric (12.9) (sometimes nitric-sulphuric or nitric-phosphoric acid mixtures are used). The calcium nitrate by-product of this reaction must be removed, or the solid fertiliser would be hygroscopic. One method of achieving this is by crystallisation, and the other is by treatment with ammonia. In the latter instance, a mixture of ammonium phosphate, ammonium nitrate and dicalcium phosphate is obtained (12.10). Alternatively, the calcium nitrate can be converted and the product left in the mixture (12.11). 

TABLE 12.4 Phosphorus Content of Typical Commercial Fertilisers (wt% P2O5)  

10Ca(NO3)2 +6H3PO4 +2ONH3 co.+4H,o 20NH4N03 +4CaC03 + 6CaHP04 (12.11) 

Other fertiliser compounds which can satisfactorily provide phosphorus and nitrogen are urea phosphate, CO(NH2)2 H3PO4, ammonium polyphosphate, [(NH4)P03] and phosphazenes such as P3N3(NH2)g (Table 12.4). Red phosphorus is slowly oxidised in damp soil and has been considered as a possible fertiliser [32,33]. 

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Helsel Z R (1992). Energy and alternatives for fertiliser and pesticide use . In Fluck RC, Energy in World Agriculture, Vol 6 Elsevier Science Publishing, Amsterdam, 177-210. 

In most of today s ethanol plants for the conversion of wheat, rye and corn, the required thermal energy is provided by natural gas, heavy fuel oil or coal. The protein-rich by-products of ethanol plants are referred to as dried distillers grains with solubles , abbreviated as DDGS, and are mostly used for animal fodder. Alternatively, they can be converted to biogas for heat and electricity production. The resulting residue can then be used as fertiliser (see Table 7.16). 

Recent developments to the hydrothermal process include improvements in yield and reaction rate and in overcoming the difficulty associated with the coproduct salt. One method of overcoming the co-product problem is to use magnesium nitrate instead of chloride, with the ammonium nitrate being utiHsed for fertiliser production [102-104]. At least one plant based on this concept is now in commercial production. While a considerable advance on the initial chloride process, the nitrate route does require close integration with a fertiliser process and thus lacks flexibility. An alternative approach being developed is to recycle the ammonium salt co-product (nitrate or chloride) and use it to leach magnesium oxide, a potentially inexpensive raw material [103]. 

Estimation of the relative effectiveness of nutrient sources can be a useful way in which to estimate their efficacy as fertilisers (Barrow 1985). The relative effectiveness of alternative nutrient sources is usually calculated by comparing the yield plateau of the response curve of the fertiliser in question to a soluble source of the same nutrients (Barrow 1985). For minerals used as nutrient inputs in organic farming systems their relative effectiveness is almost always <1 due to low solubility in soil. Organic matter inputs can also be evaluated in terms of their relative effectiveness based on their recalcitrance, but of equal importance is the extent to which they are physically protected from degradation in soil aggregates (Strong et al. 1999), which would be different in different soil types. 

The demand for hydrogen, driven by classical applications such as fertilisers or oil refining as well as new applications (synthetic fuels, fuel cells,...) is growing significantly. Presently, most of the hydrogen produced in the world uses methane or another /ossil feedstock, which is not a sustainable option, given the limited fossil resources and need to reduce C02 emissions. This stimulates the need to develop alternative processes of production which do not suffer from these drawbacks. 

Possible solutions include the use of inorganic fertilisers or imported topsoil, which can be very expensive depending on location and availability. An alternative solution is the use of organic wastes such as sewage sludge, which is already used in the UK as well as Sweden and Finland. 

Similar natural methods have been adapted to evaluate deposits of cheap rock phosphates as an alternative to expensive, often imported, phosphate fertilisers, and to find the most efficient way to use these fertiliser deposits for maximum plant growth. 

Compost — the process of composting organic matter can be accelerated by alternately adding about 5 % of hydrated lime and 5 % of a nitrogenous fertiliser to layers of compost. The hydrated lime provides an alkaline environment in which the fertiliser catalyses the conversion of the compost into humus. 

Biotechnol., 8, 203-210. van Beilen, J.B. and Poirier, Y. (2008) Production of renewable polymers from crop plants. Plant J 54, 684—701. Ledgard, S.F. and Giller, K.E. (1995) Atmospheric N2 fixation as an alternative N source, in Nitrogen Fertilisation in the Environment (ed. P.E. Bacon), Marcel Dekker, New York, 443-486. 

Ramadurai, S. 1990. Operational Experiences With NP/NPK Granulation at Coromandel Fertilisers Limited - India, IN J. J. Schultz and G. Hoffmeister (Eds.), Urea-Based NPK Plant Design and Operating Alternatives, Workshop Proceedings, pp. 21-26, SP-15, IFDC, Muscle Shoals, AL, U.S.A. 

The build-up of phosphates in the oceans or on the ocean beds may, in a few centuries, make the ocean the most economic if not the sole convenient source of supply. As an alternative to the direct mining of sea bed apatite, however, it may become possible to develop species of edible marine plants which could obtain their phosphate directly from (shallow) ocean waters, thus ranoving the necessity for fertiliser manufacture as we know it today [74]. 

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