Fertilisers and manures

Abstract This chapter explains the importance of feeding crops and ensuring that soil fertility is maintained. It discusses the main elements required by crops, both major and trace. It explains why liming is so important for crop nutrition. It deals with the calculations and sources of information needed to accurately decide on fertiliser application rates, and cost them. It describes the materials used on-farm as fertilisers and explains the differences between straights, compounds and blends, as well as dealing with liquids and solid fertilisers. There is a section on the use of oiganic manures and slurries and how to make best use of them. Finally, the chapter deals with the effect of fertilisers on the environment, during both their manufacture and application. 

Key words nitrogen, phosphate, potash, lime, fertihser manual, compounds, straights, 

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Available P was adequate but decreased in all except the synthetic fertiliser treatment. The average P budgets (kg P ha y ) were 3.8 for synthetic fertilisers only, -5.0 for synthetic fertilisers and manure, -S.7 for organic and -7.8 for biodynamic (Oehl et al. 2002). 

Nitrous oxide contributes severely to global warming and the depletion of ozone in the stratosphere (Crutzen 1981, Bouwman 1996). Almost 90% of the global atmospheric N2O is formed during the microbial transformation of nitrate (NO ) and ammonia (NH ) in soils and water. In OECD countries the agricultural contribution to N2O emissions is estimated at 58% (IPCC 2001). Soils fertilised with inorganic fertilisers and manure stores are seen as the largest sources (Chadwick et al. 1999, Brown ef al. 2002). 

Less affirmative, though not necessarily less favourable than for conventional systems, is the evidence that has been presented in fields like pollution of water resources and the food chain with pathogens (due to the more pronounced use of organic fertilisers and manure). The same applies to the emission of N2O and CH (because manure stores are seen as a major source and because, on an output unit scale, the CH,j emission potential tends to be higher in organic farming). 

The recommendations for fertiliser use were reviewed in 2010 and published by the Department for Environment, Food and Rural Affairs (Defra) in their Fertiliser Manual 8th edition (RB 209). This is essential reading for anyone involved in crop production. The new recommendations take more account of the economic use of fertilisers, the value of a wider range of oiganic manures with more emphasis on readily available nutrients, the minimisation of environmental risk associated with the use of all types of fertilisers and manures, and introduce new recommendations for biomass crops. 

The new Defra nitrogen recommendations continue to use Soil Nitrogen Supply (SNS) as their basis but the system has been fully revised with clearer definitions of soil type and revised index tables. Defra defines SNS as the amount of nitrogen (kg N/ha) in the soil (apart from that applied for the crop in manufactured fertilisers and manures) that is available for uptake by the crop throughout its entire life, taking accoimt of nitrogen losses . It is calculated using the equation ... 

Normal agricultural practices generally cause an enrichment of heavy metals in soil, particularly Zn, Cu and Cd, due to the application of manure or its derivatives, compost or sludge and inorganic fertilisers and other human activities such as... 

N2O contributes to the greenhouse effect with 4% (Schonwiese 1995). N2O emissions from agriculture come from mineral and organic N-fertilisers and from leguminous crops. The emission levels depend on the kind of fertiliser and on the application technique. The N2O emission factors for the most frequently applied forms of mineral N-fertilisers are < 0.5%, for organic manure 1.0 - 1.8% and for N from legumes, about 1% of the fixation rate. 

Conventional arable (synthetic fertiliser), organic arable (green manures) and organic mixed (green manure and manure) over 15 years. 

Nitrogen is added to the soils in organic forms in crop residues and manures and in synthetic forms (ammonium, nitrate and urea) in fertilisers. The organic nitrogen in soil is slowly mineralised by biological processes, the final stages being to ammonium, nitrate and nitrite.43 The conversion of ammonium to nitrite in soils is rapid, while the conversion of nitrite to nitrate is so rapid that nitrite is rarely detectable in soils. 

Approved inputs are listed in Annex II of the Regulation although nearly all require permission from the certifying body before they can be used. Approved fertilisers and soil conditioners include low soluble materials such as rock phosphate and lime, as well as animal manures and by-products such as bone meal and hoof/hom meal. More soluble materials such as blood meal, wood ash and potassium sulphate are also allowed, again following approval from the certification body. 

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