Fertilisers

1. Fertilisers Chapter 7

2. Fertilisers • any material added to soil to supply one or more nutrient elements: • organic or inorganic • natural or synthetic • necessary to maintain fertility because of intensive cropping • organic fertilisers have the macronutrients – especially the key three N, P and K – but also organic matter which enriches the soil • inorganic fertilisers, which are generally more processed, only provide the macro- and micronutrients Soil Analysis Ch7

3. Inorganic • a mixture of salts of the various nutrients • N:P:K grade is an important measure • the %w/w of the three elements (with P expressed as P2O5 and K expressed as K2O) • does not actually contain these forms of P and K • the standard way of reporting their levels • eg a fertiliser with a grade of 10-6-8 is composed of 10% N, the equivalent of 6% P2O5 and the equivalent of 8% K2O. Soil Analysis Ch7

4. x 0.44 %P ÷ 0.44 Fertiliser Grade ÷ 0.83 %K x 0.83 Conversion between % Class Exercise 7.1 • A fertiliser has a grade of 14.1-8.2-10.3, What are the actual levels of N, P and K? • %N = 14.1, %P = 8.2x0.44 = 3.6, %K = 10.3x0.83 = 8.5 Soil Analysis Ch7

5. Exercise 7.2 Soil Analysis Ch7

6. Commercial grade • complete – have the full set of macro- and micronutrients, • trace element mixture – micronutrients only • specific plants – formulations for plants requiring nutrients in different proportions to normal • slow release • soluble – all components are soluble, providing a quick tonic for plants, but one that is readily lost by leaching Soil Analysis Ch7

7. Organic fertilisers • Natural material, principally manure, is both: • a mulch – a slowly decomposing organic cover over the soil to retain moisture – and • a source of nutrients • NPK grade of fresh animal manure is typically around 10:5:10 • increased after drying (less water) • different for different animals • addition of organic matter to the topsoil is probably as important in manure fertilising as the provision of elemental nutrients Soil Analysis Ch7

8. natural fertiliser is processed before use • safety – the risk of disease being spread by animal waste • contamination – especially by weed seeds • wood ash is contains a high proportion of potash (K2CO3) • a good source of that element, but is strongly alkaline Soil Analysis Ch7

9. Use • overuse by commercial farming activities • pollution of groundwater and surface water through leaching of excess nutrients • lead to algal blooms, toxic algae and eutrophication • timing also important • particularly for nitrogen, which is readily lost if not immediately used • most appropriate time for nitrogen application is the plant moves into its main growth cycle • if the plant is moving towards flowering of fruiting, then nitrogen is the last thing it needs, because it will only encourage leaf growth Soil Analysis Ch7