Fertilisers

Chapter 7

Soil Analysis Ch7 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

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

Fertiliser Grade

%P

%K

x 0.44

x 0.83

÷ 0.44

÷ 0.83

Soil Analysis Ch7 5

Exercise 7.2

General Slow Release Soluble

Total N 5.0 18.0 15.0

N grade 5.0 18.0 15.0

Total P 5.5 4.8 4.0

P grade 12.5 10.9 9.1

Total K 4.1 9.1 26.0

K grade 4.9 11.0 31.3

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