Plant Nutrients - Soils and Food Security - page 184

cation exchange. Given the current high price of K, it is appropriate to
consider widely available silicate minerals as an alternative source.

d n 1 r 2 n g | 3

1 Introduction
This chapter focuses on the need for mineral fertilisers, recognising that these
are supplied by the mining industry and ultimately come from finite (but very
large) stocks of geological materials.

1.1 Geological Sources of Plant Nutrients

For healthy growth, plants require a diverse range of nutrients. Carbon
dioxide is derived from the air, as a raw material for photosynthesis, and water
is derived from surface or groundwater sources. The dominant fertiliser
nutrients that are applied to farmed plants are nitrogen (N), phosphorus (P)
and potassium (K), which make up the bulk of the global fertiliser industry
output. Minor nutrients and trace elements are also vital, and can be applied
artificially if there is evidence of deficiencies.
The fertiliser industry is dominated by the production of N, P and K
fertilisers as bulk commodities. This is big business, worth of the order of US$
70 billion for N (fixed ammonium), 1 US$ 20 billion for P, 2 and US$ 26 billion
for K. 3 By comparison, the global petroleum industry is worth of the order of
US$ 2000 billion, gas US$ 750 billion, and coal US$ 1000 billion. 4
The production of N, P and K fertilisers depends on geological resources,
but these differ in each case. 5 Nitrogen production is very closely related to
that of natural gas, on two counts - gas is a source of raw materials and a
source of the energy required to fuel the process. N fertiliser production uses
the Haber process, which involves reaction between atmospheric N and
hydrogen derived from methane, to produce ammonia and carbon dioxide,
according to the following simplified reactions:

CH 4 z2H 2 O~CO 2 z4H 2

ð 1 Þ

N 2 z3H 2 ~2NH 3

ð 2 Þ

The CO 2 produced in this process is not necessarily wasted, as it can be
combined with ammonium to produce urea, CO(NH 2 ) 2 , a common fertiliser
product. Nitrogen fertiliser production is energy-intensive, consuming 94% of
the energy used globally in the manufacture of all fertilisers (K and P use 3%
each). 5
In contrast to N, both P and K fertilisers are derived from mined rocks, with
varying amounts of processing. Phosphate fertilisers are ultimately derived
uniquely

from phosphate rock, which occurs widely across the globe.