Chemistry Reference
In-Depth Information
oil (reflecting urea manufacture's high energy requirement); DAP does so to a
lesser extent. Urea and DAP fertiliser prices peaked in 2008, reflecting changes
in the price of petroleum. KCl peaked soon afterwards, reaching almost US$
900 per tonne. Prices dropped in 2009, and have since started to rise again. At
present, the price of KCl is about US$ 500 per tonne.
d
n
1
r
2
n
g
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3
3 Nutrient Audits and Fertiliser Use Statistics -
Evidence of Need
Farmers use fertilisers to replace the nutrients that crops have removed from
the soil. After harvest, nutrients can be returned to the soil from crop residues;
they are also supplied by manures and composts, often produced locally.
Fertilisers are purchased on a market that has a limited number of suppliers
focused in geographically few locations.
Audits of plant nutrient supply underpin the use of fertilisers, and give rise
to the concept of nutrient balances. In the UK, recommended fertiliser
application rates are published by the government,
8
and these tell farmers what
applications are required for different crops so that soil fertility is maintained
from year to year. GPS-controlled precision methods can be used to take into
account natural variability in natural soil nutrient content within a single field,
to ensure that just the right amount of fertiliser is applied.
On a wider scale, nutrient audits for P
14,15
and P, K, and N
16
have been
carried out globally and for individual countries. These assess the amount of
nutrient removed by offtake (removal of a crop from the field) and compare
this with the amounts of nutrient that are supplied by different inputs. An
example is shown in Figure 4. This shows that, on an assessment of the entire
continent of Africa,
17
total nitrogen inputs match outputs, and so N is in
balance. Nitrogen fertiliser inputs supply less than 30% of total inputs.
Similarly, P is also in balance or added to excess; in this case, fertilisers make
up 50% or more of inputs. The situation with K differs; total inputs correspond
to 40% of outputs, and fertiliser use corresponds to no more than 20% of
inputs. Thus, on a continental basis, there is removal of K from African soils
that is not replenished by fertiliser inputs. This observation is supported by
data published by the FAO.
11
Of the 57 African countries considered, 47
consume no potash fertiliser, and the total imports of K fertiliser (there is no
significant indigenous production) amount to 450 000 tonnes, 1.5% of world
production in support of almost 15% of the world's population.
On a global basis for the period 1960-1996, it has been shown
16
that the net
annual rates of removal of N and P from soil progressively increase from 8 to
12 kg N ha
21
and from 2 to 4 kg P ha
21
. Much greater values are reported for
K, which is removed at rate increasing from 10 to 20 kg K per ha in this period.
These values correspond to global deficits of 18.3 Mt N, 6.75 Mt P (50 Mt
phosphate rock at a grade of 30% P
2
O
5
), and 30.56 Mt K (equivalent to 36 Mt
K
2
O). To compensate for these deficits would require an increase in world
nitrogen fertiliser production of about 15%, and an increase in mining of
