World’s Soil Resources

N P K
Nutrient inputs
Inorganic amendments Yes Yes Yes
Organic amendments Yes Yes Yes
Biological fixation Yes No No
Dry or wet deposition Yes Yes Yes
Sedimentation and run-on Yes Yes Yes
Nutrient outputs
Harvested products Yes Yes Yes
Grazed products Yes Yes Yes
Leaching Yes Generally negligible Low
Gaseous emissions Yes No No
Erosion and runoff Yes Yes Yes
Table 6.7 Major components of soil nutrient mass balances for N, P and K
The main losses are related to nutrients contained in exported harvested products (crops or fodder), and
nutrients contained in food ingested by primary grazers (Table 6.7). Nutrients may also be lost by gaseous
emissions (NH 3
, N 2
, N 2
O), through erosion and in surface runoff, or by leaching. The latter applies mostly to
NO 3
-N, to a lesser extent to NH 4
-N and K, and to a very limited extent to PO 4
-P except in coarse textured soils
saturated with P.
6.8.3 | Nutrient budgets: a matter of spatial scale
The larger the spatial scale, the more certain nutrient flows are internalized. For instance, in a selfsufficient,
well-managed farm, the net balance may be nil or close to nil. However, different parts of the farm
may well have very different balances. Likewise, in extensively-managed agropastoral systems, nutrient flows
mediated through livestock occur between rangelands and croplands. At a regional scale, the balances may
thus be nil or only slightly negative, whereas large imbalances exist within the region (see Box 6.1).
At the global scale, fertilizer use and the growing of leguminous crops have resulted in a doubling of the
rate at which N enters the terrestrial ecosystems as compared to pre-industrial levels. Likewise, the use of P
fertilizers, animal feed supplements and detergents has led to a doubling of P inputs in the environment as
compared to background P release from weathering. This is indicative of a net positive balance but hides large
regional disparities. Bouwman, Beusen and Billen (2009) calculated global soil N and P balances for the year
2000. Outputs were restricted to harvested and grazed crops and grasses, whereas inputs included manure,
fertilizers, N deposition and N fixation. These authors estimated the inputs to soils at 249 Tg N and 31 Tg P yr -1 and
losses through harvest and grazing at 93 Tg N and 16 Tg P yr -1 . Assuming no build-up of N in the soil, their model
predicted that 16 percent (41 Tg yr -1 ) of the inputs may be lost by erosion and leaching, thereby contributing
to a loss in environmental quality. In the case of P, their calculations predicted a net accumulation of P at a
rate of 12 Tg yr -1 and losses of P through leaching and erosion of 2 Tg yr -1 . On a continental scale, considering
both natural and agro-ecosystems, balances were always positive and comprised between 8.5 (North Asia)
and 35 (Europe) kg N ha -1 yr -1 , and between 0.22 (Africa) and 5.5 (Europe) kg P ha -1 yr -1 . Focusing specifically on
P and cropland, but restricting the balance calculations to fertilizer and manure inputs and harvest outputs,
highlighted large P deficits in South America, northern United States and eastern Europe. Large P surpluses
were found in the coastal United States, western and southern Europe, East Asia and southern Brazil.
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