World’s Soil Resources

Figure 6.11 Major components of the soil nutrient balance.
The red discontinuous line marks the soil volume over which the mass balance is calculated. Green arrows correspond to inputs and
red arrows to losses. ΔS represents the change in nutrient stock.
6.8.2 | Principles and components of soil nutrient balance calculations
Because the magnitude of the nutrient fluxes is often small compared to the total stock of nutrients in the
soil profile, changes in soil nutrient stocks can be rather slow and difficult to detect over short time scales
(< decades). Hence calculating nutrient balances from nutrient flows rather than from changes in nutrient
stocks has been preferred in many studies (Figure 6.11).
Table 6.6 lists the main inputs and outputs used for calculating the mass balances of N, P and K. Inorganic
amendments are mostly composed of mineral fertilizers, but also comprise urine or minerals contained in
irrigation water. Organic amendments include liquid, semi-solid or solid manures, compost, mulching
material not produced on-site, and household refuse. It also includes faeces dropped by animals. In systems
such as urban gardening, the re-use of waste water may also input organic compounds. Biological fixation by
bacteria is restricted to N. Wet deposition refers to nutrients supplied with rainwater, whereas dry deposition
refers to nutrients deposited as dust and aerosols. Dry deposition is a particularly important phenomenon
in the case of K in areas downwind of major dust producing areas (e.g. West Africa;). Sedimentation refers to
the deposition of sediment eroded upstream or to sediment deposited during river flooding. Additional fluxes
may exist in specific situations (e.g. nutrients in subsurface lateral flows ; supply of NO 3
from groundwater ().
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