Page 138 - Soil Degradation, Conservation and Remediation

5.2 Loss of Nutrients and/or Organic Matter 127

Table 5.2 Nutrient balances in some African countries
Nitrogen Phosphorus Potassium
−1
−1
−1
−1
−1
Countries (kg ha year ) (kg ha year ) (kg ha year )
−1
Benin −16 −2 −11
Botswana −2 0 −2
Cameroon −21 −2 −13
Ethiopia −47 −7 −32
Ghana −35 −4 −20
Kenya −46 −1 −36
Malawi −67 −10 −48
Mali −11 −2 −10
Nigeria −37 −4 −31
Rwanda −60 −11 −61
Senegal −16 −2 −14
Tanzania −32 −5 −21
Zimbabwe −27 2 −26
Source: http://www.fao.org/docrep/006/y5066e/y5066e06.htm

uses attribute and geographic database systems in conjunction with empirical and
mechanistic models to produce information for analyses and monitoring. The
approach builds upon previous work on nutrient balances (Stoorvogel and Smaling
1990; Smaling and Fresco 1993; Smaling et al. 1993). The nutrient balance values
are given in Table 5.2.
Soil nutrient depletion is linked to food insecurity in developing and least devel-
oped countries due to the intensification of land use for agricultural production with-
out proper application of external inputs (Henao and Baanante 1999). The absence of
nutrient replenishment of nutrient depleted soils is jeopardizing agricultural sustain-
ability in these regions (Sheldrick et al. 2002). Long-term decline in crop yields under
conditions of low-input and unbalanced fertilization in many parts of Africa, Asia, and
Latin America (FAO/UNDP/UNEP/World Bank 1997). Stoorvogel et al. (1993) and
Smaling et al. (1993) used nutrient budget approach to highlight the serious situation
of soil nutrient depletion in many African countries. Soil nutrient depletion and imbal-
ance are also widespread problems in Asia. Dobermann et al. (1995) computed
nutrient balances for rice-based farming systems in several eastern Asian countries
−1
−1
and reported that the K deficit ranged from 25 to 70 kg ha year . Lin et al. (1996)
estimated nutrient balances on 71 rice paddy farms in South China and found a
surplus of N and P but a deficit of K. Mutert (1996) reported negative balances of N, P,
and K in Bangladesh, Indonesia, Myanmar, the Philippines, Thailand, and Vietnam.

5.2.1 Leaching of Nutrients

Leaching is the downward movement of soil materials in solution and suspension
with percolating water. It distributes and removes significant quantities of materials
provided that there is a positive water balance, that is, if there is enough water excess