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5.3 Acidification 129

5.2.3 Nutrient Losses due to Erosion

Amounts of nutrient losses by water erosion are affected by soil type, cover, and
fertility; slope of the terrain; amount, method, and time of fertilizers application;
intensity, amount, and moment of occurrence of rainfall (Gascho et al. 1998); as well
as by soil management practices (Barisas et al. 1978; Schick et al. 2000; Mello 2002).
Nutrient losses are expected to be reduced in soil management systems that preserve
plant residues. However, even under such conditions, plant residues can be washed off
(Burwell et al. 1975), becoming sources of soluble nutrients (Barisas et al. 1978;
Johnson et al. 1979), which can be lost by water erosion. Ca and Mg losses by water
erosion are usually high (Bertol 1994; Schick et al. 2000). This occurs because these
elements are strongly adsorbed by colloids in the soil, which makes it easy for them
to be transported with the sediment, and also because they occur in high concentra-
tions in erosion water. Gafur et al. (2000) reported loss of 61 kg Ca, 13 kg Mg, 13 kg
K, 0.14 kg P, 0.20 kg S, 0.05 kg Cu, 6.7 kg Fe, 6.1 kg Mn, and 0.065 kg Zn due to
erosion in slashed and burnt sites in southeastern hills of Bangladesh.

5.2.4 Crop Removal of Nutrients

Crop removal of nutrients is affected by soil and climatic conditions. Soil properties
such as available soil nutrient supply, moisture content, aeration, compaction, soil
temperatures, pH, nutrient imbalances, and other factors affect uptake of plant
nutrients. Table 5.4 gives the values of nutrients removed by the harvested crops.
If residue contents of nutrients were added, these values could be multiplied by a
factor of 2–3. So, crops alone remove a substantial amount of soil nutrient which
need to be replenished by fertilizers. If residues are also removed from field, the loss
−1
becomes much higher. On an average, harvested crops remove 50–100 kg ha N,
−1
−1
10–20 kg ha P, and 15–30 kg ha K in field crops. Potassium removal amounts to
100–200 kg ha by forage crops (Table 5.3).
−1
5.3 Acidification

Soil acidity is indicated by the concentration of hydrogen (H ) ions in soil solution.
+
When (H ) ions predominate over (OH ) ions, the soil is said to be acidic. The
+
−
+
predominance of (H ) and (OH ) ions is expressed by soil pH. The pH is the nega-
−
tive logarithm of the hydrogen concentration (moles per liter), and its scale ranges
from 1 to 14. A pH of 7.0 is taken as the neutral point, with values below 7.0 being
acidic and above 7.0 being alkaline. Because the pH scale is logarithmic, soil with
a pH of 5 is 10 times more acidic than soil with a pH of 6 and is 100 times more
acidic than soil with a pH of 7. There are two types of soil acidity: active acidity
created by the predominance of (H ) ions in solution and reserve or exchangeable
+