Page 143 - Soil Degradation, Conservation and Remediation

132 5 Chemical Soil Degradation

Diammonium phosphate is the primary phosphorous fertilizer available in many
countries and has a moderate acidifying effect when applied. For every kg of
N added as DAP, 5.25 kg of limestone is needed for complete neutralization.
If phosphorus fertilizer is recommended, triple superphosphate (0-46-0) can be
used; it does not impact soil pH.
Severely acidifying fertilizers: Ammonium sulfate, monoammonium phosphate (MAP)
These fertilizers are very acidifying and should be avoided if possible. Approximately
8 kg of agricultural limestone is needed to neutralize the effects of 1 kg of these
fertilizers. For example, if the grower applied a 50 kg MAP or ammonium sulfate,
400 kg of lime would have to be applied. Usually farmers apply the fertilizers,
but not the lime. So, the soil becomes acid in the long run. The amounts of lime
required for unit amount of some fertilizers are shown below.

Nitrogen source Lime requirement, kg pure CaCO 3 per kg N applied (Glendinning 2000)
Ammonium sulfate 5.2
Anhydrous ammonia 1.8
Ammonium nitrate 1.8
Urea 1.8
MAP 5.0
DAP 3.1

5.3.1.2 Leaching of Nitrate

Leaching of nitrate is a major cause of agricultural soil acidification. Nitrate is
derived from the biological oxidation of ammonium or added as nitrate containing
fertilizers. Ammonium is obtained in soil from fertilizers and mineralization of
organic matter. The chemical processes that produce nitrate nitrogen from ammo-
nium leave the soil slightly more acidic. This acidity is neutralized by plants dis-
charging an alkaline substance as they take up nitrate nitrogen. While the plants
continue to take up all the nitrate nitrogen, the acid/alkali balance of the soil
surrounding the roots remains in balance. Leaching breaks the balance of the acid/
alkali processes and results in increased soil acidity. Nitrate leaching has important
environmental impacts. Leached nitrates from agricultural lands may pollute
ground and surface water.

5.3.1.3 Removal of Produce

Grain, pasture, and animal products are slightly alkaline, and continued removal
will lower the soil pH over time. If very little produce is removed, such as in wool
production, then the system remains almost balanced. Where a large quantity of pro-
duce is removed as in the case of hay making (particularly clover or alfalfa hay), the