Characteristics of Natural and Urban Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 109

Man-made salinity is widespread in most dry regions of the world where irri-
gation is carried out. In Australia it also is very common where removal of the
original vegetation of woodland or native scrub and perennial herbaceous ground
storey plants has led to decreased evapo-transpiration, increased rainwater acces-
sions to the groundwater, rising groundwater tables and capillary transport of salt
to the soil surface. This process is called “dry land salting” to distinguish it from
irrigation salting.
2.3.1.5 Nitrogen and Phosphorus
Similar to some compounds mentioned above, even macronutrients such as nitrogen
and phosphorus may behave like a contaminant.
Nitrogen as a biologically vital element will occur in a very large number of natu-
ral organic materials. In mineralised form as ammonia, ammonium, nitrite or nitrate
it is highly soluble in water and mobile, but in water ammonium acts like a cation
and is subject to the same exchange processes as other cations. When adsorbed as
exchangeable cation it is not mobile, but easily exchanged with other cations. All
mineral forms are subject to a wide range of biochemical processes that can reduce
the oxidised forms to N 2 (denitrification), oxidise the reduced forms (nitrification),
and convert them to new organic contaminants.
Phosphorus as another vital element in living systems has a somewhat simpler
chemistry in the soil, as in mineralised form it occurs only as the phosphate (PO 2 4 )
anion in soil. Like nitrogen, it also occurs in organic contaminants. The phosphate
anion is strongly adsorbed in most soils by the coatings of ferruginous material
around soil particles, and it can form low-solubility compounds with aluminium
hydroxide, iron oxy-hydroxides and calcium. The most common phosphate soil
minerals are fluorapatite (Ca 5 (PO 4 ) 3 F), hydroxylapatite (Ca 5 (PO 4 ) 3 OH), variscite
(AlPO 4 .2H 2 O) and strengite (Fe 3+ PO 4 .2H 2 O) and all have very low solubility.
Therefore, phosphate is generally only slightly mobile in soils which contain signif-
icant amounts of clay, the above mentioned oxy-hydroxides or calcium carbonate.
The maximum solubility of these phosphates is when the soil pH is around 6.5 (Wolt
1994 ).
The most discussed aspect of excessive concentrations of N and P in the soil
concern the off-site impact. An example in the case of nitrogen is the risk to the
groundwater quality and to streams and lakes from the inflow of high N seepage,
and in the case of phosphorus the risk of high-P contaminated soil particles washing
off the land with runoff to adjacent open water bodies. However, there are also
other risks. Clay-sized soil particles become increasingly dispersive and mobile with
increased sorbed P (Giszczak et al. 2006 ). Thus they can move more easily within
the soil and over the soil surface with runoff. Likewise, Zhang et al. ( 2003 ) found
that colloidal iron oxides in sandy soil became much more mobile in the pore water
as the amount of sorbed P increased and that other trace elements followed the trend.
Smil ( 2000 ) quotes a 1977 survey by Frissel and Kolenbrander ( 1977 ) which
showed that continuing applications of manures and fertilisers to agro-ecosystems
created annual surpluses of P in the soil of 40 kg ha − 1 in the Netherlands, 30 kg ha − 1
in France, 25 kg ha − 1 in Germany and 10 kg ha − 1 in England.