Agriculture Reference
In-Depth Information
at pH 7, so it makes a significant contribution to the total
CEC
of any soil that
is sandy and has more than 1% to 2% organic C. In clay soils, the contribution
of organic matter relative to that of the clay minerals is much less because of the
preponderance of clay relative to organic matter.
The variable negative charge on organic matter attracts cations such as Fe
3+
,
Al
3+
, and Ca
2+
. These adsorbed cations can act as an “electrostatic” bridge between
the organic matter and negatively charged clay minerals. Alternatively, negatively
charged organic polymers can be directly attracted to positively charged Fe and Al
oxides. These kinds of charge interactions are fundamental to the aggregation of
soilparticles,whichisaprerequisiteforgoodsoilstructure.
Availability of the Micronutrients and Trace Elements
Iron,Mn,Cu,andZnoccurasmetalliccations(seetable3.1),asdothetrace
metals Cd, Pb, and Ni (as Cd
2+
, Pb
2+
, and Ni
2+
). The fully charged cationic
forms exist only at low pH. As the pH increases, these hydrated metal cations
hydrolyze
3
and eventually precipitate as insoluble hydroxides. For example, the
cation Fe(H
2
O)
6
3+
, which exists at pH less than 3, hydrolyzes to the hydroxy
form Fe(OH)(H
2
O)
5
2+
as the pH increases. This hydrolysis continues with
the stepwise release of H
+
ions until neutral Fe(OH)
3
is formed between pH 4
and 5. The other micronutrient cations and trace element metals (Pb, Ni, and
Cd) behave similarly, with their hydroxides forming at higher pHs. Because of
adsorption of the hydroxyl cations by clays and precipitation of the hydroxides,
the overall effect of an increase in pH is for the availability of these elements to
decrease.
The metal cations Fe
3+
, Mn
2+
, Cu
2+
, and Zn
2+
also form complexes with
organic compounds. Although these complexes are generally insoluble (when
the organic matter is well humified), repeated applications of the metals can
raise the concentration of the soluble fraction of the complexes to a point where
they become toxic to beneficial soil organisms such as earthworms. This effect
has been observed in older vineyards that have been repeatedly sprayed with
Bordeaux mixture (a mixture of lime in copper sulfate [CuSO
4
] solution). On
the other hand, some synthetic organic compounds called chelates are used to
formsolublecomplexeswithFe,Mn,Cu,andZntoinhibittheformationof
the insoluble hydroxides and make these nutrients more plant-available. One
example is the synthetic compound ethylenediaminetetraacetic acid (EDTA),
which forms the complex Fe EDTA
−
, which is more stable than Fe(OH)
3
up
to pH 9. Similar chelation occurs between metal cations and natural organic
3
Hydrolysis occurs when a water molecule attached to a cation splits into H
+
and OH− and the H
+
ion is released.
