Agriculture Reference
In-Depth Information
The residual effect of insoluble CaCO
3
and MgCO
3
is high. The residual ef-
fect of the more soluble forms depends on the
CEC
of the soil, and in the case of
K, on the presence of mica clays that can fix K
in their structures (section 4.6.2).
Soil pH and Nutrient Availability
5.5
The Macronutrients
Soil pH influences the ratio of NH
4
and NO
3
in the mineral N pool through
its effect on nitrification. The optimum pH for nitrification in soil is 6.6-6.8, and
the rate of nitrification is slow below pH 5. Above pH 7, the rate of oxidation of
NO
2
to NO
3
decreases and NO
2
accumulates—a condition that is undesir-
able for plant growth.
The effect of pH on soil P availability is discussed in section 5.4.2.1. Soil pH
influences Ca, Mg, and K availability indirectly through its effect on mineral
weathering, carbonate dissolution, and cation exchange. As the soil pH falls be-
low 5, the concentration of exchangeable Al
3
increases and Al
3
ions displace
exchangeable Ca
2
, Mg
2
, and K
, which are then lost through leaching. Sul-
fate is also indirectly affected by soil pH. Positive charges on variably charged sur-
faces increase at low pH, so more sulfate is adsorbed and leaching is decreased.
5.5.1
The Micronutrients
Micronutrient availability is very dependent on soil pH. Over the pH range of
4-9, the elements Fe, Mn, Cu, Zn, and Co occur as cations. Mo occurs as an an-
ion, and B as the very weak boric acid (H
3
BO
3
), which dissociates at pH
8 as
follows:
H
3
BO
3
H
2
O
H
B(OH)
4
(5.9)
As discussed in chapter 4, the availability of these elements is determined by
the strength of their adsorption by clays and sesquioxides, relative to their ability
to form complexes with organic compounds, and by their tendency to precipitate
as insoluble oxides and hydroxides. The range of possible reactions is illustrated
in figure 5.8. For vines, the most important micronutrients are Fe, for chlorophyll
function, and Zn and B, for good fruit set.
The concentration of free micronutrient ions in the soil solution is normally
very low. Although the surface adsorbed component (fig. 5.8) accounts for
10%
of the total of any micronutrient in the soil, it is important because it buffers the
soil solution against depletion. The cations Fe
3
, Mn
2
, Cu
2
, and Zn
2
are ad-
sorbed on negatively charged surfaces in competition with Ca
2
, Mg
2
, and K
and also through complex formation on sesquioxide surfaces. As the pH increases,
the micronutrient cations hydrolyze, which increases their affinity for the charged
surface. The stronger retention contributes to the decrease in Fe
3
, Mn
2
, Cu
2
,
and Zn
2
availability as the pH rises. However, hydrolysis of the metal cations is
the first step in precipitation of the insoluble hydroxides, as discussed in ap-
pendix 5, thus Fe, Mn, Cu, and Zn deficiencies may occur in vineyards at a soil
pH
7.
5.5.2
