The Nutrition of Grapevines - Understanding Vineyard Soils - page 78

Similar principles apply to the mineralization of organic P and S compounds
in soil, except that the critical C-to-P and C-to-S ratios are of the order of 10 times
higher than the C-to-N ratio.
Availability of Calcium, Magnesium, Potassium, Sodium, and
Inorganic Phosphorus
heavailableformsofCa,Mg,K,andNaarethecationsCa 2+ ,Mg 2+ , K + , and
Na + . These cations are attracted to negatively charged soil particles and can be
exchanged by other cations in the soil solution; hence they are called exchangeable
cations. Although not a nutrient, Na + is included because Na + ions have an impor-
tant effect on the behavior of soil colloidal particles in water (see box 4.3, chapter
4). In addition to its organic component, half or more of the P in soil may occur in
an inorganic form that is strongly bonded to Fe and Al oxides or precipitated as an
insoluble calcium phosphate.
Within a soil's clay fraction, identified in “Calibration for Texture,” chap-
ter 2, the negatively charged particles comprise crystalline clay minerals and
organic matter. Iron and Al oxides are normally present as positively charged
particles or as “coatings” on clay surfaces. All these components of the clay
fraction are important for nutrient retention in soil, as discussed in the next
section.
RetentionofNutrientsbyClayMineralsandOxides
Although clay minerals and oxides are of varied composition, they exhibit sev-
eral basic features, outlined in box 3.3. Examples of the crystalline structure
of the common clay minerals are shown on the website www.virtual-museum.
soils.wisc.edu/displays.html. Because minerals of the clay fraction are very small
(<2micronsequivalentdiameter),theyhavealargesurfaceareaperunitvolume
(called the specific surface area). The mineral montmorillonite, which belongs
to the same group as the bentonite used to coagulate wine proteins, has the
largest specific surface area. For example, if a heaped teaspoon (about 5 g) were
spread out one layer thick, the clay particles would cover about half a football
field.
The large specific surface area and permanent negative charge make clay min-
erals the site of many chemical and physical reactions in soil. The negative charge
is measured by the number of moles of cation charge that can be held per kilo-
gram of clay. This is called the cation exchange capacity ( CEC ). However, because
organic particles can also carry a negative charge (see next section), CEC is usually
measured as the moles of cation charge per kg of soil.
Box 3.4 explains the concepts of moles and moles of charge, and table 3.3
summarizes some of the properties of the common clay minerals.