How the Soil Supplies Nutrients - Soils for Fine Wines - page 104

Summary Points 4.8

This chapter covered all aspects of how the soil supplies nutrients to plants. The
main points are summarized here.
■ The elements C, H, O, N, P, S, Ca, Mg, K, and Cl are essential for plant growth
and reproduction. They are called macronutrients because they occur in concen-
trations 1000 mg/kg of plant DM. The elements Fe, Mn, Zn, Cu, B, and Mo,
which are also essential, normally occur at concentrations 100 mg/kg DM, and
therefore are called micronutrients . Elements in concentrations 1000 mg/kg in
the soil are called trace elements . They include Cr, Se, I, and Co, which are es-
sential for animals only. Others such as Li, Be, As, Hg, Cd, Pb, and Ni are not
required by plants or animals, but are toxic at concentrations greater than a few
mg/kg.
■ Nutrients in soil-plant-animal systems are continuously cycling among a biomass
store (living plants and animals), an organic store (dead plant and animal matter),
and an inorganic store (minerals and the soil solution). Much of the biomass store
is aboveground. The organic store is concentrated in the top 15-25 cm of soil,
whereas the inorganic store is distributed through the soil profile. Within each
store, nutrients can be transformed from one chemical form to another.
■ Plants absorb nutrients from a labile pool (part of the inorganic store), consisting
of ions in solution and adsorbed by clays, sesquioxides, and organic matter. These
are called “available nutrients.” Nutrients and potentially toxic elements held in
insoluble precipitates or strongly adsorbed complexes (organic or inorganic) are
nonlabile . This subdivision is particularly important for elements such as P, Fe,
Mn, Zn, Cu, and Mo.
■ Ninety-eight percent or more of soil N is in the organic store, but it is converted
by ammonification and nitrification (collectively called mineralization ) to the min-
eral forms NH 4 and NO 3 , respectively. The reverse process—incorporation of
mineral N into microbial protein—is called immobilization .
■ A minority of plants form symbiotic associations with microorganisms and reduce
N 2 gas to NH 3 (incorporated into proteins) within their tissues. This process is
called biological nitrogen fixation . The most important symbioses of this kind in-
volve legumes and the bacterium Rhizobium . Some free-living microorganisms also
fix atmospheric N 2 . Nitrogen fixation by legume cover crops can provide an im-
portant input of N to the biomass store in vineyard soils.
■ Cations such as Ca 2 , Mg 2 , K , Na , and NH 4 are held as exchangeable cations
2

by negatively charged clays and organic matter. Anions such as H 2 PO 4 , SO 4 ,
NO 3 , and Cl are attracted to positively charged sites on the edge faces of clays
(at pH 6) and sesquioxides (at pH 8). Phosphate ions, which have a high
chemical affinity for Al and Fe, are also adsorbed on sesquioxide surfaces by li-
gand exchange . Ions such as H 2 PO 4 and MoO 4 2 that attach to surface metal
atoms by ligand exchange are specifically adsorbed and not easily released.
■ The combination of fixed negatively charged surfaces and mobile cations in the
contiguous solution comprises a diffuse double layer ( DDL ) up to 10-20 nm thick.
In detail, the DDL consists of a Stern layer of cations tightly adsorbed to the sur-
face plus a diffuse layer in which cations are accumulated and anions are in deficit,
relative to the bulk solution. The thickness of the DDL decreases with the charge
on the adsorbed cation and with an increase in ion concentration of the bulk so-
lution. An increase in DDL thickness causes flocculated domains of clay crystals
to swell in water and may lead to deflocculation.