Agriculture Reference
In-Depth Information
Nutrient Availability in the Soil
When considering their availability to grapevines, the nutrients and other ele-
mentscanbedividedintogroupsasfollows:
1. MacronutrientelementsN,S,andPthatexistmainlyinorganicforms
2. Calcium,Mg,K,Na,andP,whichbondwithvaryingstrengthas
inorganic ions to soil particles (note that P is included in both Groups 1
and 2)
3. MicronutrientelementsFe,Mn,Zn,Cu,andMo,andtraceelementsPb,
Cd, Cr, and Ni, which form insoluble compounds and also bond strongly
to mineral particles and organic matter
4. Chlorine and B (nutrients) and As (potentially toxic), which occur
mainly in solution.
Only a nutrient in the soil solution is immediately available to vine roots. Assessing
the availability of other nutrients that are distributed between solid and solution
phases is difficult because both phases can contribute to the supply. Deficiency
occurs when a nutrient's supply is insufficient to meet a vine's demand; conversely,
when supply exceeds demand or a nonnutrient element accumulates in the root
zone, toxicity may occur.
Availability of Nitrogen, Sulfur, and Organic Phosphorus
About 99% of soil N is in an organic form not directly available to a vine. Thus
asoilwithanorganicCcontentof1%,equivalentto20,000kgC/ha.15cm,
2
will have between 1333 and 2000 kg N/ha.15 cm in an organic form, depending
on the C-to-N ratio of the organic matter (discussed later). The available forms
(NH
4
+
and NO
3
−
ions) are produced through mineralization by microorgan-
isms—literally, the conversion of an organic form into an inorganic or “mineral”
form.MineralizationalsodescribestheconversionoforganicSandPintoinor-
ganic S (SO
4
2−
) and P (H
2
PO
4
−
and HPO
4
2−
), which are the plant-available forms.
The reverse process of incorporation of C, N, P, and S into living microbial tissue
is called immobilization. The role of soil microorganisms in the C cycle is dis-
cussedin“heSoilBiomass,”chapter5.Box3.2describessomeconsequencesof
N mineralization and nitrification.
2
This notation means kg C per ha to 15 cm depth.
