Agriculture Reference
In-Depth Information
ing animal waste and biosolids). These inputs must be carefully monitored so that
the potentially toxic elements do not exceed safe levels.
Concentrations of elements in soils and plants can be expressed in several dif-
ferent units of measurement. It is important to know the meaning of these units,
and the relationships between laboratory and field measurements, as explained in
box 4.1.
Box 4.1
Ways of Expressing Concentrations of Elements
The concentration of an element (e.g., K) in soil or plant material is often
expressed as
g K/g soil (or plant material). Note that
1 mg K/kg
1 part per million (ppm K)
1
g K/g
(B4.1.1)
1.33 Mg/m
3
, we
know that the mass
M
of o.d. soil per ha-0.15 m is 1995 Mg. We can use this
relationship to convert from laboratory results (mg/kg) to an element content on a
field scale as follows:
1 mg K/kg soil
From equation B3.4.4 (box 3.4), for a soil of average
BD
1.995 kg K/ha-0.15 m
2 kg K/ha-0.15 m
(B4.1.2)
An exact conversion for any given soil can be made if the soil
BD
is
measured. If the soil depth of interest is
0.15 m, the conversion factor in this
equation is correspondingly greater. For example, suppose the average mineral N
concentration in a vineyard soil was measured to a 1-m depth. The conversion
equation would be
2
1 mg N/kg soil
13.3 kg N/ha-m
(B4.1.3)
0.15
The concentration of an element in the soil is sometimes expressed as a
percentage. For example, 1% C may be written as
1 g C
0.01 kg C
1% C
19,995 kg C/ha-0.15 m
(B4.1.4)
100 g soil
kg soil
Therefore,
1% C
20,000 kg C/ha-0.15 m (B4.1.5)
Fresh or air-dry soil samples are usually used for the analysis of “available”
elements. Analysis of total element concentration may be done on o.d. soils. But
all analytical values for soils and plants should be expressed on an o.d. basis. Plant
material should be dried to constant weight at a temperature of 70°C. The
gravimetric water content
g
(see equation 3.3) is used to correct a soil analysis to
an o.d. basis. For example, suppose the soil P content is measured as 0.1 P% (air-
dry basis), and
g
for the air-dry soil is 10%. Then
0.11% P (o.d. soil) (B4.1.6)
Elemental analyses of organic manures and biosolids are often quoted as a
percentage of fresh or “wet” weight. To convert to a DM basis, we need to
measure the dry matter percentage of the fresh weight, and calculate the element
concentration as follows:
0.1% P (air-dry soil)
110/100
% P (o.d.soil)
g P
100 g fresh weight
% P (DM)
(B4.1.7)
100 g fresh weight
g DM
