Soil concentrations %, mg g1, ppm, etc may or may not reflect differences in soil content kg ha1, g

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Calculating Soil C and Nutrient Contents
Soil concentrations (, mg g-1, ppm, etc) may or
may not reflect differences in soil content (kg
ha-1, g m-2). In order to calculate soil
content, you must have 1) soil concentration 2)
horizon thickness 3) bulk density 4) percent
coarse fragment ( gt2mm) Note that both soil
concentration and soil content refer to the fine
earth (lt2 mm) fraction.
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Calculating Soil Mass Make the units cancel!!
Units in bold are those left over after
cancelling kg ha-1
A formula for soil mass is M (kg ha-1) (T,
cm)(Db, g cm-3)(1-G)(F1, 10-3 kg g-1)(F2, 108 cm2
ha-1) 1) Canceling out units, we can simplify
to M (T)(Db)(1-G)(105)
2) Where M Soil Mass (kg ha-1) T
Thickness of horizon (cm) Db Bulk density (g
cm-3) G Fraction greater than 2mm (coarse
fraction) (g g-1) F1 and F2 are conversion
factors
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Calculating Soil Nutrient Content
A formula for soil nutrient content is NC (kg
ha-1) (C, mg kg-1) (F, kg mg-1) (M, kg ha-1)
3) Where NC Soil Nutrient Content (kg
ha-1) M Soil Mass (kg ha-1) C nutrient
concentration (mg kg-1) F conversion factor to
convert concentration units to a fraction with no
units, which equals 10-6 kg mg-1 in this case
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Calculating Soil Nutrient Content
Note that if concentrations units are other than
mg kg-1, you can either convert them to mg kg-1
or change the conversion factor in equation 3.
For example, if concentration units are mg g-1 ,
the conversion factor is F 10-3 mg g-1 If
units are in percent F 10-2
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Example 1 Calculating Soil N Content
Example A soil has the following properties
Calculate soil N content in kg ha-1
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First Calculate Soil Mass
A horizon mass (kg ha-1) (10 cm) (0.9 g cm-3)
(1-0.15) (105) 765,000kg ha-1 B horizon mass
(kg ha-1) (15) (1.3) (1-0.17) (105) 1,618,500
kg ha-1 C horizon mass (kg ha-1) (35
)(1.55)(1-0.20) (105) 4,340,000 kg ha-1
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Example 1 Calculate Soil N Content Make the
units cancel!! Units in bold are those left over
after cancelling kg ha-1
A horizon N content (kg ha-1) (M)(2.14)(10-6)
(765,000 kg ha-1)(2.14 mg g-1)(10-3 g mg-1)
1,637 kg ha-1 B horizon N content (kg ha-1)
(1,618,500 kg ha-1)(1.03 mg g-1)(10-3 g mg-1)
1,667 kg ha-1 C horizon N content (kg ha-1)
(4,340,000 kg ha-1)(0.58 mg g-1)(10-3 g mg-1)
2517 kg ha-1 Total Soil N content 1,637
1,667 2,517 5821 kg ha-1
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Calculating Base Cation Contents from
Exchangeable Concentrations Expressed on a Charge
Basis

• Base cations are often expressed in cmolc kg-1,
  which is numerically the same as the old units
  used for decades in soils, meq 100g-1
• 1 meq (milliequivalent) 1 cmolc kg-1 (centimole
  of charge per kilogram)
• 1 meq 0.001 eq 0.001 molc 1 mole x
  equivalent weight
• Equivalent weight (Molar weight)/Valence

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Calculating Base Cation Contents from
Exchangeable Concentrations Expressed on a Charge
Basis
Molar and equivalent (molc) weights of cations
common on soil exchange sites ____________________
__________________________ Valence Mola
r Wt Equivalent Wt g mole-1 g
molc-1 ___________________________________________
___ Ca2 2 40.08 20.04 Mg2 2
24.31 12.155 K 1 39.10 39.10 Na
1 22.99 22.99 Al3 3 26.9
8 8.99 H 1 1.01
1.01 Can also exist as in -1, 1, or 2
valence depending on pH. Often noted as Aln
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Calculating Base Cation Contents from
Exchangeable Concentrations Expressed on a Charge
Basis
The formula for converting exchangeable cation
units from equivalents or molc to weight basis
(i.e., mg kg-1) Wtb, mg kg-1 (Mcb, cmolc
kg-1)(F1, molc cmolc-1)(EW, g molc-1)(F2, mg
g-1) Make the units cancel!! Units in bold are
those left over after cancelling mg
kg-1. Where WTb concentration of cation on
weight basis, mg kg-1 Mcb concentration of
cation in cmolc kg-1 F1 10-2 molc per cmolc EW
Equivalent weight (g molc-1) (Molar
weight)(Valence) F2 103 mg g-1
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Example 2 Calculating Soil Exchangeable Ca2
Content
Example A soil has the following properties
Calculate soil exchangeable Ca2 content in kg
ha-1
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Example 2 Calculating Soil Exchangeable Ca2
Content
Convert concentrations from a charge basis to a
weight basis A horizon Ca2 (mg kg-1) (3.15
cmolc kg-1)(0.01 molc cmolc-1)(20.04 g
molc-1)(103 mg g-1) 631.3 mg kg-1
B horizon Ca2 (mg kg-1) (1.45 cmolc
kg-1)(0.01 molc cmolc-1)(20.04 g molc-1)(103 mg
g-1) 290.6 mg kg-1
C horizon Ca2 (mg kg-1) (0.45 cmolc
kg-1)(0.01 molc cmolc-1)(20.04 g molc-1)(103 mg
g-1) 90.2 mg kg-1
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Example 2 Calculating Soil Exchangeable Ca2
Content
A horizon Ca2 content (kg ha-1)
(M)(2.14)(10-6) (765,000 kg ha-1)(631.3 mg
kg-1)(10-6 kg mg-1) 483 kg ha-1 B horizon Ca2
content (kg ha-1) (1,618,500 kg ha-1)(290.6 mg
kg-1)(10-6 kg mg-1) 470 kg ha-1 C horizon Ca2
content (kg ha-1) (4,340,000 kg ha-1)(90.2 mg
kg-1)(10-6 kg mg-1) 391 kg ha-1 Total Soil
Ca2 content 483 470 391
1344 kg ha-1
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Calculating nutrient fluxes via open bucket
collectors (precip, throughfall, snowmelt) Make
the units cancel out to kg ha-1!!
XL (kg ha-1) (Xc , mg L-1)(W, cm)(F1, cm2
ha-1)(F2, L cm-3)(F3, kg mg-1) Where XL Flux
of nutrient X, kg ha-1 Xc concentration of X in
solution (mg L-1) W water flux (cm) estimated
from collection volume F1 108 cm2 ha-1 F2
10-3 L cm-3 F3 10-6 kg mg-1
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Calculating nutrient fluxes from open bucket
collectors Example
20 cm
NO3-N concentration (XC) 0.7 mg L-1 Water
collection volume 1.2 Liters Diameter of
circular collector 20 cm Area of collector p
r2 3.1418 x (10)2 314 cm2 Water flux (1.2
L)(1000 cm3 L-1)/314 cm 3.82 cm NO3-N flux
(0.7 mg L-1)(3.82 cm)(108 cm2 ha-1)(10-3 L
cm-3)(10-6 kg mg-1) 0.27 kg ha-1
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How to calculate volume of water from depth
equivalent
Assume an area, for example, 1 ha 1 ha 108
cm2 Let Wd Depth equivalent of water in
cm Water volume (cm3 ha-1) (Wd, cm)(108 cm2
ha-1) Water volume (L ha-1) (Wd, cm)(108 cm2
ha-1)(10-3 L cm-3)
Area 108 cm2
Volume (cm3 ha-1) (Wd, cm)(108 cm2 ha-1)
Wd
Volume (L ha-1) (Wd, cm)(108 cm2 ha-1)(10-3 L
cm-3)
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Water budget from Cl balance method
The Cl balance method assumes that Cl flux in via
precipitation (kg ha-1) equals Cl out via soil
leaching H2Op x Clp x F FluxClp H2Os
x Cls x F FluxCls 1) Where H2Op cm of
water in precipitation H2Os cm of water in
soil leaching Clp weighted average Cl
concentration in precipitation (mg L-1) Cls
weighted average Cl concentration in soil
solution (mg L-1) F conversion factor to make
units come out FluxClp Flux of Cl in
precipitation, kg ha-1 FluxCls Flux of Cl via
soil leaching, kg ha-1
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Water budget from Cl balance method
Rearranging equation 1) to solve for
H2Os 2) So equation 2) shows
that soil leaching water flux is simply equal to
the ratio of weighted average Cl concentration in
precipitation to weighted average Cl
concentration in soil solution times
precipitation water flux.
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Nutrient leaching flux from soil solution data
Once you have calculated soil water flux from
the Cl method (or any other method), you
calculate nutrient leaching flux by multiplying
soil water flux by weighted average nutrient
concentration Nf (kg ha-1) (Cw, mg
L-1)(H2Os, cm)(F1, 108 cm2 ha-1)(F2, 10-3 L
cm-3)(F3, 10-6 kg mg-1) Where Nf
nutrient flux (kg ha-1) Cw weighted average
nutrient concentration in soil solution H2Os
soil water flux, cm, which is the same as Wd in
the water volume calculation earlier F1, F2,
and F3 are conversion factors to make units come
out
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How do we get weighted average concentrations?
3) Where Cw weighted average
concentration Ci concentration at time I Wi
water volume at time I (in either depth
equivalents L ha-1, or simply field collection
volume)
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Calculating nutrient fluxes Example
Calculate water flux and weighted average Cl
concentration (Clwtd) in precipitation Precipita
tion Collector Data Collector diameter 20
cm Collector area 314 cm2
Total water flux 94.15 cm
Weighted average Cl concentration
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Calculating nutrient fluxes Example
Calculate weighted average Cl concentration
(Clwtd) in soil solution Collector diameter and
area are irrelevant
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Calculating nutrient fluxes Example
Calculate soil water flux (H2Os)
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Calculating nutrient fluxes Example
Calculate weighted average soil solution NO3-N
concentration
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Calculating nutrient fluxes Example
Calculate soil solution NO3-N flux
NO3-N flux (kg ha-1) (0.26 mg L-1)(41.67
cm)(108 cm2 ha-1)(10-3 L cm-3)(10-6 kg mg-1)
1.09 kg ha-1