Units and organic matter

Units and organic matter

Pedotri's public functions all take percentages by default — sand, silt, clay, organic matter as numbers in [0, 100]. Real-world lab reports almost never come in that form. This page lists the conversions you'll actually need and shows how to plug them in.

What units do labs use?

Convention	Range	Where you see it
percent	0-100	Most US / international literature; pedotri default
g/kg	0-1000	European labs, especially for organic matter and exchangeable cations
g/g (mass fraction)	0-1	Modelling pipelines, internal MD scripts
% w/w	0-100	Synonym for "percent"; nothing special
% OC vs % OM	0-100	PTFs want organic matter; labs report organic carbon

The units= keyword

Four public functions accept a units= keyword that converts at the boundary:

import pedotri

# Same soil, three conventions, identical answer
pedotri.classify(60, 20, "USDA")                       # default %
pedotri.classify(600, 200, "USDA", units="g/kg")       # g/kg
pedotri.classify(0.60, 0.20, "USDA", units="g/g")      # mass fraction

The same keyword works on:

• pedotri.classify
• pedotri.ptf.saxton_rawls
• pedotri.ptf.wosten
• pedotri.psd.convert

When you mix sand/clay in one unit and organic matter in another, convert by hand using the pedotri.units helpers (next section). The units= keyword applies uniformly to all fraction arguments.

OC ↔ OM (Van Bemmelen)

The Van Bemmelen factor 1.724 assumes organic matter is ~58 % carbon by mass:

OM = OC × 1.724
OC = OM / 1.724

from pedotri.units import organic_carbon_to_organic_matter

oc_pct = 1.16              # % organic carbon from lab
om_pct = float(organic_carbon_to_organic_matter(oc_pct)[0])
print(om_pct)              # ~2.0

When to override the factor

The 1.724 default works for typical agricultural mineral soils. For specific contexts, consider:

Factor	Source	When to use
1.724	Van Bemmelen 1890 (default)	Generic mineral agricultural soils
1.9	Pribyl 2010	Less-decomposed organic material, surface horizons
2.0	Various forest-soil studies	Forest litter, O horizons

om_forest = float(organic_carbon_to_organic_matter(oc_pct, factor=2.0)[0])

g/kg ↔ %

Trivial but everyone gets it wrong eventually:

from pedotri.units import g_per_kg_to_percent, percent_to_g_per_kg

g_per_kg_to_percent(20)          # → array([2.0])
percent_to_g_per_kg(2)           # → array([20.0])

A real lab report → pedotri pipeline

Lab gives you (in their preferred units):

sand:    420 g/kg
silt:    380 g/kg
clay:    200 g/kg
OC:      11.5 g/kg
BD:      1.42 g/cm³

Classify with USDA and run Saxton-Rawls:

import pedotri
from pedotri.ptf import saxton_rawls
from pedotri.units import g_per_kg_to_percent, organic_carbon_to_organic_matter

sand_pct = float(g_per_kg_to_percent(420)[0])    # 42.0
clay_pct = float(g_per_kg_to_percent(200)[0])    # 20.0
oc_pct   = float(g_per_kg_to_percent(11.5)[0])   # 1.15
om_pct   = float(organic_carbon_to_organic_matter(oc_pct)[0])  # ~1.98

print(pedotri.classify(sand_pct, clay_pct, "USDA"))
# 'loam'

r = saxton_rawls(sand=sand_pct, clay=clay_pct, organic_matter=om_pct)
print(r.field_capacity, r.wilting_point, r.available_water)

Or, equivalently, push the conversion to the units= boundary and convert OC→OM upstream:

om_g_per_kg = 11.5 * 1.724
pedotri.classify(420, 200, "USDA", units="g/kg")
saxton_rawls(sand=420, clay=200, organic_matter=om_g_per_kg, units="g/kg")

Things that look like units but are not

• "Texture index" is not a unit; it's the output of a classification.
• "% < 2 µm" = clay; "% < 50 µm" = silt + clay (USDA). Don't hand pedotri a "% < 50 µm" number as silt — it's not silt.
• "Mechanical analysis" = a sand/silt/clay PSD, but the cutoffs depend on the country; see Soil-conversions.
• EC, pH, CEC are not pedotri inputs at all; the library is texture-and-OM only at the moment.

Related

• API docs: pedotri.units
• Pedotransfer-functions — PTFs expect OM, not OC
• Soil-conversions — converts triples between standards, with the same units= keyword