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Soil Moisture Converter

Converter

About This Tool

Soil Moisture Converter – VWC, GWC, Matric Potential & Irrigation Scheduling

Soil moisture is one of the most critical variables in agriculture, horticulture, environmental science, and geotechnical engineering — yet it is expressed in several fundamentally different ways. This tool converts between the most common soil water content representations: Volumetric Water Content (VWC), Gravimetric Water Content (GWC), matric potential / tension (kPa, bar, atm, psi, cm H₂O, and pF), equivalent water depth (mm), and Available Water Capacity (AWC).

Volumetric vs. Gravimetric Water Content

The two most common ways to express how much water is in soil are:

  • Volumetric Water Content (VWC, θv) — the volume of water per unit volume of total soil (m³/m³ or %). Modern capacitance, TDR, and FDR sensors report VWC directly. A reading of 25 % VWC means that 25 % of the soil volume is occupied by water.
  • Gravimetric Water Content (GWC, θg) — the mass of water per mass of oven-dry soil (g/g or %). Laboratory oven-dry methods measure GWC. The same 25 % GWC in a loam soil with a bulk density of 1.3 g/cm³ translates to a VWC of only 32.5 % — the two numbers are not interchangeable without knowing bulk density.

The conversion uses a simple relationship: θv = θg × (ρb / ρw), where ρb is the soil bulk density and ρw is the density of water (1.0 g/cm³). Use the soil texture presets (sand, loam, clay) for a quick estimate, or enter a measured bulk density from an undisturbed core sample.

Matric Potential and the pF Scale

While VWC and GWC describe how much water is in the soil, matric potential describes how tightly that water is held. Plants expend energy to extract water against this tension. Common units include kPa (kilopascals), bar, atmospheres (atm), psi, and centimetres of water (cm H₂O).

The pF scale is the base-10 logarithm of tension in cm H₂O: pF = log₁₀(tension in cm H₂O). It is logarithmic because soil water tension spans many orders of magnitude — from near zero at saturation to over 100,000 kPa in oven-dry soil. Key reference points:

  • pF 2.5 ≈ 33 kPa — Field Capacity (FC), the moisture retained after drainage
  • pF 3.0 ≈ 100 kPa — common irrigation trigger point
  • pF 4.2 ≈ 1,500 kPa — Permanent Wilting Point (PWP), plants can no longer extract water

Water Depth for Irrigation Scheduling

Irrigation engineers and agronomists often need to express soil water as a depth of water (mm) over a soil layer — for example, to calculate how much water to apply to refill the root zone. The conversion is straightforward: Water depth (mm) = VWC (fraction) × Soil layer depth (mm). A 30 cm soil layer at 25 % VWC holds 75 mm of water. This is also expressed as 75 L/m², directly linking sensor readings to irrigation volumes.

Available Water Capacity (AWC)

AWC is the amount of water a soil can hold that is accessible to plants — the difference between field capacity and the permanent wilting point: AWC = (FC − PWP) × Soil depth. A loam soil with FC = 0.30 m³/m³ and PWP = 0.10 m³/m³ over a 1,000 mm root zone holds 200 mm of plant-available water. This figure is central to deficit irrigation scheduling and crop water budgeting.

Practical Applications

This converter is used by agronomists comparing sensor outputs to lab results, irrigation engineers scheduling drip or furrow applications, environmental scientists modelling vadose zone hydrology, and students learning soil physics. The tool shows each conversion formula step-by-step and identifies the current soil moisture status (below wilting point, plant-available range, or near saturation) when field capacity and wilting point values are provided.

Tips for Accurate Conversions

  • Measure bulk density from undisturbed core samples for the most accurate VWC ↔ GWC conversions; estimated values from texture presets introduce some error.
  • Matric potential enters as a positive tension magnitude. The tool follows the convention that tensions are positive numbers (e.g. 100 kPa, not −100 kPa).
  • The pF scale is only valid for positive tension values. At saturation (zero tension), pF is mathematically undefined.
  • For bulk density presets: sandy soils ≈ 1.5–1.7 g/cm³, loams ≈ 1.3–1.4 g/cm³, clays ≈ 1.0–1.3 g/cm³. Compacted soils can exceed 1.8 g/cm³.

Frequently Asked Questions

Is the Soil Moisture Converter free?

Yes, Soil Moisture Converter is totally free :)

Can I use the Soil Moisture Converter offline?

Yes, you can install the webapp as PWA.

Is it safe to use Soil Moisture Converter?

Yes, any data related to Soil Moisture Converter only stored in your browser (if storage required). You can simply clear browser cache to clear all the stored data. We do not store any data on server.

How does the Soil Moisture Converter work?

Select your conversion mode — volumetric/gravimetric, matric potential, water depth, or available water capacity — enter the input value and any required parameters (bulk density, soil depth, field capacity, wilting point), and the tool instantly applies the appropriate soil-physics formula. Results include the converted value, relevant secondary outputs, and a step-by-step formula breakdown.

What is the difference between volumetric and gravimetric water content?

Volumetric Water Content (VWC, θv) expresses the volume of water per unit volume of soil (m³/m³ or %). Gravimetric Water Content (GWC, θg) expresses the mass of water per mass of dry soil (g/g or %). They are related by soil bulk density: θv = θg × (bulk density / water density). VWC is more common in sensor outputs; GWC is used in lab oven-dry measurements.

What bulk density value should I use?

Typical bulk density values for mineral soils range from about 1.0 g/cm³ (clay) to 1.8 g/cm³ (compacted sand). Loam soils are typically around 1.2–1.4 g/cm³. Use the soil texture presets in the tool to quickly select a representative value, or enter a measured value from a core sample for greater accuracy.

What is the pF scale and why is it logarithmic?

The pF scale is the common logarithm (base 10) of the matric potential expressed in centimetres of water (pF = log₁₀|tension in cm H₂O|). It is logarithmic because soil water tension spans many orders of magnitude — from near-zero tension at saturation to tens of thousands of kPa at oven-dry conditions. Field capacity is approximately pF 2.5 (about 33 kPa) and the permanent wilting point is approximately pF 4.2 (about 1,500 kPa).

How is Available Water Capacity (AWC) calculated?

AWC is the fraction of soil water that plants can extract. It equals the difference between field capacity (the maximum water held after drainage) and the permanent wilting point (the minimum moisture plants can access): AWC (fraction) = Field Capacity − Wilting Point. Multiplied by the soil layer depth in mm, it gives the equivalent mm of plant-available water for irrigation scheduling.

How accurate are the conversion results?

The formulas used are standard soil-physics relationships with exact unit conversion factors (1 bar = 100 kPa, 1 atm = 101.325 kPa). Accuracy depends on the input values — particularly bulk density, which varies with soil texture, organic matter, and compaction. For field use, measure bulk density from undisturbed core samples for the most reliable conversions.