Soil Field Capacity Calculator

Calculator

Soil texture

Field-drained wet mass grams

Oven dry mass grams

Core sample volume cm³

Known bulk density g/cm³, optional

Known field capacity percent, optional override

Root zone depth cm

Current volumetric water percent

Permanent wilting point percent, optional

Garden area m²

Irrigation efficiency percent

Management allowed depletion percent

Example Data Table

Soil	Wet Mass	Dry Mass	Volume	Root Depth	Current Water
Loam bed	165 g	125 g	100 cm³	30 cm	18%
Sandy vegetable row	142 g	120 g	100 cm³	25 cm	10%
Clay flower border	180 g	130 g	100 cm³	35 cm	28%

Formula Used

Gravimetric water content: θg = (wet mass − dry mass) / dry mass

Bulk density: ρb = dry mass / sample volume

Volumetric field capacity: θfc = θg × ρb

Water storage at field capacity: storage = θfc × root zone depth

Available water: available = (θfc − θwp) × root zone depth

Deficit to refill: deficit = max(0, θfc − θcurrent) × root zone depth

Gross irrigation volume: volume = deficit × area / efficiency

One millimeter of water over one square meter equals one liter.

How to Use This Calculator

Select the closest soil texture.
Enter wet mass, dry mass, and core volume when available.
Add known bulk density if a lab report gives it.
Use known field capacity only when you want an override.
Enter root depth for the crop or planting bed.
Add current water content from a sensor or estimate.
Enter garden area and irrigation efficiency.
Press calculate, CSV, or PDF for the required output.

Understanding Soil Field Capacity

Field capacity describes stored water after free drainage slows. It is not saturation. It is the water held in soil pores after gravity removes excess water. Garden roots use this reserve between watering events.

Why It Matters

Good watering starts with the root zone. A shallow bed holds less water than deep soil. Sandy soil drains fast. Clay soil can hold more water, but it may release water slowly. Loam usually gives a balanced result. Field capacity helps you set watering depth, irrigation timing, and refill volume.

Useful Measurements

A simple core sample can give strong estimates. Weigh the moist sample after drainage. Dry it fully. Then weigh it again. The mass difference is water mass. Divide water mass by dry soil mass to get gravimetric water content. Multiply that value by bulk density to estimate volumetric field capacity.

Planning Irrigation

Volumetric water content becomes practical when multiplied by root depth. The result is water depth in millimeters. One millimeter over one square meter equals one liter. This makes garden planning direct. You can move from soil science to watering volume without extra unit confusion.

Advanced Use

This calculator also compares current water content with field capacity. It estimates deficit, excess drainage, available water, and gross irrigation need. Application efficiency adjusts for losses. A drip system may have high efficiency. Sprinklers may lose more water to wind and evaporation. The management allowed depletion value shows when plants should be refilled before stress rises.

Interpreting Results

Treat the output as a planning estimate. Soil structure, compaction, organic matter, mulch, slope, and root distribution can change real performance. Field observations still matter. Check plants, soil feel, and drainage after watering. Adjust inputs when you collect better local measurements.

Garden Benefits

Knowing field capacity prevents guesswork. It helps avoid constant light watering. It also reduces deep drainage losses. Better water control supports stronger roots, steadier growth, and fewer nutrient losses. The best result comes from combining calculations with careful garden checks.

When to Recheck

Recheck field capacity after adding compost, changing beds, or compacting paths. Repeat tests each season. Fresh measurements improve watering schedules and make fertilizer decisions safer for crops. Keep notes for every garden bed and crop.

FAQs

What is soil field capacity?

It is the water remaining in soil after excess water drains away. It shows the upper useful storage point for plant roots.

Is field capacity the same as saturation?

No. Saturation means pores are filled with water. Field capacity comes later, after gravity removes free drainage water.

Why does bulk density matter?

Bulk density converts gravimetric water content into volumetric water content. This helps estimate stored water across a root zone.

Can I use texture estimates only?

Yes. Texture estimates are useful for planning. A measured core sample gives stronger local results when accuracy matters.

What is available water capacity?

It is the water held between field capacity and permanent wilting point. Plants can use much of this range.

Why include irrigation efficiency?

Not all applied water reaches roots. Efficiency adjusts for drift, evaporation, runoff, leaks, and uneven application.

What does depletion mean?

Depletion is the amount of water used from storage. Management allowed depletion helps schedule watering before plants face stress.

Should I test every garden bed?

Yes, when beds differ. Compost, compaction, texture, slope, and depth can change field capacity across the garden.