Assessing your soil’s water holding capacity and estimating how much it shares with plants
Article 3A walked you through assessing your soil by using your senses. This article takes that process a bit further. You will conduct a soil profile assessment followed by a calculation that translates your observations into a number — how many inches of plant-available water your soil can hold in the root zone.
That number will inform which strategies you implement to maximize the soil investment will pay off most. Understanding it for each area of your growing site generates a plan that is specific to your site and growing context.
SHARED VOCABULARY
Available water holding capacity (AWHC): The volume of water a soil can store that plant roots can actually extract. Expressed in inches of water per inch of soil depth (in/in). It’s the difference between a soil at field capacity (full tank) and one at permanent wilting point (empty tank). A deep loam with 36 inches of rooting depth can store 4–6 inches — enough for many vegetable crops to go 6–8 weeks without rain.
Field capacity: The moisture level a soil reaches 24–48 hours after saturation, once free-draining water has moved through. The upper limit of what roots can access.
Permanent wilting point: The moisture level at which water is held so tightly in soil pores that roots can no longer extract it. AWHC is the difference between field capacity and permanent wilting point.
Effective rooting depth: The depth to which crop roots can actually penetrate on your site, limited by the profile — not just by the crop. A restrictive layer at 18 inches means effective rooting depth is 18 inches, regardless of what the crop could do in better conditions. This is the depth you use to calculate total water storage.
Restrictive layer: Any subsurface layer that impedes root growth or water movement: hardpan, claypan, fragipan, cemented layers, or a seasonal high water table. Caps your soil’s water storage potential regardless of what the surface soil can hold.
Subsoil constraints: Any subsoil condition limiting root penetration, water storage, or drainage. Includes restrictive layers, seasonal high water tables, abrupt texture changes, and zones of chemical imbalance. See the site factors publication Section 1 for the main types and their implications for dry farming.
AWHC reference by soil texture
Use this table, along with your field observations and the calculation method in Section 1 of the publication, to estimate water storage potential at each depth interval. These are reference values — organic matter content, structure, and compaction all can change a given soil’s AWHC.
| Soil texture | Approx. AWHC (in/in) | Water resilience implications |
|---|---|---|
| Coarse sand / gravel | 0.03 – 0.07 | Very low water storage. Dries quickly. Mulch and organic matter additions have a high impact. |
| Sandy loam | 0.08 – 0.12 | Low to moderate storage. Water moves through quickly. |
| Loam | 0.14 – 0.18 | Good storage and drainage balance. Well-suited to a range of water resilience strategies, including dry farming with adequate profile depth. |
| Silt loam | 0.17 – 0.22 | High storage capacity. Excellent for dry farming where drainage is adequate. Prone to compaction. Protect structure with minimal tillage. |
| Clay loam | 0.17 – 0.22 | High potential AWHC, but compaction reduces it significantly. |
| Clay | 0.15 – 0.20 | Lower plant-available water than clay loam because water is held too tightly. Cracks as soil dries down. Structure and organic matter are critical investments. |
ACTIVITY – How to calculate your AWHC
This approach estimates how much plant-available water your soil can store. You will identify your soil texture at each horizon from your sensory assessment and/or Web Soil Survey and use the calculation method from Section 1 of the publication. The worked example below shows the process for a representative profile.
Option 1 — Online soil assessment tools (quick estimate):
Web Soil Survey and SoilWeb provide mapped soil data, including texture and AWHC estimates, for your location. While these tools are free to use, their maps are not always accurate, especially for small plots. They can, however, give you a quick estimation of soil types in your area.
Option 2 — Calculate from pulling a soil core with an auger: Using the texture you identified in Article 3A and the AWHC reference table above, work through the calculation below. The publication’s Section 1 provides the full method and reference values for a wider range of textures.
Note: These calculations assume no subsurface restrictions. Subsoil constraints can significantly complicate the calculation process. If you’ve identified restrictive layers in Article 3A, we suggest consulting a certified professional soil scientist for a more accurate estimate.
WORKED EXAMPLE — CALCULATING AWHC FOR A 60-INCH ROOT ZONE
Step 1. Estimate AWHC for each soil horizon using texture. Example for a loam surface horizon: Field capacity (FC) ≈ 28% water by volume; permanent wilting point (PWP) ≈ 14% water by volume. AWHC = FC − PWP = 0.14 inches of water per inch of soil depth.
Step 2. Multiply each horizon’s AWHC by its depth, then add all horizons to get the total for the root zone, also known as the Available Water Storage (AWS).
| Horizon Depth (in) | Texture AWHC (in/in) | Observation | AWHC (in/in) | AWHC for horizon (in) |
|---|---|---|---|---|
| A | 0–12 | Loam | 0.14 | 12 in × 0.14 = 1.7 in |
| AB | 12–20 | Loam | 0.14 | 8 in × 0.14 = 1.1 in |
| Bt1 | 20–30 | Clay loam | 0.14 | 10 in × 0.14 = 1.4 in |
| 2Bt2 | 30–60 | Clay | 0.12 | 30 in × 0.12 = 3.6 in |
| Total: | 7.8 in plant-available water in the 60-inch root zone |
Result: This soil stores approximately 7.8 inches of plant-available water in the 60-inch root zone. See the publication’s Section 1 (AWHC and yields) for how this value relates to expected yields for specific dry-farmed crops.
Now calculate your own:
| Location | Effective rooting depth (in) | Dominant texture | Total est. water storage (in) | Key limiting factor |
|---|---|---|---|---|
| Location 1: | ||||
| Location 2: | ||||
| Location 3: | ||||
| Location 4: |
Review your estimates and consider:
- What is the range of total AWHC across your farm locations? Which areas have the most storage capacity?
- Where would increasing organic matter have the highest return on water storage capacity?
Not sure about your calculations? Navigating subsoil constraints? Hiring a certified professional soil scientist is the most accurate option. Ask them specifically to provide AWHC estimates by horizon and to flag any subsoil constraints.
SEE IT IN CONTEXT – Section 4: Case Studies
The OSU Dry Farming Program case study in Section 4 includes an example where a five-foot soil core assessment revealed the soil type was different from what Web Soil Survey showed — a reminder to verify mapped data with field observation. The case studies in Section 4 show how growers in specific contexts have applied the tools in this article. They’re illustrations, not blueprints — and you may not see your climate, your scale, or your crop mix reflected in what’s currently available. Each farm is unique. If your context isn’t reflected yet, your experience is what this community needs. We’re actively building more examples. Email us at info@dryfarming.org.
From AWHC estimate to strategy
Based on your calculations from Article 3B.
| My AWHC estimate shows… | ✓ | Strategy areas worth exploring first |
|---|---|---|
| Total water storage above 5 inches, with roots reaching 30 or more inches | □ | Strong dry farming potential. Focus on variety selection and planting timing matched to your dry season length. A small-scale reduced-irrigation trial this season may help you develop more observational data and confidence. |
| Total water storage between 2.5 and 5 inches with moderate rooting depth | □ | Flexible irrigation management with soil moisture monitoring to guide decisions; cover crops and mulch to extend intervals between irrigations; explore reduced irrigation on a portion of the field to understand your soil’s actual limits. |
| Total water storage below 2.5 inches, with rooting depth as the limiting factor | □ | Addressing what’s stopping roots comes first: deep-rooted cover crops, subsoil work, NRCS earthworks consultation. As effective rooting depth increases, water storage increases with it. |
| Total water storage below 2.5 inches, with rooting depth of 24 or more inches | □ | Organic matter building over multiple seasons — cover crops, compost, reduced tillage. Each 1% increase in organic matter can meaningfully increase AWHC. Drip irrigation to make the most of current storage in the near term. |
| Storage varies significantly across the farm | □ | Match crops to soil capacity across zones. Dry farming or reduced irrigation trials belong in higher-storage areas. Intensive soil building belongs in lower-storage areas. The variation itself is useful planning information. |
PUTTING IT INTO PRACTICE
- What is the range of total AWHC across your farm locations? Which area has the most storage capacity?
- For your most limited location: is the constraint rooting depth, texture, or organic matter?
- Is there a location where your AWHC estimate suggests a different crop or management approach than what you’re currently doing?
Carry forward: Your AWHC estimates and limiting factors at each location carry into Article 3C, where you’ll connect them to specific practice strategies.
RESOURCES
Site Factors publication — — Understanding and Evaluating Site Factors Related to Dry-Farmed Vegetable Productivity. dryfarming.org
A Guide to Collecting Soil Samples for Farms and Gardens — — OSU Extension.
SWCD Soil Health Programs — — Equipment, soil testing, and grants directory for Oregon.
Web Soil Survey — — Web Soil Survey (WSS) provides soil data and information produced by the National Cooperative Soil Survey. It is operated by the USDA Natural Resources Conservation Service (NRCS)
How to Use Web Soil Survey — — American Farmland Trust video walkthrough.
SoilWeb — — Online soil survey browser, available as a phone app.
What Is Soil Health? — — SARE interactive exploration of soil health.
Estimating Soil Moisture by Feel and Appearance — — NRCS illustrative guide.
Find a Professional Soil Scientist — — For formal AWHC estimates and profile descriptions.
Dry Farming on Sandy Soils with Deep Mulch — — 26 min presentation from Jen Clark of Roots Farm in Poulsbo, Washington sharing her experience with dry farming on sandy soils using deep mulch.
Factilitator Notes (click to view)
Bridge from Article 3A (if using). Open by asking growers what they noticed in their Article 3A sensory observation. The systematic profile assessment should confirm, refine, or challenge those impressions.
The profile observation table is an invaluable activity. Many growers have never looked below 12 inches. Finding a fragipan, a moist subsoil horizon later in the season, or an abrupt texture change often changes how growers understand what’s happening where the roots are. If you can do one hands-on activity in a site visit, make it this one.
Organic matter is a lever growers can pull. For growers with limited AWHC due to low organic matter, frame the multi-season investment: each 1% increase in OM can add meaningful inches of AWHC. That makes cover cropping and adding compost translate to an economic decision.
DFI × OSU CRAFS · Water Resilience Toolkit · Grower Pathway