October 17, 2026

How to read hydrologic soil groups for your garden beds

A hydrologic soil group is a letter from A to D that rates how fast water enters a soil and moves down through it once the soil is thoroughly wet. Group A soaks water in fastest and sheds the least as runoff. Group D soaks it in slowest and sheds the most. You can find your yard's letter in a few minutes on the USDA's free Web Soil Survey. Then you check whether your beds drain like the map says, because a filled raised bed often doesn't.

What the four letters measure

Soil scientists at the USDA's Natural Resources Conservation Service rate each mapped soil for runoff potential. The rating assumes bare, unfrozen soil after a long soaking rain, the hardest case for runoff. Under those conditions water drops quickly through large pores and crawls through small ones. Texture and depth to a slow layer set most of the letter.

• Group A. Sand, loamy sand, or deep gravelly soil. Large pores let water fall through fast, so these soils dry first after rain.
• Group B. Loams and silt loams of moderate depth. Water enters at a moderate rate, and the soil holds a useful store for roots.
• Group C. Soils with more clay or a layer that slows water partway down. Heavy rain pools or runs off before it soaks in.
• Group D. Clay, soil over a shallow hard layer, or soil with a high water table. Water barely moves down, so the surface stays wet long after rain stops.

Some soils carry two letters, such as B/D. The first letter applies where drainage has lowered the water table. The second applies where the water table still sits high.

Step 1. Mark your yard on the soil map

Open Web Soil Survey and click the Area of Interest tab. Zoom to your address, pick the rectangle or polygon tool, and draw around your lot. Keep the outline tight so the report covers your yard and few of your neighbors.

Step 2. Open the hydrologic soil group rating

Go to the Soil Data Explorer tab, then Soil Properties and Qualities. Under Soil Qualities and Features, choose Hydrologic Soil Group and click View Rating. The map colors each soil unit by its letter, and a table below lists them.

Step 3. Read the letter under your garden

A small lot often crosses two map units. Find the unit under your beds and note both its letter and its soil name. The name tells you the texture, such as a silt loam or a clay. If you land on a C or D clay, our guide on compost for clay soil covers how organic matter opens up the pores.

Step 4. Test how each bed drains

The letter describes native ground, and your beds may hold something else. Dig a hole as deep as a trowel blade in each bed and in a patch of undisturbed ground nearby. Fill each hole with water and let it drain. Fill it again and time how long the second fill takes to disappear. The first fill wets the surrounding soil, so the second timing sits closer to the wet condition the letter assumes.

Compare the beds with each other. A raised bed filled with compost, bark, and peat or coir has far more large pores than native clay. It drains like a group A or B soil even over a group D yard. Its height and exposed sides let gravity, sun, and wind pull water out from several directions. An in-ground bed amended only in its top layer behaves like two soils stacked. The loose top dries fast in sun while the native subsoil below holds water, so water can perch where the two meet. Our guide on raised bed mix ratios and drying rate goes further into how the mix sets the pace.

Step 5. Watch which bed dries first

A drain test shows how fast water leaves after a soaking. It can't show how fast roots and sun draw a bed down over a dry week. For that you need readings over time.

Our own garden has four probed beds, three raised and one in the ground, all sitting on the same native soil. Raised bed 1 held the narrowest band of the four, 39% to 54% moisture, across mid-July to early September. Raised bed 2 sat a little wider, at 36% to 57%. The ground bed and raised bed 3 shared the widest spread, 32% to 62%. Over 13 days the ground bed dropped from 62% on July 24 to 32% on August 6. Our wettest day of the year brought 2.46 in of rain, and raised bed 3 topped out at 62% the next day, August 21. One map letter covered all four, and they still ran in different ranges.

Once you map a soil probe to a bed, Siltwell pushes an alert when two readings in a row from that bed drop under 25%. That alert tells you which bed ran low. Timing and amount of watering stay your call. Our guide to connecting a soil sensor per bed walks through the setup.

What can go wrong

• The mapped soil is gone. Builders often strip topsoil and spread fill around new houses. The survey then describes a soil that no longer sits in your yard.
• You test dry ground. A single fill poured into dry soil vanishes into empty pores. Clay can look like sand on that first pour.
• You misread a dual letter. A B/D soil in a low, wet corner behaves like D. Check whether water stands there after rain before you trust the first letter.
• You test after a downpour. Saturated beds drain slowly for everyone. Wait a few dry days so each bed starts from its usual state.

The hydrologic group can't tell you how fast a planted, mulched bed dries in a hot week, because the rating assumes bare soil that is already saturated. Plant roots, mulch, and shade change that pace bed by bed.

Start with the bed your drain test flagged as fastest. In a dry spell, water that bed first and check the others before you add water to them. For loam beds, our guide on loam soil watering frequency covers how often to water, and our guide on measuring soil moisture per bed compares the ways to check.