Soil series profile

Rillito soil series

Rillito is a somewhat excessively drained soil with a fine sandy loam surface, mapped on 614,254 acres in 6 counties across 2 states. Hydrologic group A.

#416 by mapped area coarse-loamy Typic Haplocalcids (Aridisols)

Drainage class: Somewhat excessively drained
Hydrologic group: A
Typical topsoil pH: 8.2
Mapped area: 614k

Typical Rillito profile

Texture and pH by depth

Depth to scale. More strongly shaded layers hold more clay. Horizon names are the soil survey's (A = topsoil, B = subsoil, C = parent material).

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Typical Rillito horizons (representative values)
Depth	Texture	pH	Clay	Sand	Organic matter
0–1 in (A)	Fine sandy loam	8.0	8%	75%	0.5%
1–18 in (Bk1)	Gravelly fine sandy loam	7.8	10%	75%	0.3%
18–30 in (Bk2)	Gravelly sandy loam	7.8	12%	70%	0.3%
30–60 in (Bk3)	Gravelly sandy loam	7.8	14%	70%	0.3%

Representative values from the most extensive Rillito map unit (Coolidge, Denure, and Rillito, dry, 0 to 5 percent slopes, survey area AZ648). Other map units of this series vary within the series range.

Stock photo for illustration, not a sample of this soil series. Credit on the Sources & method page.

What this means

What Rillito soil means for gardens, lawns and septic systems

For gardens

Typical topsoil in Rillito soils is moderately alkaline: pH 8.2 (most land between 8.0 and 8.2). Acid-loving plants such as blueberries and azaleas struggle without amended beds, and some trees and shrubs show iron chlorosis (yellow leaves with green veins).

Loamy topsoil (loam, silt loam and similar) covers 76% of mapped Rillito acres: generally the easiest soil to garden, holding moisture without staying waterlogged. The most common surface texture is fine sandy loam (32% of mapped Rillito acres).

76% of mapped Rillito acres is excessively or somewhat excessively drained: water moves through fast, so gardens need more frequent watering in dry spells.

For lawns and yards

82% of mapped acres is in hydrologic group A, where water soaks in fast. Lawns there dry out quickly; deeper, less frequent watering and organic matter help.

For a septic system

For conventional septic absorption fields, NRCS rates 2% of the acres in mapped Rillito soils very limited, 25% somewhat limited and 73% not limited.

Flooding is mapped on 9% of the acres (0% occasional, 0% frequent).

Only a licensed site evaluation or perc test at the actual lot decides whether a system is allowed and which design.

Generated from the mapped numbers above using fixed rules, not a site visit. See how these notes are written.

Where Rillito soil is found

Mapped acres

Rillito is mapped in Arizona and California.

Counties with the most mapped Rillito soil
County	Acres	Share of county land
Maricopa County, AZ	298,101	4.8%
La Paz County, AZ	113,603	4.0%
Yuma County, AZ	56,572	2.5%
Pima County, AZ	51,202	1.0%
Pinal County, AZ	3,892	0.1%
Riverside County, CA	1,582	0.1%

Septic absorption fields

NRCS rating, share of Rillito acres

Not limited 73%

Somewhat limited 25%

Very limited 2%

Septic tank absorption field rating across mapped Rillito acres. Scale 0 to 100% of mapped land.

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Septic tank absorption field rating across mapped Rillito acres.
Category	Share of acres
Not limited	73.2%
Somewhat limited	24.5%
Very limited	2.2%

Drainage, water table and flooding

Share of Rillito acres

Depth to seasonal high water table: None reported within about 6.5 feet (median where reported).
Flooding: Mapped as flooding on 9% of acres (0% occasional, 0% frequent).
Slope: Typically 0–8% (median 3%).
Topsoil organic matter: Median 0.5%.

Drainage class

Excessively drained 0%

Somewhat excessively drained 76%

Well drained 24%

Moderately well drained 0%

Somewhat poorly drained 0%

Poorly drained 0%

Very poorly drained 0%

Drainage class across mapped Rillito acres. Scale 0 to 100% of mapped land.

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Drainage class across mapped Rillito acres.
Category	Share of acres
Excessively drained	0.0%
Somewhat excessively drained	76.1%
Well drained	23.9%
Moderately well drained	0.0%
Somewhat poorly drained	0.0%
Poorly drained	0.0%
Very poorly drained	0.0%

Hydrologic soil group

A: fast infiltration (mostly sandy or gravelly) 82%

B: moderate infiltration 18%

C: slow infiltration 0%

D: very slow infiltration (clayey, shallow or wet) 0%

A/D: D as is, A if drained 0%

B/D: D as is, B if drained 0%

C/D: D as is, C if drained 0%

Hydrologic soil group across mapped Rillito acres. Scale 0 to 100% of mapped land.

Show as a table

Hydrologic soil group across mapped Rillito acres.
Category	Share of acres
A: fast infiltration (mostly sandy or gravelly)	82.0%
B: moderate infiltration	18.0%
C: slow infiltration	0.0%
D: very slow infiltration (clayey, shallow or wet)	0.0%
A/D: D as is, A if drained	0.0%
B/D: D as is, B if drained	0.0%
C/D: D as is, C if drained	0.0%

Sources & data dates

Source: USDA NRCS Soil Survey Geographic Database (SSURGO), queried through Soil Data Access. Public domain.
Data date: SSURGO as published on November 3, 2026 (most recent survey area update October 26, 2026). NRCS refreshes SSURGO each October.
Scope: All map unit components named Rillito (component kind "Series") in SSURGO: 614,254 acres.
Method: Shares are by mapped acres of Rillito components. The profile comes from the single largest Rillito component. Full methodology.

Independent reference site. No government affiliation or endorsement.

Full narrative description: USDA Official Soil Series Description for Rillito.