Soil Survey

Chapter 19

Making a soil survey

Contents of a soil survey

This short presentation focuses on the two below topics.

General soils mapshowingsoil associations

A soil survey is an inventoryof soils in a county. It showswhere different soil types occur,gives properties of them andsays whether their propertiesmay limit uses of them.

This is one of two types of soilmaps. It’s low resolution.

Differences among soils often due to topography and drainage –catena

Soil association group of different soils thatoccur together

Thus, soils in an associationmay be fairly similar, like theloess soils on campus. TheFeliciana is on the slopes andis better drained than Olivier,which is on broad flat areas.

Detailed soils map

The other type of soilmap is much higherresolution and showswhat specific soil typeis at a spot. The wholecounty is shown in aerial photos with soiltype boundaries super-imposed.

What’s in red is a square mile, or section of land(= 640 acres). The smallletters inside boundariesare abbreviations forsoil types (technically, mapping units).

Besides general and detailed soils maps

Index map

This is your guide to which detailed map sheet (aerial photo) to seefor a specific tract of land. It’s right before the aerial photos.

How would you go about mapping the soilsof county X?

Mapping Soils

Define each soil to be mappedDelineate boundaries between soilsCharacterize each soil

There are usually only a few dozensoil types in a county. The first taskin making a soil survey is to determinewhat soil types occur. This is wherean understanding of soil formation isvery useful. If you have information onthe geology of the county, you have aguide to parent material. Dig some exploratory holes on different combina-tions of parent material, vegetative coverand topographic position, and see whatwhat’s there, i.e., soil type.

A lot of lab work besides the detaileddescriptions of typical pedons for thedifferent soil types in the county.

Auger a hole where you think a certain soilis and one where you think another one is.Guess and confirm location of transitionalong a transect. More efficient than gridsampling (compare number of holes inillustration).

Sampling sites located on aerial photosand boundaries between soils drawn

Old style data input.

Newer technology

GPS and GIS

Remote sensingAerial Satellite

Spectral data correlated with soilproperties. Particularly helpful in difficult terrain.

Contents of Soil Surveys

NarrativeMapsTablesGlossary

Maps and tables are the core of a soil survey.However, explanations in narrative sections and definitions in glossary are necessary for the laymanto fully understand tables.

Mapping units

SeriesPhases of seriesConsociationComplex

The soil types delineated on aerialphotos are called mapping units.Sometimes these are soil series orminor variations of a certain seriescalled phases. This might have todo with texture (Ruston sandy loam orRuston fine sandy loam), slope(Ruston, 1 – 3 %, or Ruston, 3 – 8 %)and so forth.

In some cases, different soil typesoccur in such tight and complicatedspatial patterns that it is not feasibleto delineate them. There are 2 typesof this kind of lumping, consociation,with 2+ soil types that are similar, andcomplex, with 2+ types that are dissimilar.

Consociation consists of two or moresimilar soils

Complex consists of two or moredissimilar soils

Many tables

A few examples from St. Helena Parish

Prime Farmland

Soils best suited for meeting the Nation’sfood and fiber needs

Many conditions like

Warm, adequate rain, deep and so forth

In St. Helena Parish

15 mapping units are prime farmland

Soil Percent

Bude silt loam, 0 to 2 % slope 2.1Calhoun silt loam 0.5

12.3

Tangi silt loam, 1 to 3 % 14.1Toula silt loam, 1 to 3 % 8.7

37.7

Land capability and yields

Land capability classes

I Few limitations on croppingII SomeV Not suitable for cultivationVII Not suitable for pastureVIII Only for recreation and so forth

This is an important tablefor agricultural use of asoil. You can farm Class Isoils without risking de-grading them. More management is needed withClass II, even more with III.

Subclasses like IIe

c adverse climatee erodibles root-zone problemw wetness

Lower case letter for Class II and upgives the major limitation. Erosionand wetness are common for soilsin Louisiana. Wetness refers to ashallow water table.

In St. Helena Parish

Soil Land Soybeans BahiagrassCapability

Bude IIw 25 Bu 7.0 AUMCypressVIIwRuston IIe 30 9.5Toula IIe 25 10.0

These are estimates of relativeproductivity for possible crops andare part of the Land Capability Classtable.

Numbers are per acre. AUM = animal unit month, like 1 acre of bahiagrass can support 1 cow for 7 months or you need 12 / 7 acres per cow.

Recreational development

Limitations

Property Slight Moderate Severe

Flooding None Occasional Frequent

Slope < 8 % 8 - 15 > 15

Watertable > 2 ft 1 - 2 < 1

The next slide gives ratingsfor a few soils if used for campsites or golfing. Ratings arebased on several factors.

You don’t get much bounce androll on a fairway that is water-saturated 6” below the surface. Obviously, you don’t want afairway to flood, ever.

In St. Helena Parish

Soil Camping Golfing

Bude Severe SevereWetness (0.5) Wetness (0.5)

Cypress Severe SeverePonding (+) Ponding (+)Flooding (freq) Flooding (freq)

Ruston Slight Slight

Toula Moderate ModerateWetness (1.5) Wetness (1.5)

Expected high water table depth (ft).