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[NOTE: this first part is based on a spreadsheet from NRCS Minnesota, and is way out of date. Needs to be re-done to reflect the 2008 revisions.]


According to the National Forestry Manual a Conservation Tree/Shrub Suitability Group (CTSG) is "a physiographic unit or area having similar climatic and edaphic characteristics that control the selection and height growth of trees and shrubs." In other words, a CTSG represents the summary of climate and soil conditions in an area that affects tree or shrub growth and survival. Based on those factors, a selection of appropriate trees and shrubs can be made for windbreaks and other tree and shrub planting practices.

Climatic conditions include temperature, precipitation and altitude. Edaphic characteristics refer to soil conditions such as texture, depth, water holding capacity, pH and other chemical and physical characteristics. The summary of physiographic and edaphic conditions controls the growth rate and survival of trees and shrubs. The CTSG represents a summary of this information for use in determining appropriate trees and shrubs for use in windbreaks.

There are 10 CTSGs and several subgroups based on conditions such as

  • depth to ground water,
  • limiting layers,
  • calcium carbonate content, and
  • flooding and other conditions.

Each SSURGO soil map unit (SMU) may have one or more major soil components. When possible, each major soil component in the state has been associated with a CTSG based on its characteristics, by means of an NRCS algorithm. By extension, each soil map unit is associated with the CTSG(s) of its major soil components.

Soils favor certain trees over others. In Minnesota, the tamaracks and black spruces are found in frigid, acidic, boggy soils but not readily found in warmer, more alkaline and dryer soil types. As soils become warmer, less acidic and/or dryer, the effect on tamaracks and black spruces is expressed in decreasing tree heights and survival. At a certain point the soils change to such an extent that those trees are unsuitable to the altered conditions and other trees and shrubs are more suitable. Knowing the CTSG(s) of a SMU will ensure that appropriate trees and shrubs will be planted and avoid a practice failure based on plant selection.

Group CTSG Depth to water (inches)
during growing season
Soil depth to limiting
layer (inches)
Available Water Capacity
(AWC) (inches)
CaCO3 within
20 inches
1. Somewhat poorly drained to moderately well drained soils that are moderately deep to deep with low to high AWC. Seedling mortality may be moderate on 1K soils because the free carbonates can tie up minerals and limit their availability. 1 6 - 18 ≥ 40 ≥ 6
1D 20 - 40
1F ≥ 40
1G 20 - 40 ≥ 3
1K ≥ 40
2. Moderately deep to deep soils that are poorly or very poorly drained with low to high AWC. Also includes somewhat poorly drained to well drained soils with long duration flooding. Seedling mortality will be moderate on 2 and 2K soils groups because of poor drainage, while very poor drainage on 2F, 2H and 2C will cause them to be severe. Spring planting may be delayed because of wet conditions. Seedling mortality may be moderate on 2K soils because the free carbonates can tie up minerals and limit their availability. 2 0 - 6 ≥ 40 ≥ 6
2D 20 - 40
2F ≥ 40
2H ≥ 9
2K ≥ 6
2S ≥ 3
3. Deep loamy, moderately well drained to well drained soils with high AWC. Flooding in 3W may be frequent or occasional and brief with a high available waer capacity. Free carbonates are lacking and the soils are non-saline. 3 > 18 ≥ 40 ≥ 9
4. Deep moderately well to well drained clayey soils. Soils in this group have low to high AWC. Group 4dry can also include skeletal profiles. 4 > 18 ≥ 40 ≥ 6
4Y 3 - 5.9
4A ≥ 6
5. Deep loamy moderately well to well drained soils with low to moderate AWC. Water erosion will be severe on slopes greater than 15%. Some vegetative cover should be left during early establishment of cover on soils in 5S as they are subject to wind erosion. 5S soils may have moderate seedling mortality if moisture stress occurs during establishment. 5 > 18 ≥ 40 6 - 9
5Y 3 - 5.9
6. Moderately deep soils with a root restrictive zone at 20-40 inches. These soils are moderately well to well drained and have low to moderate AWC. 6D soils have a root restrictive layer at 10 inches. 6 > 18 20 - 40 ≥ 3
7. Moderately deep to deep sandy soils that are moderately well drained to excessively drained. These soils have low to moderate AWC. Group 7W soils have a water table influence. Seedling mortality is moderate because of the low water holding capacity causing moisture stress. Water erosion can be severe especially on slopes greater than 15%. Some vegetative cover should be left during establishment because of possible wind erosion. 7 none ≥ 20 ≥ 3
7W 18 - 78
8. Moderately deep to deep calcareous soils that are poorly drained to well drained. Seedling mortality may be moderate because the free carbonates in these soils can tie up minerals and limit their availability. Water erosion will be severe on slopes greater than 15%. 8 > 18 ≥ 20 ≥ 3
8W < 18
9. Moderately deep to deep soils that are poorly drained to well drained. These soils have moderate to high AWC and a electrical conductivity rate of 4 - 16 mmhos/cm. Seedling mortality can be severe because of salt concentrations and the very wet conditions in some soils. (Young plants have some possibility of survival if drainage has recently occurred.) 9 > 18 ≥ 20 ≥ 3
9LW < 18
10. Soils in this group do not fit criteria in Groups 1-9. This group also includes urban land, water, and misc. land types. 10

These notes reflect SSURGO data downloaded for California in May, 2013. In the SSURGO database, a map unit is either a polygon or a multipolygon, and most are multipolygons. There are 17,892 map units in California. Each map unit has from one to three major soil components associated with it. When a map unit has more than one major soil component, the relative importance of each major component in that map unit is specified by a percentage.

Of the 17,892 map units, 7,951 have more than one major soil component.

There are 28,622 major soil components in California. By means of an NRCS algorithm, the appropriate CTSG value can be calculated for a soil component. The calculated CTSG values were supplied to Calflora by Ed Tallyn, NRCS California, in June, 2013.

The Count column below respresents the number of major soil components assigned to each CTSG.

CTSG Count
1 93
1A 19
1AF 2
1F 32
1H 16
1K 1
1S 70
1SA 8
1SF 33
1SS 42
1SSA 5
1SSF 9
CTSG Count
2 20
2A 2
2AF 1
2F 22
2H 1
2HF 3
CTSG Count
3 1203
3A 102
3AF 1
3F 56
CTSG Count
4 217
4A 20
4C 507
4CA 3
4CF 16
4CK 66
4CKF 1
4F 6
4K 1
4SA 26
CTSG Count
5 2408
5A 143
5F 59
5K 63
CTSG Count
6 4844
6A 205
6D 104
6DA 2
6DK 6
6F 26
6K 129
6KF 1
CTSG Count
7 355
7A 10
7AF 4
7F 14
7K 15
7S 393
7SA 24
7SF 43
7SK 18
7SKF 2
CTSG Count
8 53
CTSG Count
9 607
9F 22
9W 14
9WF 2
CTSG Count
10 7501
rated (1)
rated (2)

We would like to utilize the SSURGO spatial data enhanced by CTSG values to determine which plants are constrained as to where they grow by soil. For plants that appear to be thus constrained, we would like to predict where they might grow as a function of soil.

This is not at all the original intention of CTSG values (see the Explanation above), and there are some challenges and conceptual leaps of faith required.

First, not all of the soils in California have been surveyed and included in SSURGO -- there are major gaps in the southeastern desert and other places. Second, looking closely at the counts above, out of 28,622 total major soil components, 13,049 (45%) of them are either not rated or assigned to CTSG 10. (CTSG 10 is a catch all for soils that do not fit into the other categories.) And of the major soil components that are assigned to a CTSG value, a majority are assigned to CTSG 6, 5, or 3.

With all of those caveats in mind, below are the procedures followed to determine which plants favor which soils.

Map units are the minimum logical entity that has a spatial aspect, so it is necessary to assign CTSG values to map units. Here is the procedure:

    For each map unit,
      order each of its major soil components by perentage,
      and collect the CTSG values of the components (if any)
      in the same order.

At this point, it is possible to pick a spot on the map, find the containing map unit, and (if the map unit has any CTSGs) look up the set of CTSGs that go with that map unit.

To find which CTSGs are favored by which plants, here is the procedure.

    For each plant,
      find a high quality location where it has been observed growing wild

      find the map unit containing the point

      look up the set of CTSGs that go with that map unit

      add a record associating the plant with each CTSG in the set, keeping a count of how many locations go with each CTSG

At this point it is possible to see how many different CTSGs go with each plant, and the count for each. To get rid of some of the noise,

    eliminate plants where the sample size (number of high quality locations) is too small (working minimum: 100)

    eliminate plant - CTSG associations where the count is too small (working minimum: 3)

Preliminary Results:
It appears that some weeds (eg. Arundo donax) are associated with many (30) CTSGs. This may indicate that where these plants can grow is not constrained by soil.

A number of native plants are associated with relatively few (2 or 3) CTSGs. This may indicate that where these plants can grow is constrained by soil.

Next Steps: