1 Soil Preparation Northern Water northernwater.org

Soil Preparation—One of the Seven Principles of Efficient Landscape Water Management

Summary

Organic soil amendments (compost) improve soil quality for plant growth by o Alleviating compaction in clay soils o Improving soil water infiltration rates o Reducing runoff o Improving soil aeration and o Improving soil water retention, especially in sandy soils

Organic soil amendments are required for construction in most Front Range municipalities Composts are classified on basis of chemical composition and maturity

o Class I (plant waste based) composts are lower in soluble salts than Class II (animal waste based) composts

o Class I compost can be used at much higher levels in soils than Class II composts o Compost maturity is an indicator of long-term stability in soils

Class I and II composts are mature Class III and IV composts will undergo considerable breakdown and much of the effect of the

added organic material will be lost to these processes. Class I and II composts are recommended.

Northern water soil test results from the Soil Preparation demonstrations indicated that o Class II composted soils had higher salts than Class I composted soils o The salt levels were not high enough to affect Kentucky Bluegrass turf at any compost level (CY/1000

ft sq) or type (Class I or Class II) Kentucky Bluegrass is affected by soil salinity from 3 mmohs/cm.

o Landscape plants would not have been affected by these soluble salt Native soluble salt levels were initially low and remained at < 1 mmohs/cm after soil amendment.

o Significantly different levels of organic matter (3, 3.9, 4.4%)were found in the 0, 3, and 6 CY/1000 ft sq treatments of the Soil Preparation plots.

Soil and compost testing is recommended before amending soils along the Front Range, particularly when amending with unclassified organic soil amendments.

o Clayey soils are numerous along the Front Range and some of the soils have high potential for elevated salinity that could affect plant growth with compost or irrigation water with elevated salts.

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Why amend soils?

Soils are amended to improve soil quality for plant growth, water conservation, and soil water retention. On the Front Range, the most common reason to amend soils is to alleviate the effects of high soil clay content. While clay in soils retains water, typically clay soils have low water infiltration rates and often have poor aeration. Water can easily run off or pond on the landscape surface. Clay soils are abundant on the Front Range (Figure 1).

Figure 1. Map of Northern Water’s area from the Front Range to the Fort Morgan area. The pink shaded areas denote soil types that have at least 20% clay content. Clayey soils are very prevalent along the Front Range. Light green shaded areas are sandy soils, which are more prevalent east of Greeley.

Clay soils can become compacted easily during construction. Organic compost is the best soil amendment for increasing clay soil infiltration rates and improving porosity when compaction has occurred. Moreover, compacted soils can impede root penetration into the soil profile, thus decreasing the effective rooting zone and soil moisture available to the plant. An amended clay soil is a key step in developing a drought-resistant landscape.

Sandy soils, though not as abundant on the Front Range as clay soils, are equally challenging to work with. Unless amended, sandy soils retain less water and have high potential for losing soluble nutrients from the root zone. Once in

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the groundwater or in return flows to streams, nutrients such as nitrates become pollutants. Irrigation frequencies are reduced when compost is added to a sandy soil and water and nutrients are better retained in the root zone where they are available to plants.

Front Range municipalities are requiring soil amendments for new construction. Denver Water, for instance, will not install water taps on new sites unless soil amendment requirements have been met. Denver Water has very detailed requirements for soil amendment, specifying Class I or Class II organic soil amendments. The classifications are indicators of the compost chemical composition and maturity, with Class I soil amendments having lower levels of salts and pH, and Class IV the highest. The Rocky Mountain Region Compost and Soil Amendment Classifications http://www.extsoilcrop.colostate.edu/Soils/documents/Classification-RockyMountainRegion.pdf show chemical attributes of each classification.

Class I compost is plant waste based. Class II compost is animal waste based. Because animal waste is usually higher in salts than plant-based compost, recommendations for use are different. Class I compost can become a high percentage of soil composition when added as an amendment, and does not need to be mixed into the soil as deeply as Class II soil amendment. Class II soil amendment should not become more than 30% of the soil composition and need to be well mixed into the top 6” of the soil profile. Because of the greater soluble salt contribution from Class II soil amendment, plants sensitive to higher salts may not thrive if the native soil (or water used to irrigate) also has high salt content. It is wise to test the soil and get specific recommendations for additional compost before adding the soil amendments. Class I and II composts are considered mature, meaning that the decomposition process is finished, nitrogen will not be tied up by the maturing/decomposing process, and there is no detrimental effect on plant growth.

Northern Water developed and maintained a Soil Preparation demonstration in the Conservation Gardens at Northern Water. The purpose was to demonstrate that organic soil amendment and tillage had a clear effect on turf ability to tolerate drought conditions. Each summer, water was withheld from the plots for at least 2 weeks.

Class I and Class II organic soil amendments at the rates of 3 cubic yards (CY)/1000 sq ft and 6 CY/1000 sq ft were added to the Nunn Clay Loam soil. The east half of the plot was tilled to 6 inches; the west half was tilled to the 15 inch depth. Control areas were tilled at the 6 and 15 inch depths, but no organic soil amendment was added to these areas.

Kentucky bluegrass sod was installed in the Soil Preparation zones in 2005. Initial problems with thatch were gradually alleviated with aeration and with power raking. Normal turf irrigation and maintenance practices were followed.

In early visual observations (Figure 2), the turf was less dense and of poorer quality in plot zones with no soil amendment. There was no clear visual difference between the 6 and 15” tillage in this photo.

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Figure 2. Turf growth at 3 compost amounts (0, 3, and 6 CY/1000 ft sq) and 6 inch and 15 inch tillage depths in one soil preparation plot.

In 2010, soil analysis was completed for the two main Soil Preparation plots, which had been managed alike. Standard soil test analysis was performed at the CSU Soil Test Lab in Fort Collins. Eight soil cores were extracted at each compost level /type and tillage depth for the 0-12” sampling depth and for the 12-24” sampling depth. The soil cores from each soil amendment type, level, and sampling depth were composited across tillage depths, dried, ground, and thoroughly mixed before laboratory analysis. Comparisons were then made among soil amendment types and levels at each sampling depth.

The main result of the soil test analysis was that the Class II amended soils had significantly (p< 0.05) higher soluble salt levels than the Class I amended soils. This result is not unexpected; however, for practical purposes, the question is whether soluble salt levels in the Class II strips were high enough to cause injury to the Kentucky bluegrass.

According to Colorado State University Cooperative Extension (http://www.ext.colostate.edu/pubs/garden/07227.html), soluble salt levels in soil need to be at least 3 mmhos/cm to affect Kentucky bluegrass growth. The salt levels in this study at the 0-12 inch sampling depth (aggregated over amendment levels) were about 10% higher in Class II amended soil than Class I amended soil (0.65 mmhos/cm for Class II and 0.59 mmhos/cm for Class I organic soil amendments), far

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less than the minimum 3 mmhos/cm level at which Kentucky bluegrass might begin to be salt-affected. Salt levels in the 12-24 inch sampling depth were about 13% higher for Class II amended soil than Class I amended soil (0.70 mmhos/cm for Class II and 0.62 mmhos/cm for Class I organic soil amendments), also still less than the minimum tolerance level for Kentucky bluegrass.

Figure 3 shows soil salt level means and confidence intervals for each compost type at each compost level. Class I compost amended soil clearly trends lower in salts than Class II amended soil. In the Class II amended soil, salts increased with increasing amounts of compost added. The highest compost level (6 CY/1000 sq ft) in Class I amended soil showed higher soil salinity than the 0 and 3 CY/1000 sq ft levels.

TypeAmount CY/1000 ft^2

Class_IClass_II630630

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

Solu

ble

Salt

s (m

mho

s/cm

)

Interval Plot of Soluble Salts95% Bonferroni Confidence Interval for the Mean

Figure 3. Soil salt level means and confidence intervals by compost amount and type.

Aggregated over all compost amounts, Class II compost amended soil had higher soil salinity than Class I amended soil (Figure 4).

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Class_IClass_II

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

Compost type

Solu

ble

Salt

s (m

mho

s/cm

)Interval Plot of Soluble Salts

95% Bonferroni Confidence Interval for the Mean

Figure 4. Soil salt level means and confidence intervals by compost type over all compost amounts.

Soil salt levels trended higher as added compost levels increased (Figure 5), aggregated over compost types.

630

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

Compost Amount (CY/1000 ft^2)

Solu

ble

Salt

s (m

mho

s/cm

)

Interval Plot of Soluble Salts95% Bonferroni Confidence Interval for the Mean

Figure 5. Soil salt level means and confidence intervals by compost amount over all compost types.

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What if this had been an area of trees, shrubs, or ornamentals? A table of ranges of minimum salt tolerances for landscape species (CMG GardenNotes #224, Table 1) shows lists of non-tolerant, slightly tolerant, moderately tolerant, and tolerant plants).

CMG GardenNotes #224 Table 2 confirms that the soil in the Conservation Gardens Soil Preparation areas will most likely have no detrimental effects on non-tolerant plants.

Additionally, it was recommended that soils and soil amendments made from manure be tested for salts before application. For a soil low in salts such as the Conservation Gardens Soil Preparation plots, an organic soil amendment at recommended rates with salt levels up to 10 mmhos/cm could be added without detrimental effects—if well mixed into the upper 6-8 inches of soil. However, in soils already higher in salts, an organic soil amendment with low salts is better.

Table 1. Salt tolerance of cultivated plants.†

Relative Salt Tolerance of Cultivated Plants

Non-Tolerant 0-2 dS/m

Slightly Tolerant 2-4 dS/m

Moderately Tolerant 4-8 dS/m

Tolerant 8-16 dS/m

begonia carrot

cotoneaster green bean

onion peas

radish raspberry red pine

rose strawberry

sugar maple Viburnum white pine

Apple Cabbage

celery cucumber

grape forsythia

Kentucky bluegrass lettuce linden

Norway maple pepper potato

red fescue red maple

snapdragon sweet corn

beet black locust

boxwood broccoli

chrysanthemum creeping bentgrass

geranium marigold

muskmelon perennial ryegrass

red oak spinach squash tomato

white ash white oak

zinnia

arborvitae asparagus

juniper Russian olive Swiss chard

Note: dS/m is the unit used to measure salt content. It measures the electrical conductivity of the soil. dS/M = mmhos/cm

†Reproduced from: http://www.ext.colostate.edu/mg/gardennotes/224.html

Colorado Master Gardener Program; CMG GardenNotes #224

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Table 2. Salt level effects on plant growth.‡

Soluble Salt Test Values For Relative Sensitivity Levels of Plants

Electrical Conductivity*

Salinity Level

Effect on Plant Growth

0 to 2 dS/m

non-saline

none

2.1 to 4 dS/m

Very slight salinity

Sensitive plants are inhibited

4.1 to 8 dS/m

Moderate salinity

Many plants are inhibited

8.1 to 16 dS/m

Strong salinity

Most cultivated plants are inhibited

over 16 dS/m

Very strong salinity

Few plants are tolerant

*saturated paste extract

dS/M = mmhos/cm

‡Reproduced from: http://www.ext.colostate.edu/mg/gardennotes/224.html

Colorado Master Gardener Program; CMG GardenNotes #224

Adding organic soil amendments is best accomplished in smaller amounts over time with soil testing annually to check on soluble salt levels in the soil, especially if the organic soil amendment is higher in salts, or the soil has elevated soluble salt levels.

Because many organic soil amendments have not been tested and classified as per the Rocky Mountain Region Compost and Soil Amendment Classifications, it is very important to test the materials and the soil before amending, as soils with higher salt levels are not uncommon on the Front Range (Figure 6).

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Figure 6. Prevalence of soils with electrical conductivity (EC) greater than 1 mmhos/cm, an indicator of salts in the soil.

Other results from the soil analysis indicated that soil organic matter was significantly different among all compost levels, ranging from 3.01% O.M. at no added compost to 4.43% O.M at 6 CY/1000 sq ft (Figure 7). There were no differences in organic matter between the Class I and Class II composts (Figure 8). Other wide differences occurred for elements P, K, Ca, Mg, and Na (Phosphorus, potassium, calcium, magnesium, and sodium) at the 0-12” and 12-24” sampling depths (Tables 3 and 4).

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630

5

4

3

2

1

0

Compost Amount (CY/1000 ft^2)

Org

anic

Mat

ter

(%)

95% Bonferroni Confidence Interval for the MeanInterval Plot of Organic Matter

Figure 7. Organic matter means and confidence intervals among compost amounts.

Class_IClass_II

4

3

2

1

0

Compost type

Org

anic

Mat

ter

(%)

95% Bonferroni Confidence Interval for the MeanInterval Plot of Organic Matter

Figure 8. Organic matter (%) across compost type.

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Table 3. Means and means grouping by Tukey’s Method at the 0-12” sampling depth.

Means (0-12” Sample Depth)

Compost Type Comparisons*

Compost Amount Comparisons*

Analysis Class I Class II 0 CY/1000 sq ft 3 CY/1000 sq ft

6 CY/1000 sq ft

Soil pH 8.01 A 8.08 B 8.03 A 8.05 A 8.06 A

Soluble Salts

mmhos/cm

0.59 A 0.65 B

0.61 A 0.61 A 0.64 A

Organic matter (%)

3.73 A 3.81 A 3.01 A 3.86 B 4.43 C

N lbs/A 3.6 A 3.9 A 3.72 A 3.73 A 3.8 A

Olsen P (ppm) 66.3 A 114.7 B 17.3 A 103.5 B 150.7 C

K (ppm) 438 A 796 B 392 A 637 B 822 C

Ca (ppm) 4971 A 5210 B 4892 A 5043 B 5337 C

Mg (ppm) 580 A 647 B 546 A 619 B 675 C

Na (ppm) 65 A 121 B 64 A 95 B 121 C

CEC 32.3 A 32.8 B 32.4 A 32.5 AB 32.7 B

*Means followed by the same letter within this category are not statistically different at the p = 0.05 level

*Means followed by the same letter within this category are not statistically different at the p = 0.05 level

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Table 4. Means and means grouping by Tukey’s Method at the 12-24” sampling depth.

Means (12-24” Sample Depth)

Compost Type Comparisons*

Compost Amount Comparisons*

Analysis Class I Class II 0 CY/1000 sq ft 3 CY/1000 sq ft

6 CY/1000 sq ft

Soil pH 8.12 A 8.12 A 8.08 A 8.14 AB 8.14 B

Soluble Salts

mmhos/cm

0.62 A 0.70 B

0.60 A 0.68 B 0.68 B

Organic matter (%)

3.07 A 2.13 A 2.04 A 2.05 A 2.21 B

N lbs/A 3.65 A 4.29 A 2.97 A 3.75 A 5.19 B

Olsen P 16.3 A 16.6 A 11.4 A 16.0 B 22.0 C

K (ppm) 336 A 428 B 331 A 390 B 426 C

Ca (ppm) 5310 A 5365 A 5298 A 5305 A 5409 B

Mg (ppm) 573 A 582 A 573 A 578 A 582 A

Na (ppm) 84 A 183B 75 A 143 B 182 C

CEC 33.8 A 32.3 B 33.0 A 33.0 AB 33.2 B

*Means followed by the same letter are not statistically different at the p = 0.05 level

*Means followed by the same letter are not statistically different at the p = 0.05 level

Notably, N levels were greatest in the 6 cu yd soil amendment in the 12-24” sampling depth. Because N is highly mobile with water, N has likely migrated into this lower soil depth with precipitation and irrigation. No N differences were found in the 0-12” soil depth. In all comparisons in Tables 3 and 4, soil test interpretations indicated that potassium (K) is sufficient to high, phosphorus is adequate to high, Ca and Mg are not deficient for turf. Soil test amounts clearly vary with either compost class or compost amount.

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http://www.coopext.colostate.edu/TRA/PLANTS/turfgrass-fertility-needs.shtml#_ftn8

P adequate to high:

http://documents.crinet.com/AgSource-Cooperative-Services/Locations/UnderSoilAnaly.pdf

Ca, Mg:

http://www.extension.umn.edu/distribution/horticulture/components/1731-complete.pdf

http://www.extension.umn.edu/distribution/horticulture/components/1731-7-essentialnutrients.pdf

What does your community recommend or require?

Municipal codes differ somewhat from community to community. The common denominator for all communities is that soil amendment is required for new construction. Amount, handling, and type are variable. Greeley, for example, currently requires organic industry-accepted, certified weed-free soil amendment at rates of at least 4 CY/1000 sq ft tilled in to at least 6 inches deep. Loveland code separates soil amendment requirements by type of planting: 3 CY/1000 sq ft is required for bluegrass and other high-water plantings, 2 CY/1000 sq ft for shrubs, perennials, and moderate to low-water plantings, and 1 cu yd/1000 sq ft for xeric and very low water plantings. Other municipal codes for soil amendment, or compost, in landscape development can be found via the following links.

Greeley:

http://www.colocode.com/greeley/greeley_18.pdf

Boulder:

http://www.colocode.com/boulder2/chapter9-9.htm

http://www.bouldercolorado.gov/files/PDS/codes/dcs/ch10.pdf

Loveland:

http://www.ci.loveland.co.us/modules/showdocument.aspx?documentid=5006

Fort Collins:

http://www.colocode.com/ftcollins/municipal/chapter12.htm#sec12d130

Longmont:

http://library.municode.com/index.aspx?clientID=14590&stateID=6&statename=Colorado

Author: Mary J. Hattendorf, Water Management & Conservation Specialist, Northern Water. Feb 2012.