Soil Ecology and Restoration Group


Soil Strength Remediation

David Bainbridge

 

One of the most common effects of disturbance is an adverse increase in soil strength. As pretreatment studies at a 1999 CSS restoration site in San Diego showed, soils were very compacted in many areas. This had been caused by the weight of vehicles, tire or track slip, tillage (part was old farmland), vegetation removal, and hoof impacts during grazing, Table 7-1. Disturbance also affects many other soil characteristics that can increase soil strength. For example, soil strength is related to soil moisture, so reduced infiltration or water movement into the soil as a result of disturbance, vegetation removal and other changes at the soil surface can also increase soil strength. A reduction in soil organic matter may also reduce soil moisture retention, which further increases soil strength and makes it more difficult for plant root extension.

 

Table 7-1
Soil strength problems at a CSS restoration site 1999

Condition
Mean depth in cm after 15 impacts
Very poor n=3
Poor n=11
Fair n=12
Good n=7
Excellent n=1
4.4
8.7
12.5
19.1
38.1

 

The increased soil strength makes it very difficult for seedling roots to penetrate the soil and plant survival and growth will be limited if no remediation is attempted. The effects of even minor increases in soil strength can be dramatic in favorable environments and catastrophic in exposed or difficult sites. Reversing these effects and reducing soil strength to improve plant survival at this restoration site was a goal of the site preparation process. Measuring soil strength precisely is difficult because it involves many factors. Fortunately, the interaction of these many factors can be compared easily using a soil penetrometer.

Examination of plots after deep ripping, tillage, irrigation, tillage and mulching showed how effective treatment can be in improving soil characteristics for seedlings and transplants. The soil conditions on treated plots were much more favorable for root growth. Soil strength has been reduced dramatically for previously compacted areas. This is a function of the ripping and also increased soil moisture. Soil under the mulch was still moist in most areas even in June. Fungal mats and spores were common in plots where soil samples were collected, and some termites were seen at the soil/mulch interface.

The comparison of soil strength before treatment and after treatment is shown in Figure 7-1, comparing means of the treated sites versus roads and motorcycle trails measured in 1999. Overall the site soil penetrability has increased 150-250%. This makes it a much more favorable site for transplants or seedling establishment.

 

Figure 7-1. Soil penetration means, showing some recompaction from mulch spreading

 

Comparing soil strength by category in relation to penetration depth after 15 impacts we can see that the overall response has been very good, Table 7-2.

 

Table 7-2
Soil strength improvement at CSS restoration site 1999-2000

Depth cm after 15 impacts Condition 9/1999 7/2000
<5 cm
5-10 cm
10-15 cm
15-20 cm
20-25 cm
>25 cm
Poor or very poor
Good or better
Very poor
Poor
Fair
Good
Very good
Excellent
9
32
35
21
0
3
41%
24%
0
6
31
6
31
25
6%
62%

 

 

The initial treatment of deep ripping has been successful and conditions for transplanting appear favorable. Soil strength will continue to be followed over the next few years.