Introduction

Gardening in areas with polluted soils poses a risk for uptake of toxic compounds from the soil. This makes it unhealthy to eat the products from the gardens. A replacement or cleaning of the soil in the potential root zone will, however, often constitute a major work as many plants extend roots to 1-2 m depth. Another possibility could be to remove the uppermost soil layer and place a so called geotextile over the polluted soil. Then other clean soil should be put on top of the geotextile, whereafter vegetables can be grown. This approach demands that the geotextile is able to stop vegetable roots from growing down into the polluted soil.

Geotextiles are normally used to separate and stabilise soil layers, and ensure draining in relation to construction works. The question is whether geotextiles can effectively stop the root growth of vegetables. Plant roots can be very efficient in penetrating for example compacted soil layers. The purpose of this work was therefore to investigate the ability of geotextiles to stop root growth of vegetables.

Materials and methods

Two greenhouse experiments (A and B) were performed at the Research Centre Aarslev. Geotextiles with relatively small pores were selected for testing (Table 1), as plant roots can be very fine (diameter down to 60-130 µm). The purpose of experiment A was to test a range of different geotextiles, and those showing the best results in experiment A was further tested with different plant species in experiment B. The vegetables were planted in transparent plastic boxes of ½ m height. A box consisted of two parts which made it possible to place a piece of geotextile in some distance over the bottom of the box. Root growth was observed by regular counting of roots growing behind the transparent walls of the box.

Table 1.
Technical data as given by the producers of the selected geotextiles. All textiles are of the type Thermic fixed felt (nonwowen).

Conclusions

None of the geotextiles were able completely to stop roots from growing into an underlying soil layer. The geotextiles tested here represent the most fit for the purpose that for the time being are commercially available.

There may be cases where it is adequate to limit, but not completely stop roots from growing into a soil layer. In these cases geotextiles with very small pores such as Typar SF 111 can be used. It is, however, possible that such textiles can be clogged with mull/silt particles that are transported with leaching soil solution. In case geotextiles are used to limit root growth, it is recommended first to test the most relevant textiles in a field experiment running over at least two growth seasons.

Results

Restriction of root growth

All geotextiles were penetrated by roots in exp. A (Fig. 1A). In generel, textiles with the smallest pores delayed root growth most, the delay being approximately 16 days for the textiles with pore sizes of 80 and 85 µm (SF 56 and T4000, repspectively) compared to the textiles with a pore size of 180 µm (SF27 and T700). The textile with poresize 55 µm (SF111) delayed root growth with approximately 30 days compared to the textiles with the largest pores. Exp. B showed results which were a bit different, as no roots were able to penetrate SF111 (Fig. 2A). This was probably due to a much slower plant development in exp. B, caused by less light during the experimental period. Overall the results show that none of the geotextiles can ensure a complete restriction of root growth of all species.

Fig. 1. Roots of carrot and lucerne growing through geotextiles (Experiment A)
Number of roots observed under different types of the geotextiles Terram og Typar SF in a greenhouse experiment with carrot and lucerne. A) shows the results calculated for each of the geotextiles (data have been pooled for carrot and lucerne), and B) shows the results for each of the two species. The number 31 indicates the number of roots >30. Bars indicate standard errors for A) 6 or for B) 18 replicates.
        

Fig. 2. Roots of white cabbage, chicory, and green pea growing through geotextiles (Experiment B)
The number of roots observed under different types of the geotextiles Terram og Typar SF in a greenhouse experiment with white cabbage, chicory, and green pea. A) shows the results calculated for each of the geotextiles excluding green pea, and B) shows the results for each of the three species. Bars indicate standard errors for A) 6 or for B) 9 replicates.

There was no difference in the ability for penetration of geotextiles between carrot and lucerne or white cabbage and chicory (Figur 1B og 2B). Green pea differed as expected from the other species by showing a weak root growth that was stoped effectively by all geotextiles.

Penetration of geotextiles

The penetrating roots showed a good ability for branching under the geotextiles. This means that penetration with just a few roots is unwanted as the root system this way will be able to distribute in the underlying soil. The experiments represents only one growth season. In case geotextiles are to be used as barriers for root growth over polluted soil, there has to be an effect over several growth seasons. This work shows, however, that vegetables will be able to penetrate geotextiles allready early in the first growth season. Furthermore observations at the end of the experiments showed that several of the tested species were able to enlarge the penetrated pores. This effect will make it easier for other roots to penetrate the following season.

There can be cases where a limitation, but not a complete restriction of root growth into a soil layer will be adequate to satisfy a certain purpose. In such cases the geotextile with the smallest pores (Typar SF111) will have a clear effect for at least one or possibly several growth seasons .

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