Forschung

Infodienst Weihenstephan - Ausgabe Januar 2010

Substratforschung

Substrate quality assessment for extensive roof greening according to FLL guideline and quality assessment of resulting drainage water

1. Material and methods

Nach oben

2. Results

You can find all charts of the results in the following pdf

Abbildungen_Infodienst-01-2010_Forschung.pdf (Dateigröße: 65 kb)

2.1.2 Air volume at maximum water capacity

In SL system, the values range from 10.3 to 73.3 %vol (fig 4). Only substrate 1 is close to the FLL limit (>10 %vol). In ML system, the values range from 5 to 25 %vol. Substrates 8 and 21 don’t reach the FLL requirements (>10 %vol)

2.1.3 Permeability

Values range from 13.6 to 440 mm/min in SL system. The low limit (60 mm/min) is not reached by most of the substrates (fig.5) with a lowest infiltration rate for substrate 1 (13.6 mm/min). On the opposite, substrates 5 showed the highest value with more than 440 mm/min, considering we could only measure values below 440 mm/min. There is a strong negative correlation between maximum water capacity and permeability (r=-0.741 p-value: 0,002). 
For ML system, the values range from 12.7 to 83.8 mm/min. FLL requirements (0.6-70 mm/min) are mainly respected, two substrates get out of the high limit (substrate 10 and 19 with 77.7 mm/min and 83.8 mm/min respectively, fig.5)

2.1.4 Particle size distribution

Particle size distribution has an influence on water holding capacity, depending often from fine particles (for particles < 0.063 mm, r=0.831, p=0.000 and 0.63-2 mm, r=0.847, p=0.000).
In SL system, substrate 1 exhibits the most impressive distribution, with particles between 0.063 and 2 mm not fitting with FLL requirements (10% mass) (fig 6). The combined clay and silt components are above the limit with 17% mass.  It seems that this substrate has an amount of fine particles to high for SL construction, what we can also notice concerning the amount of organic matter or very low permeability. Substrate 11 follows the same trend, probably because of a too high amount of fine porlith. Substrates 6, 9, 12, 17 (fig 7) and 20 match perfectly with FLL requirements.

In ML system, only substrate 4 doesn’t match with FLL requirements, because of too low proportion of fine and medium gravel. The other substrates concerning this parameter are well designed for ML system. The highest particle proportions are situated between 0.63 and 6.3 mm, with 20.3; 15.9 and 15.4 % mass respectively.

Nach oben

2.2 Chemical parameters

2.2.1 Salts

Dissolved salts are measured by electrical conductivity in water extraction. In SL, values range from 0.16 to 5.71 g/l. Only substrates 7 and 11 exceed the FLL limit (3.5 g/l) with 4.3 g/l and 5.7 g/l respectively (fig 8). In that case, an extraction in gypsum solution has been carried. The saturated CaSO4 solution avoids overestimation of salts caused by soluble CaSO4. Thus, the values obtained are much lower. The substrates 7 and 11 display in that case lowers values than FLL threshold (2.5 g/l)
In ML system, as substrates 8 and 14 with 5.1 and 5.8 g/l exceed the FLL requirements (fig 8), salt has been also followed measured in gypsum solution. Thus, they fulfil FLL requirements with 2.3 and 1.6 g/l.

2.2.2 pH

pH in SL construction ranges from 5.2 to 10.6. Substrates 2 and 5 with 5.2 and 10.6 respectively  present pH value out of the FLL requirements (fig 9). This last value might be the tolerable limit for plants. pH in expanded clay in nearly not buffered, so the value will drop down with the first strong rainfalls. Even more, substrate 5 composed by broken expanded clay has not a buffered pH and might display a strong acidification over time.
In ML system, pH values stay between 7.5 and 8.3, which can be considered as fulfilling requirements.

2.2.3 Organic content

Organic content is correlated with maximum water capacity (r= 0.491, p-value: 0,001) and ranges from 0 to 240 g/l for SL system with a FLL limit of 40 g/l (fig 10). Substrates 1 and 18 display too high organic content, especially concerning substrate 1 which has at the same time a high water holding capacity. Organic content is moreover an important parameter in SL construction due to his high potential to water retention in a substrate which needs to be efficient in terms of drainage, considering the drainage layer is within the substrate. Moreover, organic matter is subject to be structurally unstable and rapidly decay over time, allowing at the beginning of the set up a well development of the vegetation but afterwards leading to a reduction of substrate performance (decrease in nutrient and water holding capacity). In ML system, the values range from 22 to 272 g/l with a limit of 65 g/l (fig 10). Substrate 15 greatly exceeds FLL limit of 65g/l.

2.2.4 Nutrients

• Nitrogen (N)

N content has been assessed by CAT extraction through NO3-N and NH4-N (100% of the N in the substrate was composed by NO3-N). N content in SL system ranges from 0.5 to 246 mg/l (limit: 80 mg/l in SL and ML, fig. 11). The highest value is displayed by substrate 1 (246 mg/l) and is equal to 0 mg/l for substrates 5, 12, 16, 22. Thus, a suitable fertilization to ensure plant growth seems to be fundamental. In ML system, N (CAT) content never exceeds FLL limit but does not exist for substrate 17 and 21.

• Phosphor and potassium (P and K)

Concerning P content, most of the substrates in both systems exceed FLL limits of 21.8 mg/l in SL and ML (ranging from 1 to 107 mg/l in SL and 29 to 102 mg/l in ML, fig 12), which is not such a case with K content: the values range from 13 to 807 mg/l in SL and from 182 to 972 mg/l in ML (substrates n°1, 18, 20 exceed the limit, fig 13).  Moreover, in compost, P and K amount are often quite important.
High amounts of N, P or K in the substrate are due to the organic matter present inside, with a high correlation between N (CAT) content and organic matter (r=0.865, p-value=0.000). The correlation between P (CAT) and organic content is poor (r=0.246, p= 0.116). For K (CAT) the correlation is much more important (r=0.776, p=0.000). There is no strong correlation between organic content and N, P, or K for ML system.

Dipl.-Ing. agr. Laura Carrillo

Nach oben