Glass suction plate

Suction cups and suction plates have a decisive advantage over the extraction of drill core samples or other soil samples: the observation of concentration changes at the same place as often as required. The random influence of small-scale concentration differences in the soil profile on the results of different sampling times is excluded - in contrast to soil sampling and subsequent extraction.
We offer suction cups and suction plates made of the same materials, the sampling is the same or very similar with both types of equipment. The question which geometry is better for the planned application (cups or plate) depends on various other factors and must be considered in each individual case.

In our opinion, suction plates should be installed laterally in a gallery below an undisturbed soil area. This usually requires an open soil profile or a trench, which means that a corresponding amount of space is required. The use of suction plates is therefore limited if the area is small, the extraction depth is large or the substrate is problematic for installation due to its high skeleton content or pourability. In all these cases, suction plates can be installed more easily and cause less disturbance to adjacent soil areas. The most important disadvantage of suction cups is that they extract an undefined volume of soil. The size of the area sampled concentrically around the filter body depends - apart from soil moisture and hydraulic conductivity - on the sampling time and the applied negative pressure or pressure difference between the vacuum system and the water tension of the soil. The concentration measured in the solution cannot easily be converted into a seepage load.

While working with suction plates the situation might be different under certain conditions: The installation effort for suction plates is usually much greater than that for suction cups. However, suction plates have two major advantages over suction cups when calculating for example nitrate leaching quantities (NO3 load): 1. they have a larger surface area and 2. the NO3 concentrations and seepage quantities determined can be related to a defined surface area (Weihermüller 2020).

Regarding 1: The larger the sampled area, the more representative the result is, since differences in flow velocity or different NO3 loading of the pores are better represented on average.
Regarding 2: The area reference (and defined area of action) of the suction plates is a basic prerequisite for a calculation of nitrate loads from measured leachate quantities and concentrations to be permissible at all.

Soures:
Weihermüller, Lutz: In-situ soil water sampling. In: Field Measurement Methods in Soil Science, Wessel-Bothe, S./ Weihermüller, L. (eds.). Stuttgart: Borntraeger 2020, S. 99-132.