Sensors should provide information to help keep the plants alive and the amplification should help articulate the plants' needs to get people to help them do so.

should be comparative to each plant's different needs/habits. (soil moisture best), light--photocell, temperature(thermistors in soil),

soil moisture: very important. most basic. The gardener who monitors Andy Goldsworthy's trees in stones on the roof of the Museum of Jewish Heritage uses a Delmhorst KSD1 http://www.delmhorstmoisturemeter.com/delmhorstsoilmoisturemeter.asp with gypsum blocks that connects to a mobile reader, we could use it...but it is $250+!! We can look into Forestry Suppliers Inc. http://www.forestry-suppliers.com/search.asp?stext=soil%20moisture they have a wide range of soil moisture devices. There's also a wide selection of low-cost and potentially hackable devices:
proximity: The objective is to detect human presence. Perhaps a pyroelectric sensor? Check out this: http://www.glolab.com/pirparts/pirmanual.PDF
light: photocells? do we want to sense different kinds of light?
gas: Jen's sensor report: http://itp.nyu.edu/physcomp/sensors/Reports/GasSensors. Kate & Rebecca have a TGS4161 CO2 sensor and KE-25 Oxygen sensor from http://www.figaro.com.

for terrarium

humidity: Thermal Conductivity Humidity Sensors, SHT11 (Digital Capacitive Humidity Sensor), The SHTxx is a single chip relative humidity and temperature multi sensor module comprising a calibrated digital output. http://itp.nyu.edu/physcomp/sensors/Reports/SHT11
soil composition: http://www.ghcc.msfc.nasa.gov/precisionag/soils.html, assessing soil comp, Old School Methodology: separating size particles to id as: clay, silt and sand and from there examine the presence of organic matter and its ph.
molecular exchange: plant/human