More than 300 sensors - XBee

Hi everyone,
I am planning to build, for a research in the University I work in, a series of automated greenhouses with a lot of sensors in each of them.
The major problem relies on the amount of soil moisture sensors. I need to monitor at least 300 of these sensors, and log the values in a SD card, every 5 minutes or so. The sensors need to be analog.
I was thinking on building an array of several arduino megas, each with 16 of these sensors, plus some digital outputs for a relay, to control electrovalves for the watering system. With the help of XBee, I hope I can distribute the information from each part of the array to a central arduino, which will log the data into the SD card as .TXT file.
So, the question is,
is there an easier way of doing this? is a XBee network capable of such a flow of data from different nodes? I will need to assign an individual number for each part of the array, and be sure I don’t miss any data. Handshaking the nodes in every pack of data cold solve that?
In a greenhouse water is a problem, and so I would like to build this communication wirelessly.
Sorry, I’m a newbie in XBee. Any help is much apprecciated.
We have a grant for this research, so buying several arduinos is not a problem.
The temperature and humidity control & monitoring are already solved. Datalogging this, too.
Thanks a lot to everyone.

As it seems you never heard about analogic multiplexers, a simple arduino duemilanove is more than enought, it might take some time to sample all those sensors, but it will be less than the 5 min interval between logs. One 4051 connected to one analog input would give you 8 inputs, but you can connect the analogic multiplexer to another analogic multiplexer, so using that you can even connect all the sensors to one single analogic input.

Thanks Senso, I will probably use multiplexers for some of the benches (where the plants will be). Or for every one of them. But the greenhouses will be big enough to justify XBees. So, propably instead of using 18 arduinos megas (as I was planning to do, along with the same number of XBees), I will use only 4 for the moisture sensors (as there will be be 4 benches), plus one that will control and sense temperature and humidity, which will be the one logging everyting. Thanks again!

is a XBee network capable of such a flow of data from different nodes?

Yes - that is what a mesh network is perfect for - You have one co-ordinator module that all of the other modules 'talk to'.

You can use only XBee with the analogic moisture sensor as it has a A/D conversor integrated. Look at for an example of what I'm using, it's only one sensor in the Xbee, but it supports 4 sensors directly with no hdw.