Symbiotic nitrogen fixation by Rhizobium

As part of a practical we had to grow Rhizobium inoculated white clover seedlings in nitrate free agar and inoculated seedlings in nitrate free and nitrate supplemented medium. After several weeks of growth we had to determine the wet weight of the arial parts of each plant. We used the mean weights for the inoculated nitrate free, nitrate free and nitrate supplemented seedlings to calculate improvement in growth due to nodulation and also the efficiency of nodulation. (we were given two basic equations to determine these).

I don't understand how to analyse my results and hoped someone could offer some advice.

We grew 11 seedlings of each type (each in separate tubes). Two of the inoculated seedlings failed to grow at all - so when I worked out the improvement due to nodulation (using log yield inoculated - log yield uninoculated) my result is a negative number.

I realise that root nodulation is beneficial for plant growth but using mean values in my results seems to give a misleading result value.

What kind of statistical analysis should be used to analyze my results?

Any help would be appreciated - even if its direction to a useful website or book.

Many thanks in advance