Ans.
In an experiment, conclusion can be made only after verifying all the results various times as difference in experimental conditions can leads to change in results.
As in given experiment, the student tested fertilizers only in a single plant and gave conclusion, but the result may change if he performs that same experiment in a different plant or in a different experimental condition. Thus, his teacher most likely asked him for additional trail because 'it is necessary to observe data from various plants before making an conclusion that can be accepted.'