In selecting tense, your first concern is to avoid confusing the reader or whip-lashing the reader with frequent or unclear changes in tense. In any section, try to establish one tense as the main tense and then depart from that tense as logic requires.
In scientific writing, you will usually use past tense when describing what others found and what you found. For example, in an Introduction or a Discussion you might write:
'York (1998) showed that transgenic tobacco plants supported smaller numbers of rhizobacteria.' or
'In our experiments, transgenic tobacco plants supported smaller numbers of rhizobacteria.'
But use present tense when the finding is accepted as a general fact. You would say:
'Plant growth often depends on nitrogen availability', not
'Plant growth often depended on nitrogen availability.'
Similarly, you would say:
'Photosynthesis requires water and carbon dioxide' , not
'Photosynthesis required water and carbon dioxide.'
The decision to use past or present tense requires judgement about the condition of the statement - if the statement is widely accepted, you will probably use present tense. If the statement reflects a particular investigator or your own findings, use past tense to show that the finding may not always apply.
You also will use past tense when describing methods:
'To determine how transgenic tobacco plants affect microorganisms, we grew transgenic and nontransgenic tobacco plants in randomized field plots.'
Don't confuse tense and voice (past tense is different from passive voice).
Sue kicked the ball. active voice, past tense
Sue is kicking the ball. active voice, present tense
The ball will be kicked by Sue. passive voice, future tense