A farmer finds that his soil has high levels of nitrites but low levels of nitrates. What could the farmer add to the soil to most directly increase its concentration of nitrates?

Nitrobacter is added to the soil by farmers in order to increase the level of nitrates in the soil.

Explanation:

Nitrobacter is a type of bacteria which converts nitrites into nitrates. Nitrites is more toxic which cannot be used by the plants while nitrate is less toxic and used by the plants as a food source. It takes 2 to 5 weeks to increase the level of nitrates in the soil by using Nitrobacter.

this is true, assuming that the allele is completely recessive and incomplete dominance is not a factor.