Two groups of tomatoes were grown under laboratory conditions, one with humus added to the soil and one a control without humus. The leaves of the plants grown without humus were yellowish (less green)compared with those of the plants grown in humus-enriched soil. The best explanation for this difference is that

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The yellowing of the leaves suggests chlorosis, which is often caused by a deficiency of magnesium and/or iron, both of which are minerals commonly found in humus. Therefore, it is likely that the humus-enriched soil contained more of these minerals, which allowed the plants to synthesize chlorophyll more efficiently and produce greener leaves.

Two Groups of Tomatoes Were Grown Under Laboratory Conditions, One