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

Options

A. the healthy plants used the food in the decomposing leaves of the humus for energy to make chlorophyll.
B. the humus made the soil more loosely packed, so water penetrated more easily to the roots.
C. the humus contained minerals such as magnesium and iron, needed for the synthesis of chlorophyll.
D. the heat released by the decomposing leaves of the humus caused more rapid growth and chlorophyll synthesis.

Answer

**Step-by-Step Solution:**

1. **Understanding the Experiment**: Two groups of tomato plants were grown under controlled laboratory conditions. One group had humus added to the soil, while the other group did not have humus (control group).

2. **Observing the Results**: The leaves of the plants grown without humus were observed to be yellowish and less green compared to those grown in humus-enriched soil.

3. **Identifying the Key Differences**: The primary difference between the two groups is the presence of humus in the soil. Humus is known to be rich in organic matter and essential nutrients.

4. **Analyzing the Role of Humus**: Humus is a source of various minerals and nutrients that are crucial for plant growth, including magnesium and iron, which are vital for the synthesis of chlorophyll.

5. **Connecting Nutrient Deficiency to Leaf Color**: Chlorophyll is the pigment responsible for the green color in plants. A deficiency in magnesium and iron, which are necessary for chlorophyll production, leads to yellowing of leaves (a condition known as chlorosis).

6. **Conclusion**: The best explanation for the observed difference in leaf color is that the humus contains minerals such as magnesium and iron, which are needed for the synthesis of chlorophyll. Therefore, the plants grown in humus-enriched soil are healthier and greener compared to those grown without humus.

**Final Answer**: The humus contains minerals such as magnesium and iron which are needed for the synthesis of chlorophyll.

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