Many farmers and gardeners compost their plant and animal waste. The living material naturally decays in compost bins, forming a dirt-like substance that’s rich in nutrients. The next season, farmers use this substance as a natural fertilizer for their crops. A biology student has grown tomato plants for several years. Until now, he used an artificial fertilizer formulated for tomato plants. This fertilizer caused his plants to grow faster and taller than they grew in unfertilized soil. The student wants to know whether using natural compost will cause his tomato plants to grow faster and taller than his artificial fertilizer. How does the design of your experiment control for outside factors that may affect the results?