My fourth grade students at Holiday Heights Elementary noticed while measuring circumference and diameter on our broccoli plants that the plants next door in the third grade bed were much smaller. They measured these too and wrote the results down in their journals. On fourth grade broccoli C=56cm and D=21cm. On the third grade broccoli C=31cm and D=11cm. The students wanted to know why there was nearly double the difference in both measurements.
We put our heads together back in the classroom to try finding a variable that could account for the difference. After discussion they ruled out sunshine, water, spacing, and planting time since these were all the same for both planting beds. Several students asked about the soil. I explained that the soil in both plots started out the same. Another student asked about the smelly fertilizer we mixed in before we planted our bed (we had added a partial bag of organic alfalfa based fertilizer). This led to a good discussion of the importance of plants having a good source of food to grow up large and healthy. They concluded that the third grade soil did not have sufficient food for the plants to grow well.