A biology student is studying ways to grow crops in soils that are high in salts and other dissolved solids. To determine whether corn plants can tolerate salty soil conditions, she grows 50 corn plants in typical garden soil and the same amount of corn plants in garden soil plus varying amounts of salt: 0.01 kg, 0.1 kg, and 1 kg of salt. After 6 weeks of growth, there was an inverse relationship between the amount of salt in the soil and plant height: the more salt present, the shorter the plants. The student concluded that salt did have an effect on the growth of corn. What was the control in the experiment?