How can this research be improved so that she will have greater confidence in her data?

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 one corn plant in typical non-salty soil and one corn plant in the same soil plus 0.5 kilograms of salt. After 6 weeks of growth, the corn plant grown in salty soil is only about 50% of the mass and height of the corn plant grown in non-salty soil. How can this research be improved so that she will have greater confidence in her data?