Not all fertilizers deliver nutrients the same way. When two commercial plant foods are tested on tomato plants, "Shultz-Instant plants average 3.3 inches tall at the end" while Miracle-Gro plants average 3 inches. Both brands outperform plain water at 1.5 inches, confirming that added nutrients help plants grow taller, but the specific formula matters too.

Fertilizer nutrients like nitrogen help plants grow, but the dose matters more than you might expect. Bean seeds planted in perlite were watered with solutions ranging from 0% nitrogen (plain water) to 10% nitrogen for two weeks. The control group with no added nitrogen grew the tallest at 20 cm. The 0.5% and 1% groups grew only 1.5 cm. Every group above 2% nitrogen died completely. More fertilizer, it turns out, is not always better — bean plants can be killed by nitrogen doses that seem small.

Plant food comes in natural and man-made forms, and each feeds the soil differently. In a test using grass seeds, three peat cups received organic fertilizer made from fish emulsion while three others received inorganic Miracle-Gro. After two weeks of watering every other day, the inorganic group grew 1.2 mm taller on average. That result suggests man-made fertilizer breaks down into a form that grass roots can take in more quickly.

Adding plant food to soil can help a plant grow faster. When mung beans get plant food mixed in their water instead of plain water, "the fertilizer group grows tallest and fastest." The plain water groups grow at the same slower rate. This shows that the extra plant food is what makes the big change.

The amount of fertilizer matters as much as whether you use it at all. Radish plants tested at concentrations from 33% to 166% of the recommended dose show that "plants given the recommended dose grow tallest." Going above or below that target slows growth, because "both too little and too much fertilizer slow growth down."