Re: Does plant food like mriacle-gro work better on plants than just water?

Fertilizers, such as Miracle-Gro, may be beneficial, harmful or have no effect 
on plant growth depending on the situation. If the soil fertility level is 
inadequate, then adding fertilizer should increase growth. If too much 
fertilizer is applied, plant growth is often reduced due to the high salt level 
which makes water less available to the plant. If the soil already has adequate 
fertilizer, then adding additional fertilizer may not increase growth. Plant 
growth depends on many factors other than the fertilizer level, such as light, 
water, and temperature. If plant growth is limited by one or more of these 
other factors, then additional fertilizer will not increase growth. 

Miracle-Gro is no miracle product but is often beneficial. It just contains 
mineral nutrients. However, it does not provide all the mineral nutrients that 
plants require. Plants require 14 mineral nutrients: nitrogen, phosphorus, 
potassium, calcium, magnesium, sulfur, iron, manganese, boron, copper, zinc, 
molybdenum, chlorine and nickel. 

The reference has a photo of plants I grew in hydroponics with Miracle-Gro as 
the fertilizer. Plants receiving Miracle-Gro died because not all nutrients 
were provided and the pH dropped because all the nitrogen was provided as 
ammonium or urea, rather than nitrate. 

Reference

Hershey, D.R. 1990. Pardon me, but your roots are showing. Science Teacher 57
(2):42-45. (Feb.)