For those of you who put fertilizer in your water, it is important that you take care not to over fertilize your plants. Using the 10-20% run-off rule will help keep your plants healthy and keep them from being overfed.
When you add fertilizer to the water, the plant food goes into the soil. That’s good. What’s not good is when the plant food builds up in the soil. You see, there is a scientific principle called osmosis*. Osmosis allows the plant to pull in the soluble plant food from the soil because the plant fluids in the roots are stronger in minerals than the soluble plant food in the soil. If the soil builds up plant food to the point where the soil has more soluble minerals in it than the plant roots have, the soil will actually start to pull the minerals out of the roots of the plant. The plant can no longer feed and an interesting thing occurs. The plant shows signs of both overfeeding and underfeeding. To the untrained eye, the symptoms can be interpreted as the plant needing more plant food because it shows signs of being underfed. This leads many gardeners to add more plant food to the soil and make matters worse.
* Osmosis (ŏz-mō’sĭs) The movement of a solvent through a membrane separating two solutions of different concentrations. The solvent from the side of weaker concentration usually moves to the side of the stronger concentration, diluting it, until the concentrations of the solutions are equal on both sides of the membrane. The pressure exerted by the molecules of the solvent on the membrane they pass through is called osmotic pressure. Osmotic pressure is the energy driving osmosis and is important for living organisms because it allows water and nutrients dissolved in water to pass through cell membranes. -The American Heritage® Science Dictionary
As a general rule, if you add fertilizer to the water your fertilizer strength should be reduced in hot weather and raised back up again when temperatures lower. This is because you will be watering more often and plants will often transpire water faster than photosynthesis can process the plant food. The fertilizer has more chance to build-up in the soil in hot weather.
I will take this opportunity to tell you some classic symptoms that plants will show when they’re being overfed.
Leaf: The leaves of overfed plants will tend to show signs of stress by turning a very dark green and then turning to a light green fading eventually to yellow. Initial stress can also be seen in the leaf as a slight buckling, or the plant’s inability to keep its leaves flat or inability to keep the classic even shape of all of the leaves. The leaf may even show signs of obvious deformity. Plant leaf tips may start to curl, either from side to side or up and down. The leaf tips may turn brown, dry-up and die. New leaf shoots may show signs of yellowing, either in the veins or between the veins.
Advanced signs of over fertilization will take on the same signs as severe under watering or drought. Leaves will be dry to the touch and often crumble into tiny pieces when lightly grasped between the fingers and palm.
Stems: Stems will show signs of mineral deficiency by displaying yellow or purple stripes on the main stem or branches. The branches will lose their ability to bend and bounce back to their original position. They will tend to break, fracture and split under pressure.
Roots: Roots will generally not give signs of over fertilization. I will say this however; those who have let their plant’s pot sit in water may very well develop root rot. With root rot the roots will turn brown and mushy. You will see many of the same symptoms as mentioned above as the plants root lose their ability to uptake nutrients.