All plants prefer nutrients by their leaves. This way the nutrients are right where they need to be and don't need to be transferred all the way from the roots. This is much more energy efficient. That being said. Supplying nutrients via their roots give you much more stable water conditions and less change on algae. It all depends on the entire system you use. Root tabs is the easiest way to go if you want it to be low maintenance I guess.

 

Christo,

Plants will take up nutrients from both roots and shoots. If nutrients are more concentrated in the water column then most uptake will be foliar. If nutrients are more concentrated in the substrate then uptake will tend to be via roots. Like Yo-han said, if you fertilize in the substrate it will tend to keep the water column more stable and potentially give you fewer algae issues on the leaves.

 

I just read a paper on nutrient uptake. It seems aquatic plants transfer 5x more nutrients by leaves than by their roots. 5% by roots and 21% by leaf. I'll see if I can paraphrase it later on.

 

Zap,

What year was the paper written? Can you email me an electronic copy?

 

Oh man, I'll have to look back over them all and find the right one. I read about a hundred papers in the last few days so it might take me a day or so to find it.

 

From Diana Walstad's book Ecology of the Planted Aquarium mostly chapter VII:

Tests have been done in split containers with the plant roots in one container and the leaves in another.
The fertilizer can be supplied to just the substrate (root container), or to the leaf/stem container (water column dosing) or both.
Some studies were done with radioactive fertilizers so the scientists could trace where the ferts came from (substrate/ root uptake or water/ leaf-stem uptake). So that if fertilizer was supplied to both parts of the container, the radioactive material would show if the plants took it up from one part or the other.

Conclusions:
1) Different species use one or the other method quite noticeably. Some species will use either method just fine.
2) Some fertilizers are better used from one method or the other. This is not perfectly consistent among all tested plants, but the trend seems to show that most tested plants behave similarly.

A few details:
In nature most minerals, including P, Fe and most traces are more commonly available from the substrate, and most plants pick up these just fine or even prefer taking them in via roots.
In nature, harder water often means Ca, Mg, K, sulfates and chlorides. Plants take these in just fine or even prefer taking these in via leaf.

Even when Phosphorus was available in both chambers, the P was taken in through the roots and translocated to the stems and leaves.

When K was available in both chambers the plants took it in through their leaves and stems at 5 times the rate of root uptake, and some plants took in K through the leaves and OUT through the roots so the substrate gained K.
The suggestion is that plants will add K+ to the substrate to knock the NH4+ off the soil so the plant can get the N. (this is part of what we talk about when we want a high cationic exchange capacity of the soil)

Plants prefer leaf uptake of ammonium. When the water is deficient the plants will take it in via roots. When ammonium is added to the water the roots stop taking in so much. But when more ammonium is added to the root zone the plants will continue taking it in through the leaves if it is available in the water column.

Aquatic plants can utilize ammonium, nitrite and nitrate. But when ammonium is present they take that in first. This is a contrast to land plants that generally prefer nitrate. There are a few aquatic plants that prefer nitrate, but not very many. Mostly these are plants that come from extremely poor water (from the plant point of view), that is, almost no ammonium or other water column nitrogen, so they take in their nitrogen as nitrate via the roots.
When the level is not toxic, aquatic plants grow better with ammonium as the source of nitrogen. This is not to suggest that you should add ammonia or ammonium to the tank! Just that the plants are a very good addition to the bio filter that is keeping ammonia and ammonium out of the water.
When plants take in nitrate they convert it back to ammonium to use it to build their tissues.

Back to me:
To fertilize the plants in an aquarium I would suggest a couple of ideas, and you can take it from there.
1) Even if the plants have few to no roots right now, they will grow roots. So add substrate tablets now, and as these tablets slowly release their minerals the plant will be able to use them. Especially be sure that P, Fe and traces are in the tablets.
2) Unless the fish you are keeping demand really soft water supply some water column ferts. Ca, Mg, K, N are the ones the plants use best this way. A GH as low as 3 German degrees of hardness is fine for the plants as long as it represents both Ca and Mg. If you suspect that one or the other (Ca or Mg) is missing, then test to be sure and supplement as needed. Fish food is a good source of N. You will probably have to dose K. Check your tap water, and if there is K in the water then plenty of water changes might be a reasonable way to get the K to the plants.
3) When you apply ferts in one location or the other they will move into the other location. Substrate tablets slowly dissolve and tiny amounts of these fertilizers do make their way into the water. Not much (as long as the tablets are well buried) but still, some. When you apply water column ferts the substrate will take some, especially if you are using a high CEC material. This is very good- it becomes a reserve supply that the plants can tap into as needed.
4) Plants will take in pretty much all the minerals and other nutrients they need from both water and substrate, so it does not much matter very much (to the plant) which way they are supplied. Ultimately, one way or another, the nutrients will get to the plants.