The ammonium/nitrate ratio in the nutrient solution is important since it can have a significant effect on the quality and total yields of the tomato crop. Both NO3– and NH4+ are the most important sources of nitrogen in the plant. The ammonium/nitrate ratio has only become critical, and a problem, since growing without soil became commercially viable.

There are a couple of factors that nature takes care of in soil that view people know about.  The first is that NH4+ is converted by oxidation (microbes) to NO3– before it is absorbed into the plant. So in effect, although NH4+ fertilizers are applied, very little (compared to hydroponics) NH4+ was exposed to the plant. NO3- is actively absorbed by the plant, meaning, some biochemical processes are involved before NO3– can be absorbed. The problem with NH4+ is that it is a passive process and the plant does not have control over how much is absorbed. This means that toxicity levels can easily be reached within the plant causing damage. pH has a significant effect on NH4+ uptake.

Optimum uptake rates are found at neutral pH but rapidly falls when the pH of the nutrient solution or soil is lowered (becomes more acidic). The opposite is true for NO3–, uptake is better at lower pH solutions than high pH solutions. Note that there is an equilibrium in a solution between NH3(aq) and NH4+, and that the plant is more sensitive to NH3(aq) than NH4+. “aq” meaning “aqua” or in solution or water soluble.

Plant sensitivity to ammonia (or NH3) is critical during early stages of growth (especially seedlings). Growers with hydroponic systems can make life difficult by fluctuating pH levels too much. The reason is that plants may tolerate high NH4+ levels at low pH, but as soon as pH is rectified (or increased), negative effects of the high concentration of ammonia will be seen.
In essence, the plant is very sensitive to any ammonia fertilizers and pH plays a significant role in the availability of ammonia. There is light at the end of the tunnel however, it has been found that it is safe to include 10% of your NH4+ in the nutrient solution without causing any detrimental effects during crop growth, that includes the initial seedling phase.

One of the big advantages of using ammonia based fertilizers is that the ammonia component does not contribute towards electrical conductivity.  That opens up the possibility to add other nutrients to the solution.  This is especially handy when the initial EC of the water is not ideal.