Although this article was written on hydrophonics, the understanding on plant nutrients are useful in determining the fertilizer ratio.
http://www.buzzle.com/articles/the-role--measures-of-nutrients-in-plant-growth.html
The Role & Measures of Nutrients in Plant Growth
Environment plays a very important role in plant growth upto a point. Once optimal environmental levels have been achieved in the hydroponics grow room, however, it is the quality of nutrition that determines crop quality and output. The following background information will be useful in understanding of the role of nutrients in hydroponics cultivation.
Nitrogen
Plants absorb nitrogen from fertilizers in both Nitrate (NO3) and ammonium (NH4) forms. Both ammonium and nitrate forms are available in the standard fertilizer mix supplied. It should be noted however, that ammonium levels should be significantly lower than nitrate levels with a safe level being 10 to 20 times nitrogen available in the Nitrate form vis-à-vis the Ammonium form. Ammonium is readily available to plants and can build up to toxic levels in plant tissue if it is not assimilated for growth. Besides, the Nitrogen from Ammonium is difficult to leach away once it is in plant tissue. It is therefore important too ensure that ammonium content in the nutrients is carefully regulated.
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