Factors Limiting Growth in Plants
Every plant species has particular conditions where they will flourish.  There are various physical factors that affect the growth rate of plants; however, all plants share one universal growth limiting factor.  Minimally stated it is "the rate of any growth process controlled by 2 or more factors is limited by the factor in least supply".  In layman's terms, the chain of growth is only as strong as the weakest link.

Basic Factors Affecting Plant Growth
There are seven (7) fundamental factors that have the potential to limit or assist in the growth of a plant.  These seven factors are listed below:

1. pH Level - The acid/alkaline levels in soil, growth medium, nutrient solution etc.
2. Light - Intensity, color and photoperiod or length of day.
3. Atmosphere - Some sub-factors are oxygen, carbon dioxide and humidity.
4. Nutrients or Soluble Minerals - Found in the soil or hydroponic solution.
5. Temperature - Both of the air and the root zone.  Also, the variation between the high and low temperature plays a part in growth or lack thereof.
6. Soil or Hydroponic Growing Media Particle Size - The size affects the rate of drainage as well as aeration or oxygen content at the root zone
7. Water - Will effect all other factors and all other factors can effect water.

With that said, what are the optimal conditions for growing plants?

pH
For most plant life, the most favorable pH value is between 6 - 6.8.  This is not something that should consume one's time as the pH level of the hydroponic solution will change from night to day and even hour to hour.  This is caused by plants taking in the nutrients and then emitting carbon dioxide at night.  As long as the pH level maintained in the system is between 5.8 and 6.8 it will not limit the growth of the most plants. 

Light
In the summer when the sun is the brightest and plants reside outside or in a greenhouse, sunlight is generally not a limiting factor.  However, some plants do require partial shade to grow best.  

In regards to hydroponic systems where light may not be as as strong, light may be limiting factor.  One may want to employ a light meter and consult a horticulture book to measure and learn the minimum footcandles for the species in the crop.

The blue and red portions of the light spectrum are most efficient in producing plant growth.  The red spectrum may also stimulate the flowering response in various plants.  Some plants also have precise photoperiod needs, to stimulate flowering, like the Chrysanthemums.

Atmosphere
Carbon dioxide and humidity are the two most influential sub-factors in the atmosphere category.

Typically, humidity is not likely to be a limiting factor.  If the air increases in humidity, then increased ventilation must be provided.  For every 10 degrees Fahrenheit rise in temperature, the air will hold twice the moisture.  Relative humidity from 40 to 60% is beneficial for most plants.

For plants that have enough warmth, light and nutrients, the likely growth limiting factor will be carbon dioxide.  The most advantageous growth temperature for many species will increase if sufficient carbon dioxide is available. Outside air contains about 300 PPM of Carbon dioxide.  Though, with all other factors being available in adequate measure the best carbon dioxide level may be between 2500 PPM and 4000 PPM.  PLEASE NOTE: Carbon dioxide levels over 2500 PPM are toxic to humans.

Temperature
As mentioned previously there are two temperature zones that one must be aware of in a hydroponic system: Air and Root.

The most favorable air temperature for most plant species is between 65-85 degrees Fahrenheit.  If the air temperature is in excess of the range, it places stress on the water and nutrient uptake system of the plant.  Below 60 degrees Fahrenheit plant growth will be stunted considerably.

The root zone temperature is an entirely separate concern.  The optimum is about 70 to 75 degrees Fahrenheit.  This allows maximum nutrient and water intake.

Nutrient
Nutrients, also dubbed "plant food", are soluble minerals that, when mixed with water, allow the plant to feed. Needed nutrients fall into two categories: Macro and Micro elements. 

Macro elements are vital to plant health; these include:

1. Nitrogen
2. Phosphorus
3. Potassium
4. Calcium
5. Magnesium
6. Sulfur. 

Other elements needed are called Micro elements.  Included in this category are:

1. Boron
2. Manganese
3. Copper
4. Zinc
5. Molybdenum
6. Chlorine
7. Sodium
8. Cobalt
9. Silicon. 

It is important to note that some of the Micro elements are toxic to the plant at levels as low as 0.01 PPM. 

The total strength of the solution usually ranges from 500 to 3500 PPM.  There is a wide variation here as it widely depends on the plant species and temperature.  For most plants, a solution strength of 1000 PPM to 2000 PPM facilitates growth best.  A note of caution: use a lower solution strength in very hot climates. 

Nutrients are only absorbed if oxygen is readily available at the root zone.  The optimum dissolved oxygen content of the nutrient solutions 6 PPM at 70 - 75 degrees Fahrenheit.  This is realized by continual aeration of the solution. 

Media Size
The best media particle size is usually between 3 to 8 mm.  This permits rapid drainage while allowing fresh air to be drawn into the root zone.  The growing media should also be inert, to prevent interactions with the nutrient solution.

The capillary action of the media also affects the optimum size.  For example, Rock wool has 0.005 mm fibers with 96% pore space.  This works extremely well as it is porous and allows for drainage and aeration.

Through the products provided by High Nutrients, you may be able to elude many of the factors that will limit the growth of a given crop.

Additional Resources:
Carbo-Load plant sugar supplement