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Plants grown Hydroponically must have all of their required nutrients supplied in the fertilizer solution rather than obtaining them from the soil like traditional crops in the field. This allows for greater control of the growing environment, but can also lead to deficiencies if all the required elements are not in the solution.

Many studies have been done by plant physiologists over the years to determine the concentrations of nutrients that plants require and a recognised authority in this field is Dennis R. Hoagland who developed the "Hoagland's solution" which is a benchmark for optimal plant growth and is used all over the world.

The essential micro- and macronutrients which all plants require and their tissue concentrations for optimum growth are:

Macronutrients(%):

Sulfur 30

Phosphorus 60

Magnesium 80

Calcium 125

Potassium 250

Nitrogen 1000

Oxygen 30000

Carbon 40000

Hydrogen 60000

Micronutrients(ppm):

Molybdenum 0.001

Copper 0.10

Zinc 0.30

Manganese 1.0

Iron 2.0

Boron 2.0

Chlorine 3.0

(Taiz & Zeiger, 1991)

Certain plants such as rice also require Silicon for proper growth.

One problem with growing plants in Hydroponic nutrient solutions is Iron defficiency which can occur due to the Iron precipitating (coming out of solution as a solid) into insoluble Iron Hydroxide. Modern fertilizers use Chelating agents such as ethylenediaminetetraacetic acid (EDTA) which keep the iron and other trace elements in solution. Precipitation of other substances such as Calcium Phosphate can occur if concentrated nutrient solutions are prepared, so solutions are best prepared separately prior to dilution and use, usually in 2-part "A" & "B" solutions.

Most commercial Hydroponic growing operations use premixed fertilizers to reduce the chances of nutrient defficiencies occuring caused by human error and to reduce the labour involved in ordering and preparing nutrient solutions from scratch. Various recipes used for lettuce, tomatoes, and cucumbers can be found at : http://www.cals.cornell.edu/dept/flori/l...

http://www.usu.edu/~cpl/nutrwht.html

http://res.agr.ca/harrow/bk2/cuke1a.htm

http://res.agr.ca/harrow/bk/tomch9.htm

http://www.ag.arizona.edu/hydroponictoma...

http://members.tripod.com/Client_Profile...

http://nfrec-sv.ifas.ufl.edu/nutrient_so...

http://www.biotron.slu.se/bio3375.htm

http://edis.ifas.ufl.edu/scripts/htmlgen...

http://www.np.edu.sg/~dept-bio/sssc/nutr...

http://www.hbci.com/~wenonah/hydro/nitra...

http://www.atlantic.net/~elifritz/hydrop...

Check out http://www.hydromall.com/info/eleccon.ht... for some data.

Another good source for recipes is the Journal of Plant Nutrition, esp. Vol. #21. Issue #10 which compares 12 different solutions.

The units of measuring concentration are Siemens and Mhos where 2 microSiemen(uS)=2 micromho(umho)=~1 ppm(approximation). Siemens and Mhos are units of Electrical Conductivity (EC) as described in an earlier article and are reciprocals of resistance (Ohms) as the EC of solutions increases linearly with nutrient concentration. Typical nutrient concentrations are from 1000-3000 uS/cm (~500-1500 ppm) depending on size, type of, and growth rate of the plants.

You can prepare your own calibration solution by calculating using the equation C=Q/V where C=concentration desired, Q=quantity of solute, and V=volume where 1g/L=1000ppm.

Goto Best hydroponics http://www.ncf.carleton.ca/~de440/ for more info..

References: Taiz & Zeiger. Plant Physiology. Redwood City, California. The Benjamin/Cummings Publishing Company: 1991.