Hydroponic Systems

Section 1: Hydroponic Production

Because an entire course could be developed on hydroponics, we will only briefly cover the basics of this subject.

The primary difference between hydroponic production and production in substrates, such as we have already discussed, is that hydroponics uses either no substrate or a substrate that has little or no cation exchange capacity and contributes little in the way of mineral nutrients. Essentially, the substrate is an inert material, and we must supply and control all mineral nutrients by manipulating the nutrient solution. Often the actual substrate is the nutrient solution (i.e nutrient solution culture). In other cases, the material being used as the substrate will have some degree of water-holding capacity (i.e. rockwool, perlite or hydroton). Therefore, plants are either grown continuously in the nutrient solution, and thus water and nutrients are continuously available, or the nutrient solution must be supplied on a regular basis.

There are numerous types of hydroponic systems. Among these are:

Static or standing solution systems - This system actually grows the plants in a static nutrient solution. Plants are often floated on these solutions using styrofoam trays. The nutrient solution is not constantly recirculated. Air may, but not always, be bubbled into the solution to provide air to the plants' roots depending upon the crop being grown because the nutrient solution changes over time, it must periodically adjusted or replaced. This is a common system used for the production of hydroponic lettuce and other types of vegetable greens.

Flowing solution systems - This system is similar to the static system except that the solution is slowly recirculated so that adjustments to pH and nutrient content can be made.

Nutrient film technique systems -In the nutrient film system, plants are grown with their roots in a shallow enclosed trough. The dimensions of the trough may vary depending upon the crops being grown and grower preference. A thin layer, or film, of nutrient solution is continuously drained down the length of the trough to provide water and mineral elements. Because the roots are covered only with a film of solution, they also have ready access to oxygen. The nutrient solution is collected at the end of the trough, recollected and reused.

Intermittent subirrigation systems - In this system plants are grown in a pea gravel, sand or rockwool medium that provides support and a suitable environment for the roots. Nutrient solution is supplied by periodically "flooding" the bed with nutrient solution and then draining recollecting and reusing the solution.

Bag culture - Although plants are not grown in solution (and some would consider this pseudo-hydroponics), this is a common method used in greenhouse vegetable production. Plants are grown in bags of sand, perlite or rockwool. The substrate provides physical support as well as some water-holding and nutrient-holding capacity. A drip tube is used to periodically deliver nutrient solution to the plant. The solution may be recollected as it leaches from the susbstrate and reused.

One of the most common systems used for hydroponic production in greenhouses is the rockwool slab system. The substrate is a slab of rockwool in a plastic bag. The bag has slits or holes in the bottom and the slab is placed in a trough. The roots of the plant grow into the rockwool substrate. A drip tube in the top of the slab supplies small amounts of nutrient solution to the rockwool on a periodic basis. Excess solution drains from the rockwool and is collected by the trough. The trough returns the solution to storage tanks to be reused. Before the nutrient solution is reused, debris is filtered using sand filters and the solution is sterilized using ozone or ultra violet lamps (or other means). The electrical conductivity and pH of the solution is also monitored to determine any changes that need to be made to the nutrient solution or fertilization regime. If the electrical conductivity is too high, the nutrient solution may be diluted with fresh water. The crop might also be irrigated with fresh water for several cycles if necessary to lower the electrical conductivity of the solution in the rockwool slab (i.e. lower the build up of fertilizer salts). If the electrical conductivity is too low, additional mineral elements may be added from stock tanks of concentrated fertilizer solutions.

Root misting systems - This type of system uses mist nozzles that spray the roots of suspended plants with a nutrient solution on a frequent basis. The roots are contained within an opaque box or tube or allowed to hang underneath the bench to exclude light. The roots are generally sprayed for a few seconds every 2 or 3 minutes. Because the roots do not sit in a liquid solution, they have ready access to oxygen. This system is often used to produce hydroponic vegetable greens and strawberries.

The primary unique management issue in hydroponics is that of managing the nutrient solution. The nutrient solution must contain all essential elements in the correct proportion for that crop. The pH, electrical conductivities and microorganisms must be tightly controlled. Because plants will take up nutrients and excrete other compounds, the solution will change over time. Therefore, the solution must be periodically changed or adjusted to account for such changes.

There are numerous types of nutrient solutions used for different crops. However, an example of a nutrient solution is outlined in the table below.

Example of nutrient solution used in hydroponic production***
KNO3	50.55 g/L
KH2PO4	27.22 g/L
MgSO4-7H2O	49.30 g/L
NaCl	5.85 g/L
Ca(NO3)2-4H2O	118.08 g/L
Sequestrene 330 Fe*	5.00 g/L
Micronutrient concentrate**	100.00 ml/L of concentrate

* 330 Fe concentrate at 50 g/L
** Micronutrient concentrate contains 0.078 g CuSO5-H2O, 0.219 g ZnSO4-7H2O, 1.538 g MnSO-H2O, 2.850 g H3BO3 and 0.020 g MoO3 per L.
*** This serves as the concentrate stock and is diluted at a 1:200 ratio.

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