How Plants Grow in Water

Questions We Get

Many people ask us, “How do you feed the plants grown in water?” “Do you use fish poop?” “What kind of water do you use?”  Here are some answers.


Plants require lots of light.  On a scale of 1-10, plants need somewhere between 8-10, whereas we only need 1-2 to see. One of the biggest problems with growing plants is making sure they have enough light over a 24 hour period.

In addition to the intensity and duration of the light, the wavelength of the light is important too.  Blue-shifted light helps the plant to produce leaves, which is great for lettuces and herbs.  Red-shifted light helps the plant to produce fruit or go to seed. At e3garden, we use special lights to help our plants either to produce leaves or fruits.


Water quality is the most important aspect to a hydroponic system.  Without good quality water, your plants will languish or die.  We use drinking water for our systems. Yes, water out of the tap. By law, your drinking must satisfy certain safety requirements.  You can request a copy of your water quality from your utility company.  If you have your own well, you should have your water tested. Penn State offers tests for about $30. The biggest problem with well water is mineral content.  Too much of a particular mineral can prevent the plant from processing nutrients properly and it can die.

Some people wonder if you can use pond water or water from rain barrels.  The short answer is that you can, but there are issues.  Water collected in rain barrels may be coming from all sorts of “dirty” surfaces; i.e., roofs that contain asphalt tiles, tar, oil, plastic, wood fibers, etc. When water from rain barrels is used to water plants, those impurities are filtered out by the soil, at least partially. However, without soil (or a filter), these impurities go into the plants themselves.  Pond water has similar issues compounded by the fact that it may contain organisms and bacteria that can make you sick.

For these reasons, we only use drinking water from the tap and do not recommend the use of pond water or rain water in hydroponic systems. Remember too that hydroponic systems use only 10% of the water required by open field farming, which is much more sustainable.


The PH of water or soil is graded from acidic (less than 7) to alkaline (greater than 7) on a scale of 1-14.  Most plants prefer a PH of between 5.5-6.5.  This is even more important in hydroponic systems than in soil since the plants are directly in the water all of the time. We use a food grade buffered water conditioner (e.g.,  citric acid) to maintain the proper PH for our plants.  While you can use lemon juice or vinegar to accomplish the same, you may get wild gyrations in PH, which stresses the plants. Buffered solutions help to avoid this problem.


Plants require several pure elements and inorganic compounds to grow, which are divided into Macronutrients and Micronutrients.  Here are the amounts present in dry plant tissue:


  • Hydrogen (6%)
  • Carbon  (45%)
  • Oxygen  (45%)
  • Nitrogen  (1.5%)
  • Calcium (less than 1%)
  • Magnesium (less than 1%)
  • Phosphorus (less than 1%)
  • Sulfur (less than 1%)

Micronutrients (less than 1%)

  • Chlorine
  • Iron
  • Maganese
  • Boron
  • Zinc
  • Copper
  • Molybdenum

[Source: H. Resh, “Hydroponic Food Production”, CRC Press, Boca Raton, FL, 2013, p. 10]

Plants do not care where these compounds come from or how they are produced; i.e., naturally or otherwise.  Think of them as little factories that combine these ingredients in the presence of light to produce plant tissue. These elements can be transported to the plants through the soil or water. If insufficient amounts are available, fertilizers are added.  There are huge differences between adding fertilizers to soil vs. water.

Natural or organic fertilizers are made from decaying vegetation, bone meal, excrement, etc. They have have certain pros and cons.

Pros of Organic Fertilizers:

  • They improve the structure of the soil and increase its ability to hold water and nutrients.
  • They are slow-release so it’s very difficult to over fertilize your plants.
  • There’s little to no risk of toxic buildups of chemicals and salts.
  • Organic fertilizers are renewable, biodegradable, sustainable, and environmentally friendly.
  • You can make your own

Cons of Organic Fertilizers:

  • Microorganisms are required to break down and release nutrients into the soil.
  • They need warmth and moisture to do their job so the effectiveness of organic fertilizer is limited seasonally.
  • Organic fertilizers break down very slowly so they may not release nutrients as soon as you need them.
  • Nutrient ratios are often unknown and the overall percentage is often lower than synthetic fertilizers.

We are strong proponents of organic fertilizers for soil based plant growth.  However, these properties do not work for plants grown hydroponically.  Here’s why.

Meeting the Nutrient Requirements of Plants Grown in Water

Plants grown in water require nutrients that dissolve in water quickly and under most conditions.  This may seem obvious but it is sometimes overlooked.  Organic fertilizers are not soluble and therefore do not satisfy this requirement.  It is also very hard to know how much nutrient is being administered with natural fertilizers.

Hydroponic systems need soluble fertilizers.  These come in the form of salts that dissolve in the water and quickly make their way to the plants. Synthetic fertilizers are carefully formulated in the exact proportion and ratios needed to support plant growth, esp. in water.  From the plants point of view, the source of nitrogen or calcium is not important; either works equally well.

We therefore use food-grade synthetic fertilizers from reputable companies to nourish our plants.  The many disadvantages associated with synthetics pertain to its effects on soil and groundwater. Since our plants grow directly in water, these effects are not relevant.  Almost all of of the nutrients we dissolve in the water is taken up by the plants (and remember, we only use 10% of the water required by open field farmers) and do not impact soils or other ecosystems.

We believe our production choices are safe, sustainable and effective at producing great tasting and nutritious produce.