Why Is pH So Important ?

The nutrients, the water, the trace elements, the growing medium, and the gases at the roots have different electrical charges all engaged in a constant battle. All these surround the root system and are exchanging positive and negative charges, and in this way nutrients are absorbed into the roots to feed the plant.

The pH is the most important factor for water culture gardeners to monitor because the pH of a nutrient solution reads the outcome of an electrical battle that is fought between the roots and everything surrounding them. When growing in pots, the run-off is the place to measure the pH level.

Why should the pH be monitored? The protein and enzyme molecules in a plant are structured in very specific shapes in order to catalyze a chemical reaction to build the plant cells. To do so, they need reactant molecules to fit precisely into their gaps like keys to locks; this means that all the negative and positive charges have to line up exactly. In fact, plants often change their own cellular pH to stop or speed up a certain enzyme reaction.

Acids: range from pH 0 (strongest) to pH less than 7.0 ( weakest or neutral)
Bases: range from pH greater than 7.0 to ph 14.0 ( strongest )

Remember that low or acidic pH means more hydrogen (+) charges, and high or basic pH means more hydroxide (-) charges. To have the correct balance of positive and negative charges on the key as on the lock so that the roots can absorb elements in the nutrient solution more readily, the pH internally in the plant must match closely the pH in its nutrient solution. The pH of the liquid nutrients - sodium (Na+), phosphorus (p+), potassium (K+), and calcium (Ca+), plus all the other elements - will affect how well each nutrient passes through the root walls and nourishes the plant.

The electrical charges in soil particles are different from those in soilless mixes. For this reason, different pH levels for nutrient uptake are needed for each type of growing medium, depending upon each medium's CEC value.

In soilless mixes with their low CEC, the pH is absolutely critical because an
incorrect nutrient uptake due to pH imbalance is hard to identify, since it always masks itself as some other problem. Blocked out nutrients can take a week to become evident, and another week to remedy provided the correction is appropriate. If the guess is wrong, two weeks would have passed since the problem first started. This can do damage to the plant.

Ever wonder why certain plants need a different pH - some need as low as pH 4.5, others might need pH 6.5?

For the nutrient uptake for regular soils, notice that at a low pH of 4.5, iron and manganese are dominant and will go into a plant more easily than elements like nitrogen or phosphorus. Therefore, any plant that uses a lot of iron and m'anganese will do better at this low pH than at a higher pH.

On the other hand, a plant that likes a general mix of all nutrients will do best at pH 6.5. A plant that needs a lot of molybdenum would need a pH of over 7.0 to do best. Thus, the nutrients that each plant prefers decide what pH level the plant would need to get the most of what it likes.

Three main things which change the pH that the plant feels are:
1. The pH of the water you start with;
2. The growing medium (e.g. Rockwool is over pH 7.0, peat moss is
- below pH 6.0, hardened expanded clay is pH 7.59); and
3. The nutrient, since it can be mixed to form many combinations of
elements that behave in different ways and which give up their
elements to the plant at different pH levels. It is only after going through these three stages that a nutrient solution is able to readily give the plant the elements it favours.

Compare the chart How pH Affects Plant Nutrient Uptake with the one labelled Experimental Availability of Nutrients on page 63. See how for most of you r favou rite plants, the organic soil nutrient solutions should be between pH 6.0-7.0 for all the nutrients to be able to go into the roots more freely. Notice that these plants in soilless nutrient solutions could use from pH 4.5-6.5 to be able to pass all the nutrients to the roots more readily. If the pH goes too high or too low, the plants will not do well and will show confusing signs or nutrient deficiencies.

Water

In a finely tuned garden that is looking for super production, the water quality is very important because plant nutrients are mixed to parts per million, and it does not take too many other parts per million to change the nutrient for worse. Because the average tap water contains some metals and trace elements that can actively slow plant growth, it is wise to get the water analyzed at a lab before using it to make up a nutrient solution for your garden.

City water is often not pure and contains chlorine, but the chlorine level
in most places is not high enough to upset plants. If you are concerned, let the water stand for a day or so before using it. You may use hot water and let it stand until it is cool, but keep in mind that hot or boiled water has dissipated its chlorine and, unfortunately, its oxygen. Note that:
fresh water will hold at 32°F (aOC) : 14.6 PPM (14.6 mg/L) dissolved oxygen
fresh water will hold at 78.8°F (26°C) : 8.1 PPM ( 8.1 mg/L) dissolved oxygen
In reality, 10 PPM (10 mglL) is only 0.00001 % in content. The air with 21 % oxygen is in fact 2,100,000 mglL, so the oxygen content in the nutrient at best can only be 0.0010/0 of that in the air.

To make a sugar molecule for energy, a plant needs to split a hydrogen (H) atom from the water (H20) molecule and a carbon (c) atom from the carbon dioxide (C02) molecule in the air. Under advanced nutrient conditions, a plant is processing a large amount of water for the hydrogen. At these higher limits, a 24-inch plant can be transpiring 1 gallon (3.78 litres) of water to stay pumped up because the leaf stomata are wide open for taking in all the CO2 that is needed. At the first sign of water diminution, the leaf stomata will start to close, and this starts to limit the CO2, which in turn slows down growth. The evaporation of water by the leaves keeps the plant's temperature down. If water is not freely available, the plant will shut down and overheat, causing irreversible damage.

How a Plant Uses Water

The plant needs water to hold it up. The leaves suck water by evaporation I
and the roots push water with capillary action and osmotic pressure. A plant is just a big balloon pumped up with water. As soon as the water stops, the plant wilts and dies.

The plant also needs water to keep itself cool. In higher temperatures, leaf evaporation keeps the leaves cool. Stop the water, and the leaves overheat and destroy their enzymes. The plant needs water to carry the nutrients up the plant and to transport the sugars down the plant. Leaf transpiration circulates the plant's internal water supply