Water Quality

Water Quality : Its Effect on Greenhouse Nutritional Management ©1986


Ray DeBruhl, Agronomist

Transplant Systems


Establishing and maintaining proper fertility management programs in a greenhouse is essential to successful growing of any crop. Most assume that fertility management begins with the soil media and the applied fertilizer. Actually, fertility management begins with a correct analysis of the water quality.

It is necessary to understand the relationship between water quality and plant nutrition to establish a successful fertility program. When formulating a fertility program for your crop, water quality should be given top priority.

Water can actually contribute to the nutrient supply in the greenhouse. Therefore, it is imperative that you know its nutrient or chemical composition. It is this chemical composition, especially the identity and concentration of soluble ions which has a dramatic effect on the subsequent plant growth.

There are several water quality factors that should be considered when establishing your nutritional program.

Your water alkalinity will have the most dramatic impact on nutritional management. Water alkalinity is so important because it has a tremendous effect on the PH of the soil media. And like the PH of the soil in the field, the PH of the soil media in the greenhouse influences the availability of several plant nutrients.

The alkalinity level of the water, not its PH, can cause the PH of the soil media to go up or down, therefore affecting nutrient availability. By definition alkalinity is the water’s ability or capacity to neutralize acid. It is a total measurement of the bicarbonates and carbonates in the water. Alkalinity is not the same as water PH. The two are actually different. Therefore you cannot test water alkalinity with a PH kit.

Limestone, which is composed of calcium carbonate, is added to the field to raise the PH of the soil. With this in mind, it is easy to understand how water with a high alkalinity level (high carbonate content) can cause the PH of the soil media to rise.

Water with a high alkalinity increases the PH of the growing medium and reduces the availability of several minor elements such as manganese, iron, zinc, boron, and copper. You can control high alkalinity water by injecting certain mineral acids such as phosphoric, sulfuric, or nitric acid into the water or by using certain fertilizers. If treatment is done with fertilizer, then it is advisable to use one that would have an acid reaction in the soil media such as 20-10-20 or 21-5-20.

Low alkalinity can also have an adverse effect on the soil media. It can cause the PH of the soil media to decrease. The problem with low PH levels is that it can cause nutrient toxicities. When alkalinity levels are low, a fertilizer like 15-0-15 or 15-5-15 can be used which is less acidifying.

A soil media PH reading of 5.8 to 6.3 provide for the optimum level of nutrient uptake. When the media PH goes above 7, the uptake of several nutrients is restricted.

Greenhouse water that is high in alkalinity will neutralize the acids in the soil media and as a result, the PH of the soil media will rise too high causing severe nutrient deficiency. Water that is low in alkalinity can cause the soil media to decrease over time because there is not enough bicarbonates to neutralize the acid reactions to the fertilizer being used.

However, low alkalinity water seems to have less effect on media PH as compared to high alkalinity water. Therefore the greatest concern is with high alkalinity levels.

There are other important factors of water quality to consider such as other dissolved nutrients in the water like calcium, magnesium, and sodium as well as the total dissolved salts.

The most important however, is to have your water analyzed to determine the alkalinity level. The time invested in obtaining a complete water analysis will give you tremendous returns in plant quality.