Words: Lawrence Brook, General Hydroponics

Greetings Urban Gardeners.

Let’s take a look at how we can get the most out of our hydroponic nutrients, otherwise known as “nutrient management.” Now, to the skilled grower, nutrient management represents an opportunity to enhance plant growth, enjoy bigger yields and achieve higher overall crop quality. However, to the novice it can represent a difficult challenge or even a complete mystery! The difference is in knowledge, understanding, environment and equipment.

Here are six questions to test your nutrient knowledge. Do you already know the answers?

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Q1) What temperature is your nutrient solution? What is the range during a day and during a season?

Your nutrient solution should be maintained at around 68 °F (20 °C) for the best combination of oxygen content and uptake by the roots.

Q2) What is the “dissolved solids” content of the water you use to mix your nutrient and does this content vary greatly from season to season? Does your water supplier provide you with good water from one reservoir at one time of the year and bad water from a different reservoir at another?

Test your water with an EC meter before adding anything to it.

Q3) Are there any components (such as high levels of calcium or magnesium) in your water that could affect the availability of other nutrient  elements? Have you considered the presence of sodium chloride from sea-water contaminating your water supply?

Ask your water supplier to provide you with an analysis of your water. Some grow stores also offer this service.

Q4) What is the “EC” or strength of your nutrient? Do you mix special nutrient blends for different kinds of plants and for each stage of the crop’s lifecycle, or in response to different environmental conditions like high temperatures and low humidity?

If you are unable to prevent temperatures in your indoor garden from rising too high, you can decrease the stress levels for your plants by decreasing the strength of your nutrient solution.

Q5) Does the pH of your nutrient stay within a reasonable range, or does it drift up and down significantly? How quickly?

It’s quite normal for the pH of your nutrient solution to rise (say from 5.8 to 6.5) over the course of a few days – but greater changes could indicate the presence of pathogens in your nutrient solution from contamination or from sick plants that may spread disease to the rest of your crop.

Q6) Do you change your nutrient often enough to prevent imbalances from salt accumulation or deficiencies from nutrient exhaustion?

It’s really important to change your nutrient solution regularly because it helps to minimize the wastes your plants discard into the nutrient. Did you know that, as plants transpire and nutrient levels drop in your reservoir, the EC or strength of the nutrient can rise to dangerously high levels?

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Ok, so how did you get on? Were those questions easy for you or do you already feel a little out of your depth?  In either case, don’t worry, we’re going to look at all these issues in more detail.

What’s In Your Water?

Water quality is a crucial issue for all gardeners. Success comes easier with soft water. Just add the right combinations of nutrients to the water and you’re off to a great start. However, if you have very hard water, or water contaminated with sodium, sulfide, or any number of heavy metals, your first step may be to filter your water using “reverse osmosis.”

The best way to find out what’s in your water is to obtain a seasonal water analysis through a lab. If you’re on a municipal water system, call your water district and request a copy of their most recent analysis. Keep in mind that a good analysis at one time of the year does not mean that the water quality will remain good throughout the year. During dry seasons water suppliers often switch to a different reservoir with different water quality.

Another approach – highly recommended – is to check your water regularly yourself with a dissolved solids meter, also called an electrical conductivity (EC) or parts per million (ppm) meter. These instruments are one of the most important tools for a grower to use regularly. By measuring the EC (ppm) of your source water routinely before adding nutrients, you will be able to tell if your water supply is consistent, or changing.

How Does a Conductivity Meter Work?

All dissolved solids instruments work in essentially the same way: they measure the electrical conductivity of the water. It is the dissolved salts in most water that allows it to conduct electricity. Pure water is a poor conductor since there are none of the conductive salts found in impure water. Purified water will show no, or very low, salt content (conductivity) when tested with a dissolved solids meter.

It is not uncommon to find high levels of salts in well water or municipal water supplies. Calcium and magnesium carbonates are among the most common ingredients in tap water and in well water. In fact, water “hardness” is defined as a measure of the water’s content of calcium and magnesium carbonates. In some regions sulfates can also reach high levels in water supplies.

Since calcium and magnesium are important plant nutrients, water with reasonable levels of these elements can be just fine for hydroponic cultivation. However, even a good thing can become a problem if the levels are too high. Generally, a calcium content of more than 200 PPM, or 75 PPM for magnesium, is on the verge of excessive for most hydroponic applications. An excess can cause other important elements in the nutrient solution to “lock-out” and become unavailable. For example, excess calcium can bond with phosphorus to make calcium phosphate, which is not very soluble and therefore not available to the crop. If magnesium bonds with phosphate it becomes completely insoluble and unavailable to the plant. The key to success is to start with decent water and add the right combination of nutrients.

Nutrients – A Question of Quality

What makes a quality nutrient? Here are some questions to ask. Some answers may be harder to find than others!

• How pure are the ingredients?
• How consistent is the product?
• Is it properly labeled with ingredients and NPK?
• How reputable is the manufacturer?
• What ingredients are in there and how are they combined?
• Does the product contain contaminants from poor quality control?
• If it is a liquid, did the manufacturer use R/O or purified water in the blend?
• Does the manufacturer have a quality control (QC) program like safety sealed bottles so you know it is pure and has not been tampered with?
• Are there any hidden or ‘mystery’ ingredients?

Too Hot, Too Cold

The temperature of your nutrient solution is another important factor. If your solution is too cold, seeds won’t germinate, cuttings will not root and plants will grow slowly – or stop growing and die. If it’s too hot, the same seeds won’t germinate, cuttings won’t root and plants will die from oxygen deficiency, or they will succumb to pathogens that thrive in higher temperatures, or simply bite the bullet due to temperature stress. Most plants prefer a root zone temperature range between 65 and 72 °F (18 to 22 °C), cooler for winter crops, warmer for tropical crops. When adding water to your reservoir it is a good idea to allow it to come to the same temperature as the water in the root zone before starting circulation pumps.

Remember, plant roots have evolved in a soil environment where temperature changes occur slowly, tempered by the thermal mass of the earth. Rapid temperature changes in the root zone can cause shock and invite root disease.

Water pH

A subject that is often discussed but rarely understood by many growers is nutrient pH. Plants normally grow best within a pH range from 5.5 to 6.8. Generally we worry about pH and its affect on nutrient availability. For example, if pH is too high, iron may become unavailable. Even though your nutrient solution may contain an ideal amount of iron, your plants may not be able to absorb it, resulting in an iron deficiency – the plant’s leaves will yellow and weaken.

On the other hand, hydroponic plant foods usually contain special “chelates” that are designed to assure iron availability, even at higher pH ranges. The result is that your crop will grow reasonably well even at higher pH levels. Nonetheless, high pH can damage plants in other ways. The cause of a high solution pH can be fairly complex. Most city water supplies contain calcium carbonate to raise the pH of the water and prevent pipes from corroding. As a consequence you are starting with water that has an abnormal pH: typically 7.5 to 8.0 for city water.

One method of dealing with the high pH of city water is to mix in fresh nutrient, let it stand for a while to stabilize, then test and adjust the pH. With city water supplies you will often have to add a solution of pH down (usually phosphoric acid) to lower the pH to the suitable range for most plants.

As your plants grow it is a good idea to occasionally test the pH and adjust it if needed. You can safely allow pH to drift between 5.8 and 6.8 without adjustment. In fact, constantly adjusting the pH in your system to maintain a perfect pH of 6.2 can do damage. It is common for pH to drift up for a while then down and up again. This change is an indication that your plants are absorbing nutrient properly. Adjust pH only if it wanders too far, below 5.5 or above 7.0 for example.  A pH below 5.5 or above 7.0 can spell trouble but don’t overreact. An apparently sudden and dramatic shift in pH can be the result of a malfunctioning pH meter. If in doubt, double-check with a reagent (color match) pH kit before adjusting your solution. Also remember that pH meters are temperature dependent. Read and follow all of the instructions that came with your meter or test kit. I know of at least one case where a grower lost a crop due to a defective pH meter after over-correcting nutrient pH.  Use a meter as well as a color-match test kit to double-check your pH.

Big pH swings can also indicate strong microbial life in the nutrient solution and root zone.  Microbes can change the pH to meet their needs.  The best way to manage this is to introduce beneficial microbes into your nutrient solution and the plant’s root zone. These microbes are nature’s little plant helpers.  The beneficial fungi typically help plants grow bigger, stronger and more effective root systems. The beneficial bacteria typically populate the root zone and protect the roots from bad microbes and environmental stress factors.

Time For a Change?

How often should you change your nutrient solution? That’s one of the most common questions asked and one of the most difficult to answer. Many people have tried to come up with a simple, easy-to-follow rule … once a week, every two weeks – but they’re all wrong! They’re wrong because there is no simple answer. It all depends on the plant variety, the number and size of your plants, their stage of growth, the capacity of the reservoir, the kind and quality of nutrient you use, water quality, environmental conditions such as temperature and humidity, and the type of hydroponic system used; there are so many factors that the answer is not obvious. Instead of a simple answer, what we need is a procedure that takes many of these variables into account and is responsive to changing conditions.

It sounds complicated, but it’s actually quite simple. All it takes is a little monitoring and some basic record keeping.

• Start with a fresh reservoir of nutrient and make note of the date, pH, and EC or PPM of the solution.
• As you run the system, the level will drop in the reservoir. Note the EC/PPM level then top-up the reservoir with fresh water. Test again for nutrient concentration. If the nutrient strength has dropped significantly, add a bit of nutrient to bring it back up to specs. Be sure to record how much water you added to top-up the reservoir. Repeat the procedure every time you top up the system, carefully recording the amount of water added.
• When the total amount of water added equals the capacity of your reservoir it is time to drain and replace all of the nutrient solution.

For example, imagine a hydroponic system in a cool greenhouse in the spring with 24 strawberry plants and a nutrient capacity of 20 gallons. Typically, such a system might require about five gallons of added water each week. After four weeks the plants will have transpired 20 gallons – the capacity of the reservoir. You need to completely drain and replace the nutrient every four weeks in this example.

Now imagine five tomato plants with a 20 gallon reservoir. They are fully mature, growing and producing quickly. It is hot and dry. Each plant pulls in a gallon a day so you are adding five gallons a day. After four days it is time to change the nutrient. It may be best to use a larger reservoir since the EC flux in this case will be pretty high.

Fascinated? Read on in the second installment of this feature in Issue 5 …