By: Larry Brooks
Owner: General Hydroponics
To the skilled hydroponic grower, nutrient management represents an opportunity to enhance plant growth. To the novice, it represents a challenge to be dealt with. The difference is in knowledge, understanding and equipment.
Consider the following questions to test your nutrient IQ:
What temperature is your nutrient solution, what is the range during a day and during a season?
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?
Are there any components in your water that could affect the availability of nurtrients to your crop?
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 life-cycle?
Does the pH of your nutrient stay within a reasonable range?
Are there any pathogens in your nutrient from a contaminated water supply or from sick plants that may spread disease to the rest of your crop?
Do you change your nutrient often enough to prevent excesses from salt accumulation or deficiencies from nutrient exhaustion?
Did you know that an important reason to change your nutrient solution is to eliminate the wastes your plants discard into the nutrient? Did you know that as plants transpire, moisture and nutrient levels drop in your reservoir and the EC or strength of the nutrient can rise to dangerous levels?
These are only a few basic question that may help you better realize what you already know, and what you may need to learn to achieve outstanding crops every time. This discussion is especially for the advanced grower who wants to achieve the highest yields and is seriously interested in being at the leading edge of plant growing technology. Hobby growers generally don’t have to worry about all of these questions, but don’t stop reading just yet. When problems arise and a crop isn’t growing as well as it should. the problem can often be traced to nutrient management. Once you know what can go wrong, it’s easier to recognize a problem when it happens.
The root environment is what separates hydroponics from soil cultivation. In soil, plants await rainfall or irrigation, and their roots search out essential nutrients. With good, fertile soil and abundant water plants thrive.
In hydroponics, the plant roots we constantly provided with water, oxygen and nutrients–no searching for available nutrients or waiting for the next rain. The challenge for the grower is to keep up with the plants’ needs and to avoid damaging plants with excesses or deficiencies of minerals, extremes in pH and temperature, or a lack of oxygen. A few simple tools and techniques can make the difference between success and failure.
What’s In Your Water?
The first question to consider is water quality. With good, soft water it’s easy to succeed. Just add the right combinations of nutrients to the water and you’re off and growing. If you have very hard water, or water contaminated with sodium, sulfide, or any number of heavy metals, you may have to filter your water using “reverse osmosis.”
So, what’s in your water anyway? The most complete answer comes from having an analysis of your water done by a lab. If you’re on a municipal water system, call your water district and request a copy of their most recent analysis.
Another approach – highly recommended – is to check your water regularly with a dissolved solids meter, also called an electrical conductivity (EQ) or parts per million (PPM) meter. These instruments are one of the most important tools for a -grower to have and use regularly.
All of these 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, or sulfates.
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, are 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 phosphorous to make calcium phosphate, which is not very soluble and therefore not available to the crop. The key is to start with decent water and add the right combination of nutrients.
Too Hot, Too Cold
Water temperature 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 simply from temperature stress. Most plants prefer a root zone temperature range of between 65 degrees (18 C) and 80 degrees (27 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 reservoir.
Remember, plant roots have evolved in a soil environment, where temperature changes occur slowly, tempered by the thermal mass of the earth.
Plants do not like rapid temperature changes, especially in the root zone!
A subject that is often discussed but rarely understood by many growers is nutrient pH. Generally, we worry about pH and its affect on nutrient availability. For example, if pH is too high, iron may become unavailable. Eventhough your nutrient solution may have an ideal iron content, 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, advanced hydroponic plant foods contain special “chelates” that are designed to assure iron availability 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 8.0 for city water.
The beg way to deal with this is to mix fresh nutrient with your water, let stand for a while to stabilize, then test and adjust the pH. With city water supplies you will often have to add a bit of pH down (usually phosphoric acid) to lower the pH to the range for most plants, between 5.8 and 6.2.
As the 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 5.5 and 7.0 without adjustment. in fact, constantly dumping chemicals into your system to maintain a perfect pH of 5.8 to 6.0 can do a lot of 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.
A pH below 5.5 or above 7.0 can mean 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 all pH measuring methods are temperature dependent. Read and follow all of the instructions that came with your meter or test kit.
Another cause of unstable pH is poor quality growing media. Industrial grade rockwool and gravel are notorious for having very high pH levels that cause your nutrient pH to rise, often to constantly rise, often to dangerous levels.
A simple way to test a new growing medium is to put some of the medium – rockwool, gravel, soil – into a clean cup, then immerse (soak) the sample. in distilled or “deionized” (chemically pure) water. Let this sit for a little while and then test the pH of the water, note the pH and continue to let the sample sit. Test the pH occasionally for about a week until it has stabilized. Has the pH risen to 8.0, perhaps 9.0? Construction grade gravel can go as high as 10.0 – torture to roots. death to plants!
Never underestimate growing media as sources of pH problems. This is one of the primary reasons that “waterculture” hydroponic methods are gaining popularity over “media-based” hydroponics. The less medium you use, the fewer problems you will encounter with pH instability and salt accumulation. Plus, the water-culture systems require less water and nutrient than media-based methods, due to higher efficiency and reduced evaporation.
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 species, the number and size of your plants. 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. 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, spring greenhouse with 24 strawberry plants and a nutrient capacity of 20 gallons. Typically, such a system would require about 5 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.
The problem of pathogens or disease in the nutrient solution can be a serious one. It is not uncommon for this to be a regional and seasonal problem. For example, in Holland during the winter, fungi thrive in the cool and damp environment: the air is full of spores. All kinds of soil-borne diseases become endemic in the Dutch winter and growers have to work hard to avoid infestations. One of the reasons Dutch growers adopted hydroponics so readily was to avoid soilborne diseases.
Keep your growing area clean. Never allow soil to get into the nutrient stream. If soil is accidentally kicked into the reservoir, the entire crop can be at risk. Some growers will place a sponge-mat soaked with disinfectant at the doorway of the greenhouse. Everyone who enters must clean their shoes on this mat before entering. This is an effective and practical way to prevent disease organisms from entering the greenhouse and endangering the crop.
If an infected plant is introduced into a hydroponic system, the disease can race through the entire crop. By the time a problem is noticed it may be way out of control. Plant diseases are beyond the scope of this article, but the best advice is to avoid problems by working clean, planting only healthy disease free plants, and closely monitoring the crop.
If you see evidence of disease in a single plant, remove and destroy it quickly before the disease spreads. Watch the crop closely and destroy any other plants that show signs of disease. It is better to lose a few sick plants than to risk an entire crop.
If you do encounter disease problem, it in a good idea to completely drain and renew your nutrient after removing the sick plants. If it is possible there is nothing better than to flush the system by running fresh water without nutrient for a day. Then drain and refill with fresh nutrient. Flushing between every three or four nutrient changes can help maintain cleanliness in the root zone and in the hydroponic system. Periodic flushing is especially helpful for gravel systems to remove salt accumulation in the medium.
To the Limit
To some hobby growers, especially those who come to hydroponics from the ” U-plant-em-and-pray” school of outdoor gardening, the techniques described above might seem too difficult and time-consurning. Remember, hydroponics offers great control over the health and quality of plants todays grower with the interest and the skill to exercise that control. That’s what this article is all about – pushing it to the limits. Remember, too, that it is possible to produce a hydroponic garden that will out-perform any soil garden by simply following the manufacture’s instructions on system operation and nutrient changes, and paying attention to the condition of your plants. But even the most casual grower can benefit from an understanding of a few basic concepts.
Quality water is a great advantage, poor water is a challenge. Use only the highest quality plant food, designed specifically for hydroponics. Low grade plant foods and common fertilizers offer your plants poor and incomplete nutrition, cause pH drift, and sometimes contain impurities that can become toxic to hydroponic plants, Only high-quality plant food can grow superior plants. Healthy plants grow faster, generate higher yields and are resistant to disease and insect infestation. When you mix fresh nutrient always measure carefully.
Keep notes an your observations of EC drift, pH drift, total water usage, temperature range, and comments on crop health and progress. Keep an eye on pH, and an especially close watch on nutrient strengh (PPM. EC, dissolved solids). Look out for diseases and remove and destroy sick plants immediately.
Control your nutrient temperature – use high quality aquarium heaters to warm nutrient in the winter, look for “chillers” to cool your nutrient in the summer if high nutrient temperature becomes a problem. The aquaculture or fish farming people have developed excellent chillers. Fish don’t like water that’s too hot or too cold either.
Don’t be overwhelmed or intimidated. Plants can tolerate quite a lot of stress and still produce well. On the other hand, the grower who knows the questions. and how to find answers, is the one who will have consistently good crops. It is far easier to avoid problems through knowledge and proper technique than to fix them after they arise.