One of the keys to a successful hydroponic garden is having a solid nutrient management plan. Plants grown hydroponically are much more sensitive to the nutrients you give them simply because they don’t have the soil to buffer any mistakes you may make. You are taking on nature’s role and are responsible for putting together the correct nutrients, in the right amounts, with the right pH, at the right time. You must also manage your water’s quality more strictly and be aware of any chemical incompatibilities that could hurt your plants.
Essential Hydroponic Nutrients
First things first, you must understand the importance of the nutrients you are feeding your plants. They can be broken down into three categories: primary, secondary, and micronutrients. In hydroponics, instead of receiving these nutrients from the soil, plants get these nutrients from the nutrient solution they grow in.
Primary nutrients include nitrogen, phosphorous, and potassium. These are needed in large quantities for plants to thrive. The secondary nutrients are calcium, magnesium, and sulfur. They are needed in smaller quantities than the primary nutrients, but in much larger amounts than the micronutrients. Micronutrients include iron, molybdenum, boron, copper, manganese, sodium, zinc, nickel, chlorine, cobalt, aluminum, silicon, vanadium, and selenium. Plants also need carbon, hydrogen, and oxygen, however, they receive these from the air and water.
If a plant takes in too much or too little of these nutrients, they will suffer from deficiencies or toxicities. Plants will show signs to help you figure out what they need. For example, a nitrogen deficiency will cause yellowing of the older leaves and slowed growth. Too much nitrogen can cause extremely green leaves, excessive vegetative growth, and reduced fruiting.
(Check out Hydroponics: Pros and Cons of Hydroponic Gardening.)
Understanding Hydroponic Ratios and Formulas
When you go into a hydroponic shop, it is easy to be overwhelmed by the hundreds of fertilizers lining the shelves. There are many different formulas and ratios to choose from.
There will be three numbers on the front of the fertilizer bottles. This is the NPK, which stands for nitrogen, phosphorous, and potassium, respectively. These numbers represent the percentage of each nutrient in the bottle. A 7-9-5 fertilizer will have seven percent nitrogen, nine percent phosphorous, and five percent potassium in that bottle. The back label will tell you the percentage of other nutrients in the fertilizer.
These ratios will vary a little by brand, what kind of formula it is, and what stage of growth it is meant for. Fertilizers formulated for the growth stage of plant development will have more nitrogen, while the bloom stage formulas will have less nitrogen and more phosphorous.
You will also see one-part, two-part, and three-part formulas. One-part formulas have one bottle for the growth stage and one bottle for the bloom stage of growth. While they contain all the nutrients needed, they likely have smaller amounts of calcium to keep the formula stable and will need to be supplemented. One-part formulas are ideal for beginners who don’t yet understand how to adjust recipes for their plant’s specific needs.
Two-part formulas have an A and B formula for growth, and an A and B formula for bloom. By separating some of the chemicals, the manufacturer has created a more stable and complete formula that can be mixed at different rates throughout the growth cycle. Specifically, higher amounts of calcium and phosphorous can be included in the base nutrients by separating them instead of putting them together in one bottle.
Three-part formulas include a separate grow, bloom, and micro formula that will be mixed together at different ratios throughout a plant’s life. This allows for much more flexibility so that the grower can tailor nutrient recipes to their plant’s specific needs and fix deficiencies more quickly.
One of the most important things to consider in a hydroponic nutrient management program is your water. It is essential to test your water before designing your program. You will want to test for alkalinity, electrical conductivity (EC), and contaminants.
Alkalinity is measured from near zero to more than 300 parts per million (PPM). If your water’s alkalinity is high, then the pH will tend to rise in your nutrient solution. Ideally, you want your alkalinity as close to zero as possible, as it is in reverse osmosis treated water. It is important to note that alkalinity is not the same as pH. While pH can be measured at a certain point in time, alkalinity is a measure of your water’s longer-lasting pH effect. Knowing your water’s alkalinity can help you choose the proper fertilizer program. For example, you may want to use a fertilizer with greater amounts of acidic nitrogen to counter the pH rise.
Your water's EC can be a rough measurement of your water’s purity. Electrical conductivity measures the total dissolved salts in your water. If your EC is high, then you will want to use a drain-to-waste hydroponic system. Electrical conductivity should be low, ideally less than 0.25 mS/cm for closed systems. You can also filter your water using reverse osmosis if you would like to use a closed system but have a high EC.
Your lab analysis should tell you what other elements or contaminants are already present in your water. This is helpful when designing your fertilizer program. For example, if your water already has calcium or magnesium then you will not have to add as much to your nutrient solution. If your water has high levels of sodium or chloride, you know that you may need to purify your water or flush your soil more often to prevent salt build-up.
Monitoring and maintaining a proper pH is crucial in a hydroponics nutrient management system. The solution’s pH measures how acidic or basic it is. The scale ranges from zero to 14, with 7.0 being neutral. Different nutrients are available at different pH levels. In general, nutrient solutions for hydroponics should have a pH between 5.0 and 6.0, which will create a pH in the root environment of between 6.0 and 6.5. In this range, more nutrients are readily available to plants.
You can measure pH with either pH strips or a pH meter. If you use a meter, be sure to calibrate it on a regular basis, about once every week or two.
Depending on your hydroponic system and your plant’s stage of growth, you may need to adjust your pH often. A drain-to-waste system does not reuse water, so you will adjust your pH when you mix it initially. However, recirculating systems will need more frequent adjustments. As roots respire, the pH will decrease as carbon dioxide reacts with water to create carbonic acid. This will happen much more towards harvest when roots are their largest.
To adjust your pH, you will add acids and bases to your nutrient solution. Add a little at a time and measure with your meter as you go. Common acids used to decrease pH include sulfuric acid, phosphoric acid, citric acid, and nitric acid. To increase your pH, potassium hydroxide and sodium hydroxide are commonly used. It is important to consider which acids and bases you use to adjust your pH, as they will add nutrients to your solution. For example, nitric acid will add nitrogen and phosphoric acid will add phosphorous.
When mixing your nutrient solution, you have the option of creating your own program from scratch or following a pre-designed program from a fertilizer company. Commercial operations usually design their program from scratch so that they can adjust their solution as needed. To save money, many commercial operations use recirculating systems and lab-test their nutrient solution regularly so they can add only the elements that have been depleted. For the hobby grower, however, it is much easier to follow a feeding plan that has been designed by a fertilizer company. Most companies have feeding schedules available to go with their products.
Although you can simply follow the measurements on the bottle, your solution will be more precise if you mix it using a PPM or EC meter. Parts per million and EC both measure the total dissolved salts in your solution. While this measurement will not tell you exactly how much nitrogen, phosphorus, or potassium is in your solution, it will give you an idea of the overall nutrient concentration. You can use this number to add nutrients to a recirculating system in between reservoir changes. Most feeding schedules will tell you what the PPM or EC should be at each week’s stage of growth.
If you are a hydro grower, follow these principles to create an effective nutrient management plan for a successful harvest.
(For more on plant nutrients, check out The Essential Plant Nutrients.)