The Science Simplified: Fertilizing With Organic Nutrients
Understanding pH, TDS, and other keys to using natural plant foods in a hydroponic garden.
You don’t need a science degree to grow an indoor garden with organic nutrients, but you’ll get healthier, more productive plants if you learn about and pay attention to a few basic principles and helpful measurements. Even if you’ve grown using synthetic fertilizers before, you’ll find important differences with natural plant foods.
Organics differ. Let’s start with a simple explanation of what organic fertilizers are and how they work in comparison with synthetics. The ingredients in organics come from naturally derived sources, such as bird guano (that’s scientific lingo for poop) or the trimmings that are left over from fish processing. These materials are diluted and blended but otherwise are left close to their natural state. The nutrients they contain are in the form plants absorb most readily. Synthetic fertilizers are manufactured from chemicals, typically ammonium sulfate or ammonium nitrate. They contain vital nutrients, but like vitamin supplements for people, they are no substitute for the complete diet you get from eating minimally processed food.
Be microbe friendly. When you feed your hydroponic plants an organic fertilizer, you are enlisting a diverse, unseen food chain in your growing process, one that mimics the natural ecosystem. Microbes, including bacteria and tiny fungi, consume the fertilizer’s ingredients and excrete nutrients that the plants’ roots can take up and use. The plants and microbes have a codependent relationship, much like the microflora in your stomach that help you digest food. The healthier the microbe population in your hydroponic system, the more robust your plants will be. The microbes need a steady supply of oxygen to stay active and reproduce vigorously, so organic fertilizers are most effective in hydroponic systems that regularly pump air into the nutrient solution. (By the way, the salts in synthetic fertilizers dehydrate microbes.)
Use a gradual release. The microbes’ process of transforming organic matter (the raw ingredients of organic fertilizers) into nutrients is slow and steady. That means your plants are fed a continuous, even supply, not brief spikes of excessive levels of nutrients that lead to unsustainable growth, as is often the case with synthetics. Growing with organic nutrients takes a little patience, but your plants will reach the same size and productivity as they would with chemical plant foods.
Keep a pH balance. In a hydroponic system, your plants get the nutrients and water they need at the same time in a solution that you provide. The pH of the solution impacts how well the plants absorb the nutrients. Technically, pH stands for “potential for hydrogen,” and it is a measure of acidity versus alkalinity on a scale of 0 (highly acidic) to 14 (highly alkaline), with 7 being neutral. Each nutrient has a range of pH at which it is available for plants to absorb. For instance, when the pH is below 5.5, nitrogen is what experienced growers call “locked out.” Phosphorus, on the other hand, is less available to plants when the pH goes above 7.5. When the pH of your nutrient solution is too low or too high, your plants may suffer from nutrient deficiencies even when you are giving them a sufficient amount because the roots are not taking up what’s available to them. Because the most critical nutrients peak from 5.5 to 6.5, that’s the ideal range for feeding hydroponic plants.
You can test the pH of your nutrient solution with simple kits that include drops or coated strips of paper and a color chart, like many people do with swimming pools and aquariums. Since you will want to test the pH of every batch of nutrient solution you mix up, you might want to invest in an electronic meter, which can give you instant readings. A basic but effective model costs around $35.
If you find that you need to adjust the pH of your organic nutrient solution, steer clear of chemical treatments, such as hydrochloric acid or sodium hydroxide, which many commercial growers use. These products will kill the microbes that your plants depend on. Instead, you can work with all-natural white vinegar (acid) or baking soda (alkali) to get the solution into the healthy, slightly acidic range. Add only small amounts of these potent substances at a time, so you don’t push the pH too far in the opposite direction.
Monitor PPM, TDS, and EC. The concentration of essential nutrients in your fertilizer solution is an important measurement that can help you ensure that your plants get exactly what they need. When the concentration is too low, your plants will be undernourished. When it’s too high, you risk nutrient burn or simply wasting fertilizer your plants will never use. Parts per million (PPM) is the measurement of soluble minerals in your nutrient solution. The labels of organic fertilizers include the recommended PPM for hydroponic growers. It’s important that you follow them carefully because natural plant foods contain more solids (ingredients that need to be broken down by microbes) than synthetics, which dissolve more readily in water.
To check the PPM of your solution, you can use meters that measure the total dissolved solids (TDS) or electrical conductivity (EC). Flowering plants typically require a TDS nutrient strength of about 1,500 PPM to 2,000 PPM. If you use an EC meter, you want a reading of about 1.8 (1 EC equals approximately 900 PPM).
Be aware that distilled or purified water has a neutral pH (7.0) and nearly 0 PPM of soluble minerals. All other water sources—especially tap water—contain minerals that affect the pH and PPM. That’s why, if you’re giving your plants healthy, organic fertilizers, it’s best to mix your nutrient solution with the purest water possible.