There are numerous reasons, like climate and space, that influence the decision to either grow indoors or outdoors. No matter which location you choose, however, there are significant differences in the techniques for each situation. This is especially true when it comes to the type and amount of nutrient you use and at which frequency you apply it. There are several issues that recommend some change between indoor and outdoor nutrient formulas.

Soil and Grow Media

Before we discuss the nutrients themselves, let’s look at grow media and soil, as they directly impact feeding. (Note, when we discuss indoor hydroponic systems in this article, we are not talking about hydroponic water culture, as this is a subject of its own.) Though the amount of nutrient used by crops grown in outdoor soil and indoor potting media doesn’t differ, the nutrient level remaining in these grow medias does.

As outdoor soils are often very slow to percolate, the nutrients tend to remain much longer. When these soils also contain high clay content, the nutrients are not leached off or washed through as quickly. Also, the plants’ root systems are below ground level, so gravity works to maintain moisture and nutrients. However, when outdoor grows use raised beds or raised rows, these will effectively drain faster and require possible changes to nutrient formula and application timing.

Indoor grow media generally lacks the sand, silt, and clay particles that make up outdoor soil (sand does not in itself bind nutrients and hold water well, but clay does have that ability). This has a real impact of the kind of nutrient formula to use. Grow media used for indoor application also often percolate quicker. This allows for faster drainage, as well as an accelerated dilution and rinsing away of nutrients. This in turn may reveal the benefits to be found using slower release nutrients. The root systems in indoor gardens are always elevated, so gravity accelerates leaching. Add to this the enhanced percolation rate of the grow media itself and the result is that the nutrients applied indoor are more quickly washed through and eliminated.

Indoor grow mediums also differ from each other. For example, the various indoor grow medias have substantially different water retention rates. These things affect how often your indoor crops require watering and feeding. As such, any discussion about nutrient formulas for indoor grows must take into consideration which media is used.

Environmental Considerations

Most, but not all, outdoor grows will receive more photosynthetic active radiation (PAR) than indoor ones. There are, of course, some awesome lighting systems available that can provide every bit as much of this valuable light. Still, when growing indoors, it might be beneficial to formulate a nutrient mix that promotes the development of chlorophyll to compensate for the synthetic light. Remember that chlorophyll is at the heart of photosynthesis.

Also, most outdoor soil has been subjected to rain, wind, animals, and every other aspect of nature for an immeasurable amount of time. Because of this, outdoor soil is generally loaded with many minerals and nutrients. However, the amount of these naturally available nutrients varies so dramatically, it is very important to know the composition and nature of your outdoor soil before deciding what nutrient formula to use.

Indoor vs. Outdoor Nutrients

Again, the same nutrients are generally required to produce a healthy and productive plant, no matter if it is grown indoors or outdoors.

Due to the nutrient-leaching aspect of most indoor hydroponic systems, you need to replenish your nutrient levels two to three times more frequently. This is a typical nutrient depletion estimate; it’s not specific to any single method or set-up. Recirculating indoor systems are different, however. As it’s normal to refresh the water reservoir and re-establish nutrient levels in these systems, this also impacts the frequency and amount of nutrients to add. If the nutrients applied to your crop are being recirculated by your grow system, then their removal will be primarily based on plant utilization and the requirement to flush the system. What is most certain is that nutrient levels will decrease much faster in non-recirculating hydroponic systems and so adding nutrients will be needed more often.

Most manufacturers provide recommendations regarding the application amounts and timing for their fertilizers. Some will give multiple recommendations based on different planting applications, such as planting into the ground, raised beds, or pots. However, because there is such a diversity of drainage and percolation rates for grow media, it’s difficult for a manufacturer to accurate anticipate exactly what you’re using and therefore need as far as a schedule goes. Ultimately, the grower needs to know the percolation rate for their grow media. A key to selecting the right fertilizer for your crop is to match instructions found on that fertilizer with your growing set-up.

When grow media systems have faster percolation, using a slow release fertilizer can have nice benefits. Some of these slow release fertilizers are made by coating the nutrient. A very thin layer of plastic film is impregnated with a release agent and then used to coat a granule of fertilizer. This then retards the release of that granule, allowing a variety of release times following application. Most of these slow feed fertilizers are released by adequate temperature and water. In grow medias that are watered more frequently, this will cause the nutrients to be released more quickly, which is beneficial.

Also, some nutrients change state during their exposure to soil or various grow media. Calcium, phosphorus, and magnesium precipitate more quickly than other nutrients. This means that they turn from their ion form (salts), which are plant available, into solids or other forms that are not plant available. If you have established a living soil/media, one that is teaming with biological life, the microbe activity will work to convert the non-ionic form of nutrient back into that form where the plant can uptake it.

Testing the nutrient levels of leaf tissue, grow media, or soil is the best way to discover whether the fertilizer or application rate you’ve been using is optimum for your crop. Testing nutrient levels in the soil or nutrient tank is great for staying on top of providing the proper nutrient levels as you go. There are many meters and test kits available to help you discover the rate of nutrient depletion. Plant tissue testing, on the other hand, will provide important information of the level of these nutrients that are getting into the plant. These two techniques are meant to work together to give the most complete picture of plant health and nutrition.

The primary differences between indoor and outdoor nutrients should then be their ability to compensate for the differences in percolation rates of the grow media or soil and the resulting changes in application timing. Knowing this will help the grower to get the maximum yield from their crop. For more information, click on the QR code.