#PlantsMakePeopleHappy, Behind The Scenes, How-to, Interview, Plant Care

Fertilizer 101

March 29, 2017

Christopher Satch, head of plant science and education here at The Sill, gives us the 4-1-1 on fertilizer – just in time for the start of the growing season… spring!  


The fresh smell of soil in the morning really does it for me. It really does it for your plants, too. Fertilization is one of those things that tends to get overlooked by most novices, but it can be quite important for the long-term health of your plant. Fertilizer should be thought of as vitamins for plants – not plant food (plants make their own food via light and photosynthesis). There are a few rules surrounding fertilization, and even the types of fertilizer that you should use.

Rule 1

Fertilization should follow growth, and the pace of growth. Spring is the start of the growing season. If you’re going to fertilize your plants – it’s best to do it in the springtime, when those vitamins will really come in handy. Use a slightly weaker dilution than the package recommends. Like with watering, it’s always better to under-fertilize than to over-fertilize! Do not fertilize if you’ve just repotted – new potting soil will provide enough new nutrients for your plant. (You can fertilize a month after repotting).


Plants that grow faster should be fertilized more often than plants that grow slowly. For example, a begonia should be fertilized more regularly than a snake plant, and even more regularly than a cactus. That being said, if new growth on a plant you’ve had for a while is visibly smaller than previous growth, if the plant has been stagnant for months (not to be confused with a plant being dormant in the winter!), or if there is a clear indication of nutrient deficiency – you can fertilize your plant.

Rule 2

Plants that do more – ought to be fertilized more heavily. Fruits, veggies, and spices all need the most fertilizer because those plants are in production and fruiting regularly. For every leaf or fruit that you take from a plant, you’re also taking all the nutrients that went into that product, i.e. that leaf or fruit. It goes without saying that the plant needs the nutrients to grow the leaf or fruit in the first place. Flowering plants need a little less fertilizer than crop-producing plants. And other plants that just grow vegetatively need less.


Rule 3

Know thy NPK values! What are NPK values? It’s the ratio of the three most-consumed macronutrients that plants need (that should be in your fertilizer) – nitrogen, phosphorus, and potassium, respectively. These values will usually be given on the label of the fertilizer box, for example: you’ll see 10-15-10, or a similar variation, on the label. If a value is not given, then we’d skip that fertilizer altogether and find another brand to use.

Rule 4

Know thy micronutrients! Micro-nutrients are just as important as macro-nutrients. Why micro? Because plants need less of them – even though they are just as important. Micronutrients include: calcium, magnesium, boron, iron, zinc, sulfur, nickel, manganese, copper, and molybdenum (but not necessarily in that order). Each micronutrient serves a role in plant enzymatic, cellular, and developmental functions. For example, calcium is involved in cell-wall thickening, and lack of calcium can lead to necrotic buds as well as mottled growth. You generally don’t have to worry about these for your houseplant. For your outdoor plants though, you do have to worry about these.


Rule 5

Know the difference between organic fertilizers and chemical fertilizers. Organic fertilizers are made from some decaying organism, whether it be a fish emulsion, bat guano, or kelp – it’s coming from some other organism. Great in theory, but tough in practice, to make sure that organism naturally provides the right amount of nutrients for your plant. Chemical fertilizers are actually made from ground up minerals, which allows them to be formulated to be the correct amount of each macro and micro nutrient.


Not convinced to go with a chemical fertilizer over an organic fertilizer? Remember that everything is a chemical of some sort – even water is technically a chemical. So, both organic and ‘chemical’ fertilizers accomplish the same job, just in different ways. Fish emulsion and chemical fertilizers deliver the same nitrates, the same potassium ions, and the same phosphates to plants. So, is one “better” than the other? Not really. Chemical fertilizers just happen to be more concentrated, and are usually more affordable. But it’s totally a personal preference.

Questions about fertilizing? Leave a comment below.


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  • Betsy Weir April 6, 2017 at 2:32 PM

    Can you suggest some chemical fertilizer options? Thanks!

    • Erin Marino April 6, 2017 at 2:39 PM

      Hi Betsy! We don’t endorse any particular brands – but members of our team use FoxFarm, Miracle Grow, and Schultz. Hope that helps 🙂