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Fertilizer-- Why, how, when and what kind? This is not quite as cut and dry as some folks might have us believe, and there is a LOT to this topic, but I’m going to share some basics about fertilizing plants and my personal approach, including the few guidelines I live by when it comes to feeding my garden. So let’s jump in!
Fertilizer Basics- NPK and more
While I’m sure many of you savvy gardeners are familiar with the information I’m going to share, it feels a bit remiss to NOT give a brief explanation of fertilizer components and ratios.
Plants require at least 14 mineral elements for their nutrition, which are generally obtained by the soil—or in the case of depleted soil, exogenous fertilizer.
These are the primary macronutrients: nitrogen, phosphorus, potassium, and secondary macronutrients: calcium, magnesium, and sulfur. These are coined macronutrients because of the relatively large amount plants require.
Then there are the trace elements or micronutrients, required in very small amounts, which are: boron, copper, chlorine, iron, manganese, molybdenum, nickel and zinc.
In the US, fertilizers are labeled with three numbers that indicate the guaranteed analysis or the fertilizer grade. The three numbers, give the percentage by weight of nitrogen (N), phosphorus (P) expressed as phosphate (P2O5), and potassium (K) in the form of potash (K2O). Often, to simplify matters, these numbers are said to represent nitrogen (N), phosphorus (P) and potassium (K) or N-P-K, and are often calculated based on the amount that’s immediately water-soluble or available to plants.
For example, if the net weight of a 18-4-10 fertilizer as pictured was 100 lbs., then there are 18 pounds of nitrogen (N), 4 pounds of phosphate (P2O5), and 10 pounds of potash (K2O). The remaining weight (the total must add up to 100% or 100 lbs.) is made up of an inert material or nutrient carrier that aids in the application of the nutrients. A complete fertilizer contains all 3 primary macronutrients, a balanced fertilizer contains equal amounts of all 3.
Different types of plants require different amounts of macro and micronutrients- for example, root crops generally fare better with a little more relative phosphorous and potassium and a little less nitrogen than other crops, as excess nitrogen can lead to big, lush top growth at the expense of root growth. Corn, on the other hand, is a notorious nitrogen hog. Legumes like peas & beans can actually fix their own nitrogen and application of excess nitrogen can reduce yields. If you’ve watched some of my planting guide type videos, you’ll often hear me suggest certain ratios of fertilizer for certain plants based on the plant’s nutrition needs. But this is really a generalized recommendation. It’s hard to hone in on specific nutrient needs without first taking other factors into consideration.
The FIRST and in my opinion MOST IMPORTANT FACTOR is the SOIL itself.
TEST YOUR SOIL
Knowing the specific levels of nutrients and minerals your own soil contains is key to optimally feeding your plants.
Applying fertilizer before you know your own soil’s makeup and what nutrients you actually need is a waste of your time and money! If your soil is already high in phosphorous (P) and Potassium (K), there’s no sense in adding a fertilizer high in P & K to your soil- in fact, it can be harmful.
For example, according to Texas A&M: “The buildup of phosphorus in lawns, gardens, pastures, and croplands can cause plants to grow poorly and even die. Excessive soil phosphorus reduces the plant's ability to take up required micronutrients, particularly iron and zinc, even when soil tests show there are adequate amounts of those nutrients in the soil.” Phosphorous has led to algal overgrowths in many large bodies of water due to fertilizer run-off. We’ve unfortunately seen a lot of this in agricultural areas of Ohio, with lakes becoming too unhealthy to fish from or swim in.
A good soil test will give you a range of measures about your soil including all of those macro and micronutrients I mentioned earlier, pH, humus or organic matter percentage, as well as compare your numbers to an optimal range so you can see where your soil measures up. A great test will give you recommendations on how to improve those areas where our soil doesn’t fall into the ideal range.
One of my favorite testing companies to use when starting with a brand new plot of soil is Kinsey Agricultural Services (kinseyag.com), as they give incredibly detailed recommendations for amending soil. I then like to do a ‘maintenance’ test every couple of years to make sure I’m on the right track toward improving my levels and find the ease and convenience of RxSoil testing is awesome for this. With RxSoil’s testing process they include everything I need- I just pop my soil sample in, mail it and very quickly I can access my results online. RX Soil Testing (use my code growfully for 10% off your order): https://rxsoil.com/nutrients?source=growfully
FEED YOUR SOIL
Once I know where my soil stands (and honestly, even before I have those test results in my hands) my goal is to FEED THE SOIL not the plants. I want to MINIMIZE (or eliminate) my fertilizer use by building better soil. This is important because plants don’t grow independently in a garden environment. They form relationships other members of the soil web including fungi, beneficial bacteria, nematodes, insects and worms. Healthy soil is teaming with microscopic life and these members of the soil community have some very important jobs. Microbes are essential for nutrient cycling- the exchange of organic and inorganic matter throughout an ecosystem- and help plants in a big way by breaking down bound nutrients and ‘feeding’ them back to the plant roots. Microbes also help to break down organic matter in the soil- crop residue, leaf mulch, grass clippings for example, and release the nutrients locked up in the organic matter into usable forms for plants.
If your soil microbiome is not healthy and robust it will lack the microbial life to properly break down nutrients and assist plants with uptake.
So how does one feed the soil and build a healthy soil microbiome? I do this through a variety of tactics. I’m not strictly no-dig (check out THIS video to see why: https://youtu.be/uk3NZ9MEodA), but do I try to minimize soil disruption as much as possible. My goal with new beds is for them to become essentially a ‘no dig’ area within 2 years of creation. I also avoid walking on beds or any unneeded compaction to beds (especially important with my heavy clay soil), so I create garden spaces with devoted walkways and slightly raised planting areas.
Avoiding disruption and compaction of the soil is very important because in order to build a healthy soil microbiome all of those wonderful microbes have to be left undisturbed in order to grow and flourish and build the networks that are needed. Every time that the soil is disrupted, turned over, tilled, you’re essentially destroying a lot of the work the microbes have done in building a healthy soil web up to that point.
I extensively utilize cover crops and green manures, as well as natural mulches like chopped leaves, grass clippings, alfalfa hay and wood chips- I try to really mix it up with many types of organic matter additions, and always aim to keep the soil surface covered. Keep in mind, the addition of organic matter is an ongoing process. It will improve over time, but I still add generous helpings every single growing season, multiple times a season. In addition to diversifying my sources of organic material, I also try to diversify my plantings as much as possible.
And, I focus on the addition of beneficial, nutrient rich soil amendments over fertilizers when possible. Manure and compost being 2 that I’ve found to be the most helpful. These both help to provide plants with a boost of nutrition and feed soil microbes and attract things like earthworms. I don’t find that bagged compost or manure from the garden store is as beneficial in this regard. It can still be helpful for improving soil structure and providing a little nutrition boost, but is not microbially rich like the homemade, or farm-made stuff is.
According to the University of Maryland Extension:
Soil organic matter releases plant-available nutrients slowly during the growing season. Your reliance on organic or synthetic fertilizers will probably decrease as your organic matter content increases.
Aim for a soil organic matter content of 5-10% (it's measured by weight by soil testing labs). Soils in this range are fertile, easy to work, and have a large number of earthworms.
But the extension also mentions:
Soil organic matter may not supply sufficient nutrients at particular times of the season and at particular stages of plant development. For example, in the early spring and when fruits start to form. The peak for nitrogen release typically occurs in July if soils have adequate moisture.
And I have found this to be the case. Which is why I still rely on fertilizers to some degree.
Which Plants Need What?
When opting for fertilizer it’s important to know who needs what (as in the example that I gave earlier with root crops versus corn) and know your heavy feeders. This can be a bit frustrating at first, because you may notice, like I did, that every source you like at gives slightly different information about which plants are heavy, medium and light feeders. For example, the University of Georgia extension lists corn as a medium feeder, but the University of Maryland lists it as a heavy feeder. In general, heavy feeders are those plants which have higher nutrient needs and often take longer to reach maturity.
I tend to agree with Jean Martin-Fortier’s pleasantly simplistic categorization in his book ‘The Market Gardener’. He lists:
Heavy Feeders: Solanacea, Curcurbit, some Brassica, Onions, Garlic
Light Feeders: root vegetables, mixed greens, lettuce
Peas & Beans- receive no fertilizer
For a bit more detail on which plants are heavy, medium and light feeders, the University of Maryland Extension has a good list: Fertilizing Vegetables | University of Maryland Extension (umd.edu)
My Fertilizer Approach
My general rule of thumb for feeding plants is this: For seeds started indoors, I start seedlings on a very low dose, all-natural liquid fertilizer, added each time I water, after seedlings have their first set of true leaves and/or are several inches tall. I use the same fertilizer for every type of seedling I start. I’m still testing out new formulations to find my favorite, but most of these liquid fertilizers are derived from things like molasses, fish emulsion—even composted, liquified food scraps. My homemade seed starting mix also has a little boost in the form of earthworm castings.
When plants are ready to go out into the garden, I typically provide one helping of plant food at transplant time or when direct sown seedlings are a few inches tall. Except for my very heavy feeders, I will rarely apply additional fertilizer unless my plants are showing me signs of deficiencies (this could show up as weak growth, yellowing leaves)
Alliums- I typically try to feed garlic and onions 2 times in the spring, once in early spring when the greens start growing and once just as the bulbs really start to swell ( honestly though, some years I’m lucky to get one feeding in).
Brassicas- broccoli, cabbage, cauliflower, kale, kohlrabi-1 dose of fertilizer at planting time
Solanaceous crops- tomato, eggplant, pepper- 1 dose of fertilizer at planting time
Lettuce and other leafy greens (arugula, spinach, mizuna…)- if I’m transplanting, I may give them a dose of fertilizer at transplant time. Often when I direct sow, I don’t fertilize at all.
Corn- I’ll typically try to side dress with composted manure and/or fertilizer when it’s a foot or so tall. Corn is such a nitrogen hog that I often find it needs a second dose of nitrogen rich fertilizer mid-season.
Cucurbits- cucumber, melons, pumpkins, squash- 1 dose of fertilizer at transplant time. If I’m direct sowing, I try to remember to fertilize once seedlings are up about 6” tall.
Legumes- peas & beans- I personally don’t fertilize these at all anymore. I find they do fine without any additional fertilizer. In areas where legumes have not recently been grown, I like to apply a rhizobial inoculant to encourage the colonization of these beneficial bacteria on legume roots.
Direct Sown Root crops- carrot, beet, turnip, parsnip- I try to remember to side dress seedlings when they are a couple of inches tall (I don’t always remember). I don’t feed radishes at all.
Potatoes- I always add plenty of compost and composted manure to the planting area. I also typically try to feed them mid-season.
Sweet Potatoes- I work in fertilizer or compost into the planting area at planting time
Perennial veggies like asparagus and rhubarb- I like to add a top dressing of compost or manure yearly.
Herbs & annual flowers- I do not feed
In regard to the specific fertilizer I use, I’ve had a lot of questions about the “Magic Powder” I apply to my transplants in many of my videos. I almost exclusively use Garden’s Alive’s line of plant foods in my garden. I like Garden’s Alive’s fertilizers because they are naturally derived- using ingredients such as alfalfa meal, blood & bone meal, kelp meal, molasses, trace minerals and humic acid, and they have various formulas tailored toward specific garden plants. For example they have formulations like Tomatoes Alive!, Root Crops Alive!, and Sweet Corn Alive! I don’t have much personal experience with other brands, but Espoma is another line of naturally derived fertilizers with a similar product offering that I have used and liked and I’ve had good results with Neptune’s Harvest line (but many of these are fish-based and very stinky)!
One further factor to consider when choosing fertilizer is:
Bioavailability of Nutrients to Plants
We want to make sure plants can efficiently uptake the fertilizer you’re providing (or the nutrients that might already be in the soil).
A great example of this, which many gardeners are familiar with, is Blossom End Rot in tomatoes. BER is widely known as a calcium deficiency, but this isn’t the full story. BER can occur even when there is sufficient calcium in the soil. It’s often an issue of insufficient calcium uptake on the part of the plant. Calcium is only moved into the plant with ample, consistent water supply- so if water is inconsistent, tomato plants may not be able to uptake calcium effectively. BER may also occur in plants which have root damage, or plants that are putting on new growth faster than the roots can uptake calcium. Overall, a plant’s ability to uptake nutrients is a complex topic, but I want to briefly share several factors which can affect nutrient uptake. First off is:
Soil pH Affects Nutrient Availability
1. Most macronutrients are most readily available in soil with a close to neutral pH= about 6.0 -7.0. Another reason that soil test is important!
2. However, it’s a fine balance- micronutrients like iron, manganese, copper, zinc and boron become ‘inactivated’ in neutral to slightly alkaline soil and are readily available to plants in slightly acidic soil.
3. In extremely acidic soils (in the 4.0-5.0 range) some nutrients such as manganese and iron may be too readily available, and plants take up too much- often with toxic results. Acidic soil can also impede the activity of bacteria which break down soil organic matter—as a result you end up with an accumulation of lots of organic matter with nitrogen locked up inside that matter because it can’t break down and release the nutrients.
4. Highly alkaline soils tend to ‘bind up’ most nutrients making them less available to plants.
5. Side Note- Chelated fertilizers can help in situations where micronutrient availability is hindered, until the issues are remedied.
2nd is a:
HEALTHY SOIL MICROBIOME
I already delved into this back when I talked about the need for healthy soil. But Bacteria & Fungi and other microbes form critical relationships with plants, relationships needed for efficient uptake of nutrients. One of my favorite examples is Mycorrhizae. A mycorrhizal association occurs when a plant and the fungus have a mutually beneficial relationship, where the fungus facilitates water and nutrient uptake in the plant, and the plant provides food and nutrients created by photosynthesis to the fungus. You can dump all the fertilizer you want on your soil, but if it can’t be broken down and/or utilized effectively by the plants, it’s a waste of money.
SOIL WATER CONTENT
As in the case of Blossom End Rot, the amount of water in the soil can affect the uptake of some nutrients in plants. In general, adequate soil moisture results in a more efficient uptake of nutrients by plant roots. Conversely, too much water can have the effect of ‘washing out’ or diluting soil nutrient content. This is commonly seen in very sandy soils as well as container plantings.
Consider also the choice between
LIQUID VS GRANULAR FERTILIZER Pros and cons of granular and liquid fertilizers - MSU Extension
Both forms have their benefits, so it’s best to consider your own garden conditions when choosing.
Nutrients can be more ‘mobile’ in a soil water solution, i.e., liquid fertilizer. This may make less mobile nutrients like phosphorous easier for plants to uptake than a granular version.
Liquids also tend to be quite fast acting- and can make a good emergency fix for stressed plants. Foliar spray can be particularly effective.
Granular fertilizers tend to be longer lasting- this is why I can often apply one dose at transplanting time, and it continues to provide for my plants for the entire season.
Liquids can be easier for immature root systems to uptake ( a reason I like using them with my indoor seedlings).
So—definitely not a cut & dry topic, but hopefully I’ve answered some of your questions about my approach to fertilizer! Feel free to ask questions in the comments below. Happy gardening!