Plants have different nutritional needs. Some plants require nutrient rich soils, while others thrive in lean, nutrient poor soils. It is not possible to give a recommendation that would work for all garden plants. I hope you can bear with me through this somewhat lengthy explanation.
If chosen for your site conditions, plants may need little or no additional food. However, many plants do need a nutritional boost from time to time. The goal is to provide enough nutrients to meet your plant’s needs without promoting plant disease or using an excess that can leach into ground water or cause other environmental problems.
Plants need three key nutrients to grow: nitrogen (indicated by the chemical symbol N), phosphorus (P), and potassium (K). These are called primary nutrients. Plants also need more moderate supplies of several secondary nutrients: calcium (Ca), magnesium (Mg), and sulfur (S). Finally, plants need trace amounts of micronutrients such as: boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn).
Different soils have different nutrient levels. The nutrients in your soil may be very different from those in mine. My sandy soil in south Oakville has low fertility, while my son’s garden, just twenty minutes north of me, has a nutrient-rich clay loam. Nasturtiums in my garden are full of flowers, while nasturtiums in his garden have beautiful foliage but few flowers. Different nutritional profiles can favour very different growth patterns and flowering behaviour in plants. Soil tests can be performed, but you can also just observe plants in your area to learn what grows well and choose plants suited to your conditions.
Some plants, like certain legumes (peas, clovers, beans, etc), form nodules in their root systems that support special “nitrogen fixing” bacteria. These bacteria use nitrogen from the air and feed it to their plant. Such plants do not need nitrogen rich soils or nitrogen supplements. In my garden, a nitrogen-fixer like blue false indigo (Baptisia australis) does very well with no need for fertilizers. In fact, fertilizers can be a real problem for certain plants.
One of the most common questions we receive during the growing season is: “Why is my plant not flowering?” Often our response is “You are feeding it too much.” From nasturtiums to lilacs, hydrangeas and peonies, too much of those primary nutrients can result in no flowers. This is also the case for some vegetables crops like tomatoes and certain vine crops (like cucumbers, squash, melons or grapes). They grow like crazy, but produce fewer flowers and therefore fewer fruits. These plants do quite well in soils with average fertility and require little if any soil supplements.
If plants are native to your area, that is if they have evolved to thrive in your local soils, they won’t need soil amendments. However, if you “clean up” your garden year after year and don’t allow plants to recycle their nutrients, you will deplete your soil over time. Additions of leaf mould or other composted material may be needed. Plants that generally do well with low fertility include: butterfly weed (Asclepias tuberosa), asters, pinks (Dianthus spp.), rock roses (Helianthemum spp.), sea holly (Eryngium spp.), bee balm (Monarda didyma), speedwell (Veronica spp.), coneflowers (Echinacea spp. and Rudbeckia spp.) and most ornamental grasses (from Shinn, “Perennials not to Feed”).
Then there are plants that are “heavy feeders.” Crops like potatoes, corn, and vegetables in the mustard family (cabbage, broccoli, and cauliflower) as well as flowers like delphiniums, mums, lilies, and astilbe thrive in nutrient rich soils. If your soil is on the lean side, you will need to add amendments or fertilize, if you want them to flourish.
So should you use “natural” or “synthetic chemical” supplements? Frankly, it doesn’t make much difference to the plant, but your choice does have different environmental impacts. Generally, the most sustainable choice is to amend your garden soils with compost (recycled natural materials, e.g. leaves, food waste, manure, etc. ) or to use compost teas and natural mulches. These provide a wide variety of nutrients. (Use of compost is not without issues and can be overused, but that is for another discussion).
If soil tests indicate a particular need, a synthetic fertilizer may be better at targeting the problem. For hydroponic systems synthetics are needed. For indoor plants, you can use compost (e.g. vermi-compost), but commercial fertilizers are generally more convenient.
Commercial fertilizers come in many forms, but all provide the primary nutrients. Prominently featured on the package will be the N-P-K ratio, which indicates the percentage by volume of nitrogen (N ), phosphorus (P), and potassium (K) in the product. (You can purchase supplements for secondary nutrients, but this should be done only if soil analysis or a particular plant disorder suggests the need.). Nutrient ratios are also often printed on bags of compost. For instance, I recently purchased cow manure with a balanced ratio: 0.5 0.5 0.5. Other composts will have variable N-P-K ratios. (Here are some typical examples https://www.allotment-garden.org/composts-fertilisers/npk-nutritional-values-animal-manures-compost/).
Generally, a balanced fertilizer or soil amendment will be fine for most applications. Some plants may require fertilizers with more or less of the “big three,” based on soil conditions. Our knowledge continues to grow. We used to advise people to add fertilizers with high phosphate to encourage root growth and better blooming, but that has turned out to be one of those garden myths that will take some time to dispel. We continue to learn.
If you are growing plants in containers and using commercial potting mixes, you may need to add supplements. Read the soil package. Some commercial soils have slow release fertilizers already added. If after six to eight weeks, plants are not performing well, then consider adding a general purpose granular fertilizer, compost tea, or a soluble plant foods.
A note of caution with fertilizers. Certain diseases, like powdery mildew, can be made worse if plants are fed. If you have a disease in your garden, read before adding supplements. The bottom line is to fertilize only when required, choose the supplement wisely and time applications to reduce disease pressures.
And a final note on the environmental impact of fertilizers. Production and application of fertilizers can contribute to the emission of greenhouse gases and the eutrophication of water bodies. In addition, soil amendments, regardless of the source, alter your soil biota, the organisms that live in you soil. Soils and their processes are extremely complex and we are just coming to understand the importance of healthy soils. It is important to protect soil and water health. Always consider the greater good, when making your garden choices.