Nutrients & Fertilizers

All vegetable plants require 13 essential soil nutrients to thrive, including the nutrients nitrogen, phosphorus, and potassium, plus the ten micronutrients needed in smaller amounts.

All 13 essential nutrients are available in the backyard or commercial compost.

Nutrients are not foods; plants make their food by photosynthesizing. They need nutrients from the air and soil to grow, and fertilizers play an essential role in this process during the growth cycle. 

There are six primary nutrients that plants require to grow. The air provides carbon (from carbon dioxide CO2), hydrogen (from water (H20) and oxygen. The soil supplies nitrogen N, phosphorus P, and potassium K.

  1. Nitrogen (N) promotes vital leaf and stem growth and dark green colours, such as broccoli, cabbage, lettuce, other greens, and herbs.
  2. Phosphorus (P) promotes root and early plant growth, which helps anchor and strengthen plants, and it helps with setting blossoms and developing fruit and seed formation. Phosphorus is essential for cucumbers, peppers, squashes, tomatoes— any edible vegetable that grows after a flower has been pollinated.
  3. Potassium (K), also known as potash, regulates water flow in plant cells, promotes plant root vigour and disease and stress resistance, and enhances the flavour. Plants deficient in potassium may display stunted leaves and fruit and be extra sensitive to drought.
Trace minerals

In addition to the three primary soil nutrients N, P, K, numerous trace minerals are needed for overall plant health as well, although in smaller quantities:

  • Calcium (Ca) improves general plant vigour and promotes the growth of young roots and shoots.
  • Magnesium (Mg) regulates plants' uptake of nutrients, aids seed formation, and contributes to leaves' dark green colour—necessary for effective photosynthesis.
  • Sulfur (S) helps a plant to maintain its dark green colour and encourages vigorous plant growth.

If the soil is rich in nutrients and the microbial life aids in the plant's uptake of these nutrients, adding more can disturb that healthy ecosystem. As garden vegetables grow, they undergo an intensive process that strips nutrients from the soil.

Balance of Nutrients 

Too much or too little nutrients can harm plants. For example, tomatoes deficient in calcium develop blossom-end rot. Excessive nitrogen causes high leaf growth in peppers but few flowers or fruits.

pH Controls Nutrient Uptake

Soil is rated on a pH scale, pH of 1 being most acidic and 14 being most alkaline. Most vegetables grow well with lightly acidic soil with a pH between 6 and 7.

Impact of pH on phosphorus (P), potassium (K) and magnesium (Mg)I

If the pH is out of range, P and K will not be absorbed even if they're present in high concentrations.

If the pH is too low, manganese may increase to toxic levels.

pH has a significant influence on the actions of all minerals.

Since I grow my vegetables in a small garden, I purchase organically prepared soil from a good nursery with a clear idea of what I wish to raise. Hence, my vegetable garden soil has a high probability of being of the right texture and containing most essential nutrients and trace elements from the getgo.

However, nutrients are used up as plants grow and must be replaced judiciously. The best way to do this is to test your soil, at least annually (see below) 


Fertilizers are used to supplement nutrients lacking in the original soil or used up as plants grow. 

The type of fertilizer used and its concentration depend on the soil composition and the plants being grown.

Therefore, test the soil to determine the pH level and nutrient content to optimize vegetable growth. In addition, the soil test results can be used to determine the soil nutrient composition to create the plants' best possible growing environment.

Soil Testing Methods

The only way to determine whether your soil will be to your plant's liking is to test it, which is not complicated. There are two ways that you can test your soil:

  1. Use a do-it-yourself kit: A basic pH test kit can be purchased at most nurseries to measure soil's acidity and alkalinity and primary nutrient content to give a rough picture of a soil's pH and nutrient levels in a small well-growing garden with soil purchased from a nursery.
  2. A complete laboratory soil test is a good investment if plot soil is used. 

A soil lab provides accurate pH level and nutrient content of a soil to determine how much and what kind of fertilizer to use to optimize the soil content for vegetable growing.

Types of Fertilizers

Organic fertilizers 

Organic fertilizers consist of ingredients such as manure, blood meal, cottonseed meal, feather meal, crab meal, and so forth. They work with soil microbes that break down fertilizers to release nutrients that plants can absorb.

Organic fertilizers do not add excess salts and acid to the soil; they encourage healthy soil biology rich in microbial activity.

Some organically based fertilizers suited for spring application contain small amounts of synthetic fertilizers to ensure the immediate availability of nutrients.

The N-P-K ratio of immediately available nutrients listed on the packaging of organic fertilizers is lower than that of synthetic fertilizers and take longer to be absorbed by plants.

Synthetic fertilizers are made in labs and derived from ammonium nitrate, ammonium phosphate, superphosphate, and potassium sulfate. They:

  • speed plant growth and can enhance the bloom rate in flowering plants.
  • are high in salts and can be detrimental to beneficial microorganisms
  • can "burn" foliage and damage plants when applied too heavily.
  • boost plants but do little to improve the soil's long-term health, texture, or fertility
  • are highly water-soluble and leach out into streams and pond

Commercial fertilizers are labelled with three numbers that indicate the fertilizer's nutrient ratio.

  • The first number indicates the percentage of nitrogen (N)
  • The second number shows the percentage of phosphate (the type of phosphorus,( P)
  • The third number represents the percentage of potash (the form of potassium used, (K)

For example, a 5-10-5 fertilizer contains 5 per cent nitrogen, 10 per cent phosphate, and 5 per cent potash, and it's called a complete fertilizer because it has some of each type of nutrient.

In contrast, a bone meal has an analysis of 4-12-0. Thus, it's a good source of phosphate (for early root and plant growth) but doesn't provide any potash (K).

Granular Fertilizers Vs. Soluble Fertilizers

Granular fertilizers are solids that must be worked into the soil. They must be given time (and water) for dissolving to become available to plants.

Slow-release fertilizers are a subset of granular formulations and contain a portion of the fertilizer that is not immediately available to the plant. Slow-release fertilizers have a polymer coating that breaks down slowly as moisture from the soil is absorbed, and nutrients (N, K, P) dissolve back into the soil.

Nutrients are metered out over several weeks. Therefore, slow-release fertilizers are applied less frequently. 

Soluble fertilizers, sometimes called "liquid feed," are sold as ready-to-use solutions or packaged dry-milled materials that need to be dissolved in water.

These tend to be quick-release fertilizers that are high in nitrogen and result in fast green growth.

I aim to build my soil's long-term health and fertility using granular organic fertilizers and supplement it with an additional water-soluble fertilizer during plant growth.

Plant Supplements

Plants can also get "vitamins" and nutrients from these soil amendments:

 Alfalfa meal is a source of readily available nitrogen for plant growth and also feeds soil organisms. In addition, it contains vitamins, folic acid, and trace minerals. 

Potassium is vital for carrots, radishes, turnips, onions, and garlic. Add greens (made from glauconite, an ocean mineral high in potassium and iron), wood ashes, gypsum, or kelp (dried, ground-up seaweed) to increase potassium.

Kelp has the additional benefit of helping the soil hold moisture, thus reducing drought and frost.

Because most soils already contain potassium, the third number in the fertilizer ratio tends to be the smallest.

  • A blood meal additive is very high in fast-release nitrogen. It also repels deer.
  • Fish meal, a by-product of fish farming, is an excellent source of nitrogen and potassium.
  • Kelp meal—dried, ground-up seaweed—provides trace minerals, amino acids, and enzymes that stimulate plant and root growth and benefit microbial life. In addition, improving soil structure can help the soil hold moisture and reduce damage due to drought and frost.
  • Soybean meal contains high amounts of nitrogen and potassium that are slowly released as it breaks down. Look for organic sources; most commercially grown soybeans are genetically modified.
  • Animal manure—As all-around fertilizer, barnyard or stable manure— often free from farmers and stables—is a good source of organic matter and nutrients if you haul it away. Never use it fresh when it is too "hot" or raw and could burn plants. Use only aged or composted animal manure. I tend to avoid using it on my vegetables.
Approach to fertilizing vegetables and herbs

Different vegetables have different fertilizing requirements, which have been scientifically worked out over the years and are readily available.

Slow-release fertilizers have a granular appearance and are either worked into the soil at planting time or sprinkled on top of the soil, perfect for container plants or new bedding plants.

Decomposed, well-rotted compost is an excellent fertilizer that should be worked into the soil in the fall or spring. 

Compost teas use compost in bags that are soaked in water for a day or two, leaving the nutrient-packed water perfect for soil, roots, foliage, and transplants.

Although some plants have particular fertilizing requirements, it's unnecessary to have a different fertilizer for each plant.

A general balanced fertilizer (10-10-10) is helpful for nearly any plant.  

However, a healthy, well-drained, and rich soil may not need to fertilize at all. Only apply if required, e.g. during fruiting. Resist using fertilizers to ease the guilty conscience of neglecting your garden. There is no substitute for sound soil and appropriate watering.