Wikipedia defines fertilizer as “any organic or inorganic material of natural or synthetic origin (other than liming materials) that is added to a soil to supply one or more plant nutrients essential to the growth of plants.”

Many of the nutrients essential to plant growth already exist in sufficient amounts in your soil and require no further supplementation. While others quickly can become depleted and need to be re-added back into the soil on a regular basis. 

Increasing your soils fertility is more complex than just dumping a bunch of fertilizer into it. The plant nutrients have a symbiotic relationship with each other and with the beneficial microorganisms that live in the soil. In addition, a soils pH level, its structure and even the weather can affect the balance and availability of soil nutrients.

For a complete discussion on soil nutrients visit my blog post 18 Essential Soil Nutrients For Plant Health. There I discuss the 18 nutrients and their roles in plant health and more.

Natural and Organic Fertilizer Verses Synthetic

Organic Fertilizers:

• Are slower to leach from the soil.
• Have the added advantage of improving your soil’s structure.
• Encourage the build-up of beneficial soil organisms that serve to break down nutrients into a form that can easily be absorbed by the plants. These soil organisms also control soil borne funguses and diseases.
• Leave no residue of harmful chemical salts.
• Do not burn plants.

Synthetic Fertilizers:

• May cause soil imbalances.
• Many of the synthetic fertilizers add dangerous amounts of salt to the soil.
• Some have low solubility.
• Some are a danger to pets and wildlife.
• Can contaminate ground water.
• May burn your plants.
• Destroy beneficial soil organisms.

What the numbers mean 20-20-20 

The numbers you see represented on fertilizer packages and containers correspond to these 3 nutrients. The first number representing Nitrogen(N), the second number representing Phosphorus (P), and the third number Potassium (K). 

Some Natural Fertilizers And Their Fertilizer Values

• Compost:  1.5 to 3.5 – 0.5 to 1 – 1 to 2
• Manure:  0.5 – 0.3 to 0.5 – 0.5
• Bone meal:  0 to 6 – 12 to 27 – 0 (raw)           0 to 4 -12 to 34 – 0 (steamed)
• Blood meal:  1.5 to 3.5 – 0.5 to 1 – 1 to 2
• Alfalfa meal:  0.5 – 0.3 to 0.5 – 0.5
• Liquid seaweed: 0.5 – 0.3 to 0.5 – 0.5
• Dolomitic limestone: provides calcium and magnesium
• Cheated iron: Iron is a micronutrient essential for the formation of chlorophyll. Insufficient amounts in the soil will create chlorosis.
• Commercially prepared blends: These totally vary but the best ratios are generally well balanced like 20-20-20

Additional Natural Fertilizers Include:

• Fish emulsion
• Compost tea
• Gypsum
• Epsom salts: provide magnesium
• Soybean meal
• Greensand
• Banana peels
• Sulfur

When to Fertilize 
Proper timing is essential because the nutrients must be present in the soil when the plants need it most. This critical time is when plants are at their most active stage of growth and

flowering. If applied to early it may be leached from the soil before plants need it. If applied to late, it may not be available on time.

Perennials: fertilize at the first signs of growth and stop fertilizing in early fall as plant growth begins to slow.

Trees and shrubs: May be fertilized annually at the beginning of the season when they begin actively growing.

Annuals: can be fertilized their entire growing season.

Roses: fertilize at the first signs of growth and again right before flowering. Stop fertilizing by the end of August to give the plant a chance to harden off new growth before winter.

Photo Credit: photo/slide created by the author.

Updated on Dec. 5, 2021

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