Natural fertilizers may come from organic sources (plant or animal waste) or mineral sources (rock powders). These fertilizers have not been chemically processed.
Before they release their nutrients, most natural fertilizers have to be broken down by organisms in the soil. This means that in addition to feeding the plants, they encourage biological activity in the soil. Another advantage to this means of degrading fertilizers is that there is less risk of leaching and burning plants’ roots.
Most natural fertilizers are slow-acting but long-lasting in the soil. Fast-acting natural fertilizers include fish emulsion, liquid kelp, sulphate of potash magnesia, sodium nitrate and blood meal.
Examples of natural organic fertilizers
N.B.: The application rates shown are indicative only. They should be adjusted according to your soil test results, your plants’ needs and the instructions on the product label.The percentages of nutrients contained in the fertilizers are all approximate.
Blood meal (12-2-0)
This high-nitrogen fertilizer also contains some phosphorous. It is fast-acting and long-lasting. It is used along with compost. It should be scratched into the surface.
Application rate: 5 kg/100 m2
Feather meal (13-0-0)
This fertilizer releases nitrogen over a period of about 140 days. It is used along with compost. It should be scratched into the surface.
Application rate: 5 kg/100 m2
Meat and bone meal (8-4-0)
This high-nitrogen and -phosphorous fertilizer is used along with compost. It can raise soil pH slightly. It should be scratched into the surface.
Application rate: 5 kg/100 m2
Bone meal (2-22-0)
This fertilizer is high in phosphorous and also contains calcium. It is used early in the season, during seeding, setting out and planting. It is slow-acting and long-lasting in the soil: 50% of the fertilizer is degraded in the first year, and the rest in 4 years.
Since bone meal attracts animals, some gardeners like to use mineral phosphate, also known as rock phosphate, instead. Mineral phosphate contains 27% phosphorous (0-27-0), but only 7% of it is available to plants. This product degrades even more slowly than bone meal (7 years). It also raises soil alkalinity, because it is high in calcium.
Bone meal can also be replaced by shrimp (8,5-6-1,2) or crab meal (4,7-5,9-0,3).
These products should be well mixed into the soil or added to compost.
Application rates:
- bone meal: 10 to 25 kg/100 m2
- mineral phosphate: 10 kg/100 m2
- shrimp or crab meal: 5 to 10 kg/100 m2
Kelp meal (2-4-10)
Kelp meal is high in potassium and contains several micronutrients. It is used to boost plant growth and hardiness. It is mixed into the soil in spring or added to compost.
Given that it is rich in micronutrients, this fertilizer should be used in moderation.
Application rate: 1 kg/100 m2
Fish emulsion (5-1-1, 5-2-1, 5-4-1)
Fish emulsion may be used to promote strong growth in spring or to correct deficiencies throughout the growing season. It is used mainly for foliar feeding, but can also be added when watering. Read the label carefully, because some brands contain chemical fertilizers.
Application rate: 10 ml/l water
Liquid seaweed
Liquid seaweed is high in minor elements and potassium. It also contains nitrogen, phosphorous, growth hormones, amino acids, natural antibiotics and enzymes. It is used to stimulate plant growth and flowering and to increase resistance to insect pests and stress (wind, cold, drought, transplanting). It can also be used to pep up diseased or weakened plants and to meet the requirements of particularly demanding plants.
Liquid seaweed is usually used for foliar feeding, but can also be added when watering.
Application rate: 10 ml/l water
Examples of natural mineral fertilizers
Sulphate of potash magnesia (Sul-Po-Mag) (0-0-22; 22% S, 11% Mg)
This fast-acting fertilizer is high in potassium, sulphur and magnesium. It should be mixed with other fertilizers to avoid burning plants’ roots.
Application rate: 1 kg/100 m2
Epsom salts
Epsom salts is magnesium sulphate (10% Mg and 13% S). It is used mainly as a foliar spray to correct magnesium deficiency problems. It is also used to encourage growth and blooming in roses and for greener foliage.
Application rates: 5 g/l water
Roses: As a foliar spray: 1 Tbsp/4 l water (one application after the leaves open in spring and once again when the plant blooms). Soil: ½ cup of granules at the base of the plant (one application in spring before the buds open and once again in fall before the leaves drop).
Basalt (Bio-Rock) (0-0-4)
In addition to supplying magnesium, calcium, potassium and micronutrients, basalt is high in silica, a mineral that improves plants’ resistance to insect pests, fungal diseases and drought. Basalt also neutralizes overly acidic or alkaline pH levels and promotes water and nutrient retention in sandy soils. It is spread on the ground or mixed with compost.
Application rate: 10 kg/100 m2
Mica (0-0-10)
Mica is high in potassium and magnesium (20%). It is mixed with compost or worked into the soil.
Application rate: 10 kg/100 m2
Borax
Borax is very high in boron. It is used to correct deficiencies. This fertilizer should be used in small amounts because it can easily become toxic.
Application rate: 100 g/100 m2