THE GREEN THUMB ALMANAC 

Your Online Gardening Information Center

Fertilizers

On This Page:

  • Plant Nutrition
  • Primary & Secondary Nutrients
  • Slow Release or Liquid Fertilizer?
  • Organic Fertilizers & Amendments
  • Make Your Own Fertilizer Recipe
Like all living things, plants need proper nutrition to grow.  There are 19 nutrients considered essential for plant growth.  Carbon, hydrogen and oxygen are supplied by air and water.  Plant roots absorb the remaining 16 from the surrounding soil.  While some soils may already contain these nutrients, they may not be in a form available for plant growth.  The benefits of regular fertilizer application are:  Vigorous growth, vibrant color, bigger blooms and healthier plants which are more resistant to droughts, insects and diseases.

Plant Nutrition

The mineral nutrients are divided into three groups based on their relative abundance in plants:

  • Primary Nutrients (or Major Nutrients)
  • Secondary Nutrients (or Minor Nutrients)
  • Trace Nutrients (or Micronutrients)


Although the major nutrients are needed in the greates quantities, a deficiency of any one nutrient can prevent growth or make plants more susceptible towards disease.

Reading A Label

Whether in a bag or a box, organic or not, most all fertilizers have three analysis numbers listed somewhere on  the package, e.g. 5-3-3.  These are the percentages (by weight) of the Primary Nutrients: Nitrogen, Phosphorus and Potassium, in that order.  You will hear this called the  N-P-K ratio.  The letters represent their listing in the periodic table.
Primary Nutrient
 Function
 Contained In
 Excess
 Deficiency
Nitrogen (N)
 Leaf growth, important for protein & amino acid production.
Manure, fish & ocean plants, compost & blood meal
 Luxuriant leafy growth, dark green leaves, few flowers or fruit..
 Light green to  yellow older, lower leaves, yellow leaves may fall off, stunted growth
 Phosphorus (P)
 Seedling hardiness, fruit & root production, helps in disease resistance
 Bonemeal, ground rock phosphate, fish emulsion, compost, super & triple phosphate
 May cause micronutrient deficiencies
 Red- to purple-colored leaves, dwarfed plants, small leaves, few fruits, leaves dropping early
 Potassium (K)
 Regulates water movement in the plant, helps with stress, cold & heat tolerance
 Potash, greensand, compost, manure, kelp
 May create magnesium deficiency
 Lower leaves mottled, dead areas in leaf, yellowing begins at leaf edge & continues toward the center, weak stems

Where Did the Phosphorus Go?

You may have noticed, when looking at fertilizer products recently, that Phosphorus is slowing disappearing from the store shelves.  Why is this?  The uptake of phosphorus by plants is highly dependent upon the pH of the soil.  Soil pH should be neutral for phosphorus to be taken up efficiently by plant roots.  Over the years fertilizer products have included varying amounts of phosphorus (i.e., "Bloom Booster" fertilizers with very high percentages of phosphorus).  It has been determined that we have been adding too much phosphorus to our soils.  What we should have been doing is adjusting our soil pH so that the plants can take up the phosphorus which occurs naturally.  Phosphorus is expensive to produce, and in excess, is harmful to our environment (Canada has already banned it, the U.S. may soon follow).

Some Secondary & Trace Nutrients

 Nutrient
 Function
 Contained In
Calcium (Ca)
 Cell-wall & enzyme production.
 Bonemeal, eggshells, limestone, milk, gypsum, ground oyster shells.
 Magnesium (Mg)
 Chlorophyll production & respiration.
 Organic matter, Epsom salts, liquid kelp, seaweed extract.
 Sulfur (S)
 Protein production, helps maintain pH.
 Ground sulfur, gypsum.
 Boron (B)
 Cell-wall formation, carbohydrate transportation.
 Organic matter, seaweed.
 Copper (Cu)
 Enzyme & photosynthesis regulation.
 Seaweed, kelp, manure, compost.
 Iron (Fe)
 Chlorophyll formation.
 Organic matter, bonemeal, iron chelate, iron sulfate.
 Maganese (Mn)
 Helps in photosynthesis & respiration.
 Manganese sulphate, organic matter.
 Molybdenum (Mo)
 Nitrogen fixation, nitrogen metabolism.
 Usually present in soil but often triggered by low pH or low organic-matter content.
 Zinc (Zn)
 Chlorohyll formation, bud development.
 Zinc sulfate, organic matter.

Slow Release Pellets or
Liquid Fertilizer? 
Not sure which to use?

Slow Release Pellets (like Osmocote) are great to use at the time of planting.  Mix it in with the soil in your pots or in the hole if planting in the ground.  This will ensure that the fertilizer is down where the plant roots can get it easily.  Read your labels!  Most slow-release fertilizers will say something like "lasts for 4 months!"  Reading the fine print will tell you that it will last for 4 months at 70 degrees F.  As the temperature increases, so does the rate at which "slow-release" fertilizers release their nutrients!
Liquid fertilizers are also good.  A benefit of liquid fertilizers is that you can combine your watering and fertilizing tasks into one activity.  The down side is that any fertilizer not used immediately by the plant will leach away.  When using liquid fertilizer remember that you must use it every time you water.  If you are using an inorganic liquid fertilizer to water containers (you know, the ones that turn your water blue) you must remember to occasionally flush your pots with fresh clear water.  This will help to eliminate salt build-up in your pot.

Organics

Organic fertilizers and soil amendments are any material derived from plant, animal or mineral origin which offer nutritional value to plants.  In order for organics to work they must first be digested by microbes found in the soil and released in a form that plants can absorb through their roots.  This process has several benefits:

Improves Soil Structure - digestion of nutrients by microbes creates humus which enhances the soil's capacity to hold water, air and nutrients.

Long-Lasting, Slow Release - this process make take several months.  Microbial activity rises with soil temperatures ensuring that plants receive the proper amount of nutrition when they need it.

Safe to Use - organics are non-toxic and safe for the enivronment.  Organics contain secondary and trace nutrients often not contained within inorganic fertilizers.

Won't Burn - organics have very low salt content which lowers the risk of "burning" plants during drought or through over-fertilization.

Won't Leach Out of Soil - plants only absorb nutrients when they need it.  Unused inorganic fertilizers will leach out of the soil, organics however, do not leach out of the soil and are consistently available when the plants need them.

Organic Soil Amendments


Peat Moss:  Low in nutrients but is a good soil conditioner.

Manure:  Is both a soil conditioner and fertilizer.  Never use fresh...compost first.  Rabbit and poultry manure are twice as rich as that of horses, cows and pigs.  Sheep and goats sit somewhere between the two.  Good source of nitrogen.

Sewage Sludge:  Milwaukee's Milorganite is a good commercial fertilizer.  Not recommended for use around vegetables as they tend to be higher in heavy metals.

Leaf Mold:  Composted leaves are high in micronutrients and is a good soil conditioner.

Fish Emulsion:  High in nitrogen.  Can be absorbed through leaves or roots.  Also has positive effect on plant growth and overall plant health.  Can be a little smelly.

Bonemeal:  Contains mostly phosphorus.  Very slow acting.

Sawdust:  Works as a soil contitioner but ties up soil nitrogen until it becomes composted.  Do not use cedar, redwood or walnut sawdust as they contain chemicals which inhibit the growth of other plants.

Wood Ash:  Added to soils to raise a low pH.  Can be spread around plants to keep slugs at bay.

Blood Meal:  Nitrogen fertilizer.

Compost:  Excellent soil conditioner.  Compost creates carbon-dioxide which helps plants grow.
Feeling thrifty?  You can make your own fertilizer at home from some common household items.  Try some of the following recipes:

General Foliage Plant Food:
1/4 tsp instant tea
1/2 Tbsp ammonia
1/2 Tbsp hydrogen peroxide
1 capful bourbon
Put in 1 gallon warm water.
Label the container.  When using, mix 1 cup of this mixture with a gallon of water.  For flowering house plants, use vodka instead of bourbon.
Natural Plant Food:
5 drops liquid fish fertilizer
4 tsp instant tea
1/2 tsp liquid dish soap
Put in 1 gallon of warm water.
Label the container.  Use this mixture at full strength.

Balanced Chemical Plant Food:
4 tsp instant tea
1/10 tsp 15-30-15 fertilizer
1/2 tsp ammonia
1/2 tsp liquid dish soap
Place in 1 gallon of water.
Label the container.  Use this mixture at full strength.