What Fertilizers Are And What They Do

Gardener’s Supply has the privilege of serving Kern County.  Because of this we are able to custom blend fertilizers for our soil conditions, unlike box stores that sell the same generic fertilizer all across the country. When you see a bag of fertilizer there will always be three numbers on the label.  These numbers tells you the balance of nitrogen, phosphorus, and potassium contained in the fertilizer.


(The first number) is necessary for leaf growth and green color.  There are several different types of nitrogen which are broken into 3 main categories.  These categories are fast release, slow release, and immediate release.

The most common sources of nitrogen are urea and ammonium sulfate. These are “fast release” nitrogen sources because they are broken down readily by soil bacteria when the soils are warm.  These fertilizers usually last only 4 weeks and give spurts of growth.  The use of fertilizers with all fast release nitrogen will promote fungus diseases when the daytime temperatures are above 90 degrees.

A second source of nitrogen is sulfur-coated urea (S.C.U.).  This is a “slow release” nitrogen – breaking down slowly in warm soils; it lasts twice as long as “fast release” nitrogen, giving green color with slow growth.   These fertilizers can last 8-10 weeks. Because this source of nitrogen gives a slow constant green, rather than a spurt of growth, fewer applications are needed which means fewer clippings to bag.

Lastly, there is calcium nitrate, which is a “immediate release.”  This source of nitrogen is used in winter blends of fertilizers.


(The second number) deals with rooting, fruiting, and flowering.  It is necessary for stem and root development as it lengthens and thickens the roots. It is necessary for flower development in ornamentals as well as fruit and vegetable production in plants and trees.

In Bermuda grasses, phosphorus promotes lateral branching to thicken thin areas and cover bare spots.


(The third number) is necessary for thickening the cell wall of the plant.  The cell wall is necessary for plant health and disease prevention. Potassium also enables the plant to metabolize nitrogen and phosphorus.  So when there is little potassium usually poor growth and general decline in plant health is a result.

Potassium in fertilizers comes from two sources: potassium chloride or potassium sulfate.  The vast majority of fertilizers are formulated with potassium chloride (also called “muriate of potash”) because it is a cheaper form of potassium. However, using potassium chloride means the fertilizer is adding chloride to your soil.  Chlorides are toxic to plants and contribute to the alkalinity of soil.[1] Alkaline soil causes poor growth, browning of shrub and tree leaves, and in extreme cases, it can cause the death of plants.

Knowing this we custom blend the fertilizers we sell so that they contain potassium sulfate.  Sulfate (a sulfur derivative) is a plant nutrient, the fourth most important nutrient to plants. Sulfates help the plant metabolize fertilizer and does not contribute to the buildup of alkaline or salts in the soil.  The advantages of sulfates over chlorides has been demonstrated time and time again showing that a sulfate base will outperform chlorides every time.


 There are different fertilizers for different plants in different situations.  A maintenance fertilizer should be a 4-1-2 or 3-1-2 ratio (such as 20-5-10 or 16-8-8).  For shrubs, flowering plants, and a corrective lawn fertilizer, a 1-3-3 ratio is best (such as 7-20-20).  Trees are special; they need fertilizer with a 1-1-1 ratio (such as 10-10-10).

Gardener’s Supply also adds an iron sucrate to all our fertilizer blends, as iron helps turn a plant green.  Iron sucrate is a non-staining form of iron, unlike iron chelate which will stain sidewalks.

[1] Which is problematic as soil in Kern County is already alkaline.

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