Fertilizers improve the supply of nutrients in the soil, directly affecting
plant growth. Soil amendments improve a soil's physical condition (e.g.
soil structure, water infiltration), indirectly affecting plant growth.
The terminology used in describing fertilizers and soil amendments is
confusing. The definitions below explain some common concepts.
Synthetic fertilizers vs. natural organic fertilizers
Quick release vs. slow release fertilizers
Fertilizer-pesticide combination products
Organic soil amendments modify the soil structure as they decompose, allowing
it to absorb and retain water and nutrients more efficiently. Soil-borne
insects, worms, fungi, and other organisms help decompose organic material,
but require energy from available nitrogen in the soil to do so. For this
reason, the soil must often be augmented with nitrogen when undecomposed
(not composted) organic amendments are applied. Many products classified
as natural organic fertilizers are also organic soil amendments.
Examples: hay, straw, peat moss, leaf mold, and sawdust
Gypsum is an example of a nonorganic amendment added to soil to improve
water infiltration on high-sodium soils.
fertilizers vs. natural organic fertilizers
fertilizers are chemically manufactured materials containing one or more
of the primary nutrients necessary for plant growth: nitrogen, phosphorus,
Natural organic fertilizers are derived from either plant or animal products
containing a significant quantity of one or more of the primary nutrients
necessary for plant growth: nitrogen, phosphorus, and potassium. Nutrient
content must be labeled on the package. Most natural organic fertilizers
also provide significant quantities of organic matter, so they can also
be classified as soil amendments.
Examples: manures, sewage sludge, and bone meal
release vs. slow release fertilizers
Quick release fertilizers, also known as "fast-acting" fertilizers,
are water soluble chemicals that once applied are readily available to
the plant. If properly applied, plants green up quickly. However, if too
much is applied, quick release fertilizers have a tendency to burn turf.
These materials are easily leached with rain or over-irrigation and require
frequent application. They are the least expensive fertilizers and are
always synthetic products.
Examples: ammonium nitrate, ammonium sulfate, calcium nitrate, and urea
Slow release fertilizers, sometimes called water-insoluble types, release
nitrogen over time. They are often applied at higher rates and less frequently
than quick release formulas. The initial response of turf is slower than
quick release types but these materials continue to provide nutrients
over a period of 8 to 10 weeks or more. They will not burn the turf even
if applied at high rates and are not prone to leaching. These products
are a little more expensive and include certain synthetic fertilizer products
and all natural organic fertilizers.
Examples: sulfur-coated urea, urea formaldehyde, Isobutylidene diurea (IBDU),
and organic fertilizers
Some fertilizers are formulated with both water insoluble (slow release)
and water soluble (quick release) nitrogen. Plants green up quickly and
continue to receive nutrients over a period of time. For the most effective
product, at least one-fourth of the nitrogen should be in the water insoluble,
or slow release form.
Some fertilizer products contain a herbicide or insecticide for weed
or insect control. However, pesticides are often not warranted at the
time fertilizer is applied. Avoid these combination materials and apply
pesticides separately and only if needed.