By Darbie Granberry
University of
Georgia
If you want to make the most of your garden, you’ll need to make
the best use of fertilizers, including compost and other organic
products.
Fertilizers fall mainly into two classes: organic or
inorganic.
Inorganic fertilizers, sometimes called synthetic or mineral
fertilizers, are store-bought mixtures of inorganic nutrients
such as nitrates, phosphates and potassium.
They have much higher concentrations of plant nutrients than
organic fertilizers. The nutrients in them are released into the
soil soon after you apply them, too.
Organic fertilizers come from plants or animals. The plant
nutrients in them have been taken up by living plants or animals
and chemically bound in their tissues and by-products.
Slow release
Because they’re integrated into complex organic molecules, these
nutrients are in relatively low concentrations. They’re not water
soluble. As a result, plants can’t take them up as soon as you
apply them.
Organisms in the soil have to break down the organic matter
before the bound nutrients are released for plants’ roots to take
them up.
Besides the slow-release nutrients, organic fertilizers typically
have generous amounts of helpful organic matter.
Compost is an excellent organic fertilizer for your garden.
Simply put, compost is decayed organic matter. Through the
composting process, plant and animal materials are broken down
into smaller particles. The final product is 35 percent to 45
percent organic matter. It resembles potting media.
What’s in it?
The nutrient content varies, depending on what material was used
and exactly how it was composted.
Generally, though, it’s 1 percent to 2 percent nitrogen, 0.2
percent to 1 percent phosphorus, 0.5 percent to 1.5 percent
potassium and 0.05 percent to 2 percent calcium. Besides these
nutrients, compost also contains small amounts of micronutrients
such as boron, copper, manganese and zinc.
If they aren’t composted, organic fertilizers from animal manure
can harbor weed seed and pathogenic organisms.
Because of the heat generated during composting, though, compost
is free of viable weed seeds. This helps keep troublesome weeds
from getting a start in your garden.
Safety factor
Perhaps even more important, the high heat phase of composting
destroys plant and human pathogens. Getting rid of these critters
helps safeguard the health and well-being of you and your garden
veggies.
Nutrients in inorganic fertilizers are released into the soil
quickly. But they also start leaching from the soil as soon as
you apply them.
Heavy rains or watering early in the season can move nutrients
deep down, beyond the reach of plant roots. Unless you apply more
fertilizer, plants become stunted. This lowers the quality and
yields of your garden crops.
Compost releases its nutrients slowly, over many months. This
keeps them from leaching and makes sure they’re available to
plants all season long.
Other benefits
Compost has a lot of organic matter, which improves garden soils
and helps your garden vegetables grow better. Besides providing
its nutrients over a long time, organic matter:
- Improves soil structure and reduces soil compaction.
- Increases soil aeration.
- Helps soil to hold moisture and nutrients.
- Supports beneficial soil organisms.
Don’t rule out the use of inorganic fertilizer.
You usually have to apply organic matter many times over several
years to build up the soil’s organic content and nutrient
reserves. Until then, most gardens will do better if you apply
both organic and inorganic fertilizer.
Moderate amounts of inorganic fertilizer can be especially
helpful in getting garden vegetables off to a good start. As
always, know the nutrient content of the product you use. And
base your application rates on your plants’ needs.
(Darbie Granberry is a horticulturist with the University of
Georgia College of Agricultural and Environmental Sciences.)
