do orGAnIc nutrIent sources reduce envIronMentAL IMPActs? It dePends!
It is commonly assumed that the use of organic nutrient sources always results in lower environmental impacts. This is general y true, but only if good management practices are followed. For example, in temperate climates a plowed alfalfa sod releases a lot of organic nitrogen that can easily meet all the needs of the following corn crop. But if the plowing is done too early—for example, in the early fall—much of the organic N is mineralized in the following months when the soil is still warm and then lost through leaching or denitrification over the winter and spring. A study in Sweden compared conventional and organic crop production and found similar nitrate leaching losses. Organic sources like manure may create a problem with nutrient runoff if left on the surface, or with leaching when applied in the fal . So, even when using organic nutrient sources, good agronomic management and careful consideration of environmental impacts are essential.
field may also result in significant nitrate leaching below
quantity of compost to a soil might supply a crop’s N
the zone that roots can reach.) Sediments lost by erosion
needs but serve to enrich the soil in unneeded P, creating
from fields fertilized with commercial fertilizers probably
a greater pollution potential.
will contain more available nutrients than those from
One of the drawbacks to organic materials is the
fields fertilized with organic sources, resulting in more
variable amounts and uncertain timing of nutrient
severe water pollution. Of course, soils overloaded with
release for plants to use. The value of manure as a nutri-
either inorganic or organic sources of nutrients can be
ent source depends on the type of animal, its diet, and
large sources of pollution. The key to wisely using either
how the manure is handled. For cover crops, the N con-
commercial fertilizers or organic sources is not applying
tribution depends on the species, the amount of growth
more nutrients than the crop can use and applying in
in the spring, and the weather. Also, manures typically
ways that minimize losses to the environment.
Organic sources of nutrients have many other good
orGAnIc fArMInG vs. orGAnIc
qualities, too. Compared to commercial fertilizers that
nutrIent sources
only “feed the plants,” organic materials also “feed
We’ve used the term “organic sources” of nutrients to
the soil.” They are also sources of soil organic matter,
refer to nutrients contained in crop residues, manures,
providing food for soil organisms that aid in forming
and composts. These types of materials are used by all
aggregates and humus. Organic sources can provide a
farmers—“conventional” and “organic.” Both also use
more slow-release source of fertility, and the N avail-
limestone and a few other materials. However, most
ability is frequently more evenly matched to the needs of
of the commercial fertilizers listed in table 18.1 are not
growing plants. Sources like manures or crop residues
al owed in organic production. In place of sources such
commonly contain all the needed nutrients, including the
as urea, anhydrous ammonia, diammonium phosphate,
micronutrients, but they may not be present in the proper
concentrated superphosphate, and muriate of potash,
proportion for a particular soil and crop; thus, routine
organic farmers use products that come directly from
soil testing is important. Poultry manure, for example,
minerals, such as greensand, granite dust, and rock
has about the same levels of N and P, but plants take up
phosphate. Other organic products come from parts
three to five times more N than P. During the composting
of organisms, such as bone meal, fish meal, soybean
process a lot of N is commonly lost, making the compost
meal, and bloodmeal (see table 18.2).
much richer in P relative to N. Thus, applying a large
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Building SoilS for Better CropS: SuStainaBle Soil ManageMent