Environmental Engineering Reference
In-Depth Information
products dominate organic fertilizer markets. They are safe and easy to
use. Municipal sewage is first dried, then heated and finally granulated.
The finished product is usually free of pathogenic bacteria and diseases
and seldom has an unpleasant odour. Sometimes sewage-based products
contain excessive levels of heavy metals, especially if produced in regions
with heavy industrialization. Only reputable products from frequently
monitored sources should be applied. Animal manure, blood meal,
poultry feathers, plant by-products and numerous other fertilizers are
produced around the world. Even though nitrogen and other nutrients
from natural fertilizers are substantially more expensive than those from
synthetic fertilizers, their usage has steadily increased.
Synthetic organics
Urea is a low-cost nitrogen source for turf-grass and is the most widely
used. Commonly used granulated urea is a fast-release, completely sol-
uble product. Granulated urea looks similar to ammonium nitrate and
contains even more nitrogen, but as an organic compound it has much
less potential to burn turf-grass leaves. After application, it dissolves in
the soil water but cannot be taken up by plants before the enzyme urease,
which is naturally present in the soil, breaks urea's molecules and re-
leases ammonia. Ammonia reacts with water, forming ammonium ions
(NH
4
). Ammonium ions are held by soil colloids and in this form are
available for plants. Unfortunately, part of the ammonia can be lost from
the soil in its gaseous form through the process of volatilization. Some-
times leaching also occurs before dissolved urea is converted to ammonia
ions. Volatilization losses occur especially in sandy soils and leaching
under high-rainfall (monsoon) conditions. Even though urea's leaf-burn-
ing potential is lower than those of inorganic carriers, it is still a concern.
To reduce leaf-burning potential, as well as volatilization and leaching
losses, fertilizer researchers have developed numerous ways to slow
down the release of nitrogen from the urea.
One major approach is to combine urea chemically with some other
relatively inexpensive compound. Another is to encapsulate urea pellets
within less permeable material.
Examples of products that have resulted from the first approach are
urea formaldehyde (also called UF), triazone and isobutylidine diurea
(called IBDU). Examples of encapsulated products are sulphur-coated
urea, resin-coated urea and combinations of the two (Fig. 5.5). Most of
the urea products described below are used frequently, and the majority
of them are commercially available in most countries. Unlike urea or
ammonium nitrate, these types of fertilizers are sold under particular
trade names, which may differ in different countries. The fertilizer label
