Geoscience Reference
In-Depth Information
11.3
Plants' Needs for Individual Nutrients
Nitrogen
, N, is frequently a limiting plant nutrient. However, it is not an all-
embracing general rule. For example, the vegetative growth of tomatoes, cucum-
bers, squash, and melons will react to an excess of nitrogen at the expense of
fruiting. Trees and shrubs have a relatively low need for soil nitrogen. On the other
hand, potatoes, corn, and vegetables like cabbage, broccoli, and caulifl ower are
heavy feeders and benefi t from high soil nitrogen levels. Nitrogen is used by plants
in two forms, as
ammonium
(NH
4
+
) and
nitrate
(NO
3
−
). They are needed for making
amino acids from which proteins are subsequently produced.
The principal transformations of N forms are described as nitrifi cation and deni-
trifi cation. In the fi rst step of nitrifi cation, special oxidizing bacteria use oxygen to
transform ammonia into nitrite. In the second step, the nitrite is oxidized into nitrate
mainly by
Nitrobacter
bacteria. Nitrifi cation is important in transferring nitrogen
into the form dominantly used by plants. Its use is realized if the soil water contains
adequate amounts of dissolved oxygen.
Because ammonium is positively charged, it is attracted to negatively charged
soil particles, mainly clay minerals (see Sect.
5.2.2
). Being less soluble than nitrate,
it is more resistant to leaching (movement down through the soil profi le). Nitrate,
being negatively charged, readily leaches below the root zone with excess rain or
irrigation on sandy soils. We prevent water pollution by avoiding overfertilization of
nitrogen, particularly on sandy soils.
If oxygen is absent, an opposite process prevails - nitrates are reduced to nitrites,
next to ammonia, and then to nitrogen gas. This reduction of nitrates to gaseous
nitrogen by microorganisms in a series of biochemical reactions is called denitrifi -
cation. This process is wasteful because plant-available soil nitrogen is lost to atmo-
sphere. In soils with high organic matter and anaerobic soil conditions (waterlogged
or ill drained), the rate of denitrifi cation is even more wasteful.
Denitrifi cation leads to the loss of nitrogen from soil and reduces an essential
nutrient for plant growth. In relation to soil fertility and agricultural productivity, it
is an undesirable process. On the other hand, denitrifi cation is of major ecological
importance in view of the fact that without it, the supply of nitrogen including that
in the atmosphere would start to become depleted.
Although soil microorganisms release nitrogen bound in the organic matter of
plant residues and decayed roots, the release rates are very slow. Therefore, rates of
nitrate availability from this source are not adequate for modern-day crop produc-
tion that is now practiced in the way prognosed by the German chemist Justus von
Liebig. Together with ammonifi cation, nitrifi cation forms a mineralization process
that refers to the complete decomposition of organic material with the release of
available nitrogen compounds. This decomposition replenishes the nitrogen cycle
(Fig.
11.4
).
Phosphorus
, P, is one of the primary nutrients in plant growth. It occurs in four
different aqueous ionic forms surrounded by tetrahedral arrangements of four atoms
of oxygen with their eventual one or two or three bonds to hydrogen. The forms and
