Agriculture Reference
In-Depth Information
14.3.5 FERTILIZATION METHOD AND NITRATE LEACHING
Reexamination of the nitrate concentration profiles in the vadose zone un-
der the greenhouses suggests categorizing the potential down-leaching of
nitrate according to the fertilization method rather than the general agro-
technical regime of organic versus conventional agriculture. As such the
differences in nitrate profiles may be attributed to solid versus liquid fer-
tilizer application. Solid fertilization refers to the application of organic
matter such as compost directly to the soil, primarily before planting,
as commonly practiced in organic agriculture. On the other hand, liquid
fertilization refers to application of fertilizers via the irrigation system
throughout the growing season (fertigation), as is commonly practiced in
conventional agriculture. Under these two categories, the differences in ni-
trate concentrations in deep sections (>1 m) of the vadose zone were much
more significant (P = 0.0002), showing a tremendous increase in nitrate
concentration with depth in farms relying on solid fertilizer application
(Fig. 8). Examination of the total nitrogen (TN) amount that was observed
in the sediment samples from the upper section of the vadose zone un-
der all of the greenhouses, organic and conventional, showed very simi-
lar values. Note that among the main nitrogen forms in the soil (nitrate,
ammonium and organic-N), nitrate is much more mobile compared with
other forms. Moreover, under aerobic conditions, which typically prevail
in the vadose zone of semi-arid regions, nitrate is considered stable, and
therefore, its transport across the vadose zone to groundwater persists.
Ammonium and nitrite concentration in pore-water samples were either
negligible or below detection limit and therefore not discussed here.
The observations of enhanced down-leaching of nitrate under intensive
farms that rely on solid fertilizers may be attributed to unsynchronized
nutrient release from the compost to the soil with respect to the nutri-
ent uptake capacity of the plants. During the early stages of the grow-
ing season, an excess of irrigation water, with respect to the plant water
and the nutrient uptake, creates a nutrient surplus in the soil (Pang and
Letey, 2000). At this stage, the soil water content remains high as does
the nutrient concentration, which is released from the compost to the wet
soil. However, in the early stages of the growing season water and nutri-
ent consumption by the undeveloped roots of the young plants is limited.
