3.5 Fertilization method and nitrate leaching
Reexamination of the nitrate concentration profiles in the vadose
zone under 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 fertilizer 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 nitrate 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 under all of the greenhouses,
organic and conventional, showed very similar 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