Agriculture Reference
In-Depth Information
) found fibrous
root concentrations in the wet core under the trickle nozzle to be increased by
four or five times compared with those in corresponding unirrigated (rainfed)
zones. This concentration of roots makes it likely that local nutrient reserves
will be depleted quickly and also offers the opportunity for very effective supply
to the tree through the trickle irrigation system (fertigation).
In fertigation the fertilizer may be added to the irrigation water in small
doses at frequent intervals (discontinuous fertigation) or by means of a fertilizer
pump that injects a steady fertilizer concentration (continuous or proportional
fertigation).
Because their chemical characteristics differ, mineral nutrients are not
equally distributed in the soil when supplied by trickle irrigation.
Nitrates and urea do not react with soil exchange sites and are not held in
soils, so, whether previously incorporated into the soil or added in the trickle
irrigation stream, move with soluble salts to the wetted front unless intercepted
by the roots (Elfving,
-
% of that irrigated by surface irrigation. Goode et al. (
). They are therefore
best provided in small doses on a frequent or 'proportional' basis. Nitrate status
in the soil at any time reflects addition in the irrigation water, removal by the
plant and losses from leaching and denitrification. The ammonium form of N
is not so subject to immediate leaching losses because it will fixtemporarily on
exchange sites in the soil. It is, however, practically immobile in high pH soils
with a high cation exchange capacity (CEC) and calcium carbonate content. It
is fixed very close to the soil surface under the drippers in such soils although it
can penetrate to a depth of
; Klein and Weinbaum,
cm in sandy soils. NH may also lead to clogging
of drippers as a result of forming precipitates with Ca or Mg salts. Nitrogen
use efficiency is generally greater in fertigated trees than in those with furrow
irrigation.
Potassium is less mobile than nitrate (Goode et al. ,
), but moves both
laterally and downwards and is subject to both binding and leaching. It is
best supplied by continuous fertigation. Supplied in this way it can correct K
deficiency more readily than by trenching large quantities of fertilizer into the
soil every few years.
There can be asymmetric distribution of P, Ca and K and, to a lesser extent,
Mg and N in the foliage of apple trees given fertigation in a humid zone climate
where trickle irrigation is essentially supplementary (Goode et al. ,
).
days using small doses of
FeEDDHA chelate in the fertigation water. Much less is needed than when
applied with conventional soil application and sprinkler irrigation (Klein and
Weinbaum,
Iron chlorosis can be corrected within
-
).
Nutrients not intercepted by the roots may leach and reach aquifers, e.g.
following winter rains, or may accumulate at the wetted front in very dry
climates, leading to salinity problems. Other potential problems include soil
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