Agriculture Reference
In-Depth Information
16000
0.7
14000
0.6
12000
0.5
10000
0.4
8000
0.3
6000
0.2
4000
Control
188 SSP
376 SSP
0.01M CaCl
2
-P
0.1
2000
0
0.0
0
10
20
30
40
Olsen P (mg/L)
Current Opinion in Biotechnology
Fig. 4.1 The gap between the agronomic optimum in Olsen P for pasture production (
green up
arrow
) and a potential threshold in Olsen P for P loss in subsurface drainage (as estimated by
0.01 M CaCl
2
-P;
red down arrow
) shows there is little justification in exceeding the agronomic
optimum. Data are from plots receiving different rates of superphosphate (SSP; kg ha
1
year
1
)in
a trial in Canterbury, New Zealand (Data from McDowell (
2012
) and used with permission)
recovered by the plant. P
i
transfer from the soil to the root proceeds mainly by
diffusion rather than mass flow, with slow diffusion rates at around 10
15
ms
1
and
a concentration gradient as the driving force towards the roots (Hinsinger
2001
;
Rausch and Bucher
2002
) resulting in a depletion zone of 1-2 mm around the root
(Jungk
2001
). Soil P
i
concentrations are extremely low, being generally
<
10
μ
M
and typically around 2
M (Bieleski
1973
; Barber
1984
; Holford
1997
), whereas in
plants, concentrations of over 40 mM can be achieved (Bollons and Barraclough
1997
). Hence, plants take up P
i
faster than it is supplied by diffusion (Bieleski
1973
).
Soil pH, buffer capacity, soil moisture and soil structure affect P
i
solubility and
sorptivity (Holford
1997
; Syers et al.
2008
); P
i
can be absorbed on the surface of
clay minerals, Fe- and Al-hydrous oxide surfaces and organic matter complexes or
be fixed in acidic soils as Al-/Fe phosphates or Ca/Mg-phosphates in alkaline soils
(Barber
1984
; Bahl and Singh
1986
; Holford
1997
; Hinsinger
2001
).
Significant amounts of soil phosphorus (20-80 %) is bound in organic forms
such as nucleic acids, phospholipids and predominately monophosphate esters, as
phytic acid and derivatives (Richardson
1994
). These organic forms have to be
mineralised and/or solubilised into inorganic forms in order to be available for
plants; a process which is either microbiological or plant mediated.
μ
