Agriculture Reference
In-Depth Information
Table 7.6 Phosphorus levels in the soil and runoff water (rainfall simulations) from field trial
treatment plots after the fifth vegetable crop. Within columns values followed by the same letter
are not significantly different at p = 0.05; ** p < 0.01: *** p < 0.001. (Adapted from Chan et al. 2010 )
Treatment
Soil Colwell P
Soil CaCl 2 P
Soil total P
Runoff total P
Runoff
soluble P
mg kg −1
mg L −1
Farmers practice
—low P
235b
17.6ab
800b
14.5ab
2.88c
Compost (125 t ha −1 )
—low P
99c
3.2cd
600c
4.6c
0.53d
Mixed (62.5 t ha −1 )
—low P
116c
3.5cd
500cd
10.4bc
0.82d
Nil input control
26d
0.5d
300d
4.1c
0.07e
Significance
***
***
***
**
***
farmer practice treatment, and that this translated into significantly lower levels of
soluble P in the runoff water (Table 7.6 ). On this basis, Chan et al. ( 2010 ) concluded
that replacing poultry manure with blended garden organics compost could pose
less risk to water quality from vegetable production systems. But nevertheless, soil
P levels can also build up over time with such large applications of blended garden
organics compost (i.e., 125 dry t ha −1 ), and this in the end becomes the environ-
mental limit for this system when using low contaminant compost. It is also worth
noting that the second compost applied in this experiment (Table 7.1 ) had almost
double the amount of total and available P which was believed to be due to a higher
proportion of poultry manure in the compost blend (i.e., around 20 % instead of
the 10 % for compost 1), and this compost application resulted in a proportionally
larger increase in soil P levels (Dougherty and Chan 2014 ). Thus, the soil P level
results from the CROA experiment indicate that although such large applications of
compost have great value in rejuvenating soil quality, environmental considerations
suggest that their application at such rates should not be done on a continual basis,
but rather as an occasional treatment to rejuvenate soil quality. Although, as with
all farming systems, the soil nutrient levels and properties need to be monitored
and adjustments made to inputs accordingly. The effect of a large initial application
of compost followed by regular smaller targeted applications (e.g., along the plant
lines in beds) should be explored further.
It is apparent from the level of exchangeable K in the compost treatment soil at
the start of the first crop broccoli (Table 7.3 ) that the blended garden organics com-
post product supplied large reserves of available K to the soil raising it to 1.2 cmol
(+) kg −1 soil from its original level of 0.29 cmol (+) kg −1 soil (Table 7.1 ). However,
by the fourth crop of this experiment (Table 7.3 ), the substantial reserves of K that
were present in the soil at the start of the experiment for the compost treatment were
almost halved to 0.62 cmol (+) kg −1 . This loss of K reserves reflects the susceptibil-
ity of K to leaching when applied at large rates (Huang 2005 ). As such, it does raise
the question of whether applying compost at such large rates is the most efficient
way of using the nutrient reserves within the compost product, from a nutrient use
efficiency perspective.
 
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