Agriculture Reference
In-Depth Information
The field trial records suggest that the Evanylo and Sherony (
2002
) assump-
tion of 10 % of the total N in compost being available for the first crop, provided a
reasonable estimate of the plant available N (PAN) supply for the blended garden
organics compost (i.e., composted blend of 90 % garden organics and 10 % chicken
manure) used in this field trial, as evident in the petiole sap test results and crop
monitoring for crop 1. In addition, the full compost and the incorporated crop 1 resi-
dues provided sufficient PAN supply for crop 2 as well, for the full compost treat-
ment. For the repeat application of 125 dry t ha
−1
of compost after the harvesting of
crop 5, sufficient PAN was supplied for the first crop (i.e., crop 6 capsicum) only,
with supplementary inorganic N (urea) required for all subsequent crops. How-
ever, it is worth noting that the compost treatments only received half the available
N fertiliser that was applied to the farmer practice treatments (i.e., only the same
amount of urea as the farmer practice treatments, but with no poultry manure N) and
this represented an N fertiliser use efficiency gain in the compost treatment rela-
tive to farmer practice. This may have been due to improved N cycling by the soil
microorganisms in the compost treatment soils. However, the PAN supply for this
compost product was dependent on prompt incorporation of the compost immedi-
ately following delivery and spreading, so as to minimise N loss to the atmosphere
via ammonia volatilisation. At some of our demonstration sites where the same
product was used but was not immediately incorporated into the soil, supplemen-
tary inorganic N fertilizer was required for the first crop. Other demonstration tri-
als using composts of predominantly garden organics (i.e., > 90 % garden organics
green-waste), revealed significant N immobilisation in the soil (i.e., 'N drawdown')
throughout the early crop phases and a real need for inorganic N fertilizer. For these
composts it is best to assume negligible PAN supply and a need for supplementary
inorganic N fertilizer. In contrast, studies have found that composts derived from
largely vegetable food waste (e.g. source separated municipal solid waste compost)
or animal manures (composted broiler litter) generally have been found to yield
> 10 % of their total N as PAN for the first crop (Pratt and Castellanos
1981
; Sims
and Stehouwer
2008
). This is generally correlated with their total N content and
their C/N ratio which usually reflects differences in their molecular composition
which influences their decomposition and N mineralisation (Eldridge et al.
2013
).
Given the difficulty in predicting the mineral N or PAN supply from composts,
sound advice for compost use in agriculture and horticulture, would be to always
monitor crop condition carefully, and be ready to apply supplementary inorganic N
fertilizer when required. The use of inorganic N fertilizer strips (i.e., applying inor-
ganic N fertiliser at the recommended rate to a small area of the field) is also highly
recommended as a strategy for the early detection of 'N drawdown' or inadequate
N supply symptoms in crops in fields receiving compost. This practice can allow an
early response to crop N deficiency with inorganic N fertilizer applications and as
such minimise the risk of 'N drawdown' impacts on crop yield.
