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Initial Concept of SSNM
The concept of SSNM for rice was
developed in the mid-1990s and then evaluated from 1997 to 2000
in about 200 irrigated rice farms at eight sites in six Asian
countries. SSNM aimed at dynamic field-specific management of N,
P, and K fertilizers to optimize the supply and crop demand for
nutrients. The crop’s need for fertilizer N, P, or K was
determined from the gap between the crop demand for sufficient
nutrient to achieve a yield target and the nutrient supply from
indigenous sources. A modification of the QUEFTS model (Janssen
et al. 1990) was used to predict the amount of fertilizer N, P,
and K required for a specific yield target as follows:
a.
Establish a yield target for average climatic conditions.
This yield
target could be based either on a percentage (for example
70–80%) of the potential yield estimated with a crop growth
model or on yields currently achievable by farmers practicing
good crop management.
b.
Estimate crop demand for N, P, and K for a target yield.
Based on a
large database of modern rice varieties with harvest indices of
0.45 to 0.55, the balanced plant nutrient requirement to produce
a metric ton (1,000 kg) of unmilled rice was estimated as 15 kg
N, 2.6 kg P (6 kg P2O5), and 15 kg K (18 kg K2O) for the linear
portion of the relationship between grain yield and nutrient
accumulation in the mature crop.
c.
Estimate field-specific indigenous supply of N, P, and K.
The indigenous supply is the cumulative crop uptake of a nutrient
from all sources other than fertilizer (that includes soil,
crop residues, manures, irrigation water, rainfall, and
atmospheric deposition). It is determined by the
nutrient omission plot
technique, whereby the indigenous supply of a
nutrient is estimated by its accumulation in a crop not
fertilized with the nutrient of interest but fertilized with
sufficient amounts of other nutrients to ensure they do not
limit yield. Indigenous K supply, for example, is determined
in a K omission plot receiving no K fertilizer but
sufficient N and P to ensure they do not limit yield.
d.
Establish recovery efficiencies for fertilizer N, P, and K.
Crop recovery
efficiencies of 0.4 to 0.6 kg kg−1 for fertilizer N, 0.2 to 0.3
kg kg−1 for fertilizer P, and 0.4 to 0.5 kg kg−1 for fertilizer
K were used as targets.
e. Estimate
optimal N, P, and K fertilizer rates.
The estimated crop demand for N, P, and K to optimally
achieve the yield target; the estimated indigenous supply of
N, P, and K; and targeted recovery efficiencies for
fertilizer N, P, and K were used to determine optimized
fertilizer N, P, and K rates for filling the gap between
crop demand for a yield target and indigenous supply.
The
achievements in the initial conceptualization and development of
SSNM before 2001 are documented in a book by Dobermann et al.
(2004).
Evolution of SSNM (2001 to present)
From 2001 to 2004, the Reaching Toward Optimal Productivity (RTOP)
workgroup of the Irrigated Rice Research Consortium (IRRC)
collaborated with national agriculture research and extension
systems in eight Asian countries to systematically transform the
initial SSNM concept into an inclusive, simplified framework for the
dynamic plant-need-based management of N, P, and K.
The SSNM approach now enables:
·
Dynamic adjustments in fertilizer N, P, and K management to
accommodate field- and season-specific conditions.
·
Effective use of indigenous nutrients.
·
Efficient fertilizer N management through the use of the
leaf color chart
(LCC),
which helps ensure that N is applied at the time and in the
amount needed by the rice crop.
·
Use of the
nutrient omission plot technique
to determine the requirements for P and K fertilizers.
·
Use of micronutrients based on local recommendations.
References
Janssen BH, Guiking FCT, van der Eijk D, Smaling EMA, Wolf J, van
Reuler H. 1990.
A system for
quantitative evaluation of the fertility of tropical soils (QUEFTS).
Geoderma 46: 299-318.
Dobermann A,
Witt C, Dawe D (eds). 2004. Increasing the productivity of intensive
rice systems through site-specific nutrient management. Enfield, NH
(USA) and Los Baños (Philippines): Science Publishers, Inc., and
International Rice Research Institute (IRRI).
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