SITE-SPECIFIC NUTRIENT MANAGEMENT (SSNM)
Nutrient Omission Plot Technique
Fig. 1. A nutrient omission plot study conducted in a farmer’s field. (Photo provided by Sarlan Abdulrachman.)
Background
The nutrient omission plot technique is a tool for determining the amount of fertilizer N, P, and K required for attaining a yield target (Fig. 1). In this technique, four 25-m2 plots with the following treatments are placed in a farmer’s field:
1. Full fertilization: NPK applied
2. N omission (–N): No N applied, PK applied
3. P omission (–P): No P applied, NK applied
4. K omission (–K): No K applied, NP applied
Fertilizer N, P, and K are applied at sufficiently high rates to ensure that yield is not limited by an insufficient supply of the added nutrients. Grain yield in the plot with full fertilization and relatively good crop management can be used to estimate an attainable yield target. Nutrient-limited yields are determined from plots in which the nutrient of interest is not added. For example, the N-limited yield is determined in an N omission plot receiving no N fertilizer but sufficient P and K to ensure that they do not limit yield. The K-limited yield is determined in a K omission plot receiving no K fertilizer but a sufficient supply of other nutrients.
The difference in grain yields between a fully fertilized plot and an N omission plot illustrates the deficit between the crop demand for N and indigenous supply of N, which must be met by fertilizers. Similarly, the difference in grain yields between a fully fertilized plot and a K omission plot illustrates the deficit between the crop demand for K and indigenous supply of K, which must be met by fertilizers.
Fertilizer N, P, and K must be applied at sufficiently high rates to eliminate limitations to crop growth. The addition of fertilizer N, P, and K is consequently adjusted based on the yield anticipated with full fertilization. More fertilizer N, P, and K are applied at higher anticipated yields (Table 1).
The rates of added N, P, and K in omission plots are completely unrelated to fertilizer practices of farmers, recommendations of extension, and the SSNM recommendation. They do not reflect a rate to be recommended for use by farmers.
Table 1. Fertilizer addition to omission plots based on an estimate of the yield attainable with full fertilization of N, P, and K.
|
Plot |
Fertilizer addition (kg ha−1) |
||||||||
|
Attainable yield: <6 t ha−1 |
Attainable yield: 6 to 7 t ha−1 |
Attainable yield: >7 t ha−1 |
|||||||
|
N |
P2O5 |
K2O |
N |
P2O5 |
K2O |
N |
P2O5 |
K2O |
|
|
NPK |
90 |
50 |
70 |
130 |
50 |
70 |
180 |
75 |
100 |
|
–N |
-- |
50 |
70 |
-- |
50 |
70 |
-- |
75 |
100 |
|
–P |
90 |
-- |
70 |
130 |
-- |
70 |
180 |
-- |
100 |
|
–K |
90 |
50 |
-- |
130 |
50 |
-- |
180 |
75 |
-- |
Nitrogen is typically applied as urea in equal doses at the following instances: Within 14 days after transplanting (DAT) or 21 days after sowing (DAS), at active tillering, and at panicle initiation. Phosphorus is applied basally as a fertilizer containing only P, such as single superphosphate (SSP) or triple superphosphate (TSP). Half of potassium is applied basally and half at panicle initiation as muriate of potash (MOP, 60% K2O). For rice-growing areas known to be deficient in zinc, broadcast zinc sulfate at about 5 kg Zn ha−1 to all four plots.
The four plots in the nutrient omission plot technique comprise one experimental unit. At least five experimental units or replications of the omission plots are typically distributed within a rice-growing area representing a relatively homogenous soil type or land management history. The grain yields obtained with the nutrient omission plot technique can then be used for developing location-specific N, P, and K management for the rice-growing area.
Layout and installation
1. Select a farmer’s field with a relatively representative level of soil fertility and history of previous fertilizer use.
2. Choose a location away from trees and edges.
3. Select five farmers’ fields to serve as replicates in a given location representing each soil type or rice-growing environments.
4. After land preparation and before transplanting or sowing rice, measure out four plots of 5 ´ 5 m (25 m2) area within a farmer’s field. Do not place the plots in a corner of the field. Place the plots near an irrigation canal to ensure that irrigation water flows directly into the plots rather than across plots or between plots (Fig. 2).
5. Construct soil bunds of sufficient height around each plot to prevent movement of water between plots. Also place another soil bund around each experimental unit of four plots to prevent contamination with water containing fertilizer from the farmer’s field adjacent to the plots.
6. Randomly assign the four omission plot treatments to the four plots. Apply preweighed fertilizers to the treatment plots at the rates and times indicated above.
7. If farmers use farmyard manure, apply the same amount as farmers to the omission plots. Ensure that farmers do not apply additional fertilizers or organic material to the omission plots.
8. Use high-quality seeds and establish rice using the common practice, either by transplanting or sowing, at the selected location. Use recommended nursery management practices for transplanted rice and recommended seed rates for wet-seeded rice.
9. Employ timely and effective practices for water, weed, and pest management.
Fig. 2. An illustration of the field layout for nutrient omission plots in farmer’s field.
Management of omission plots
Farmers can handle crop management operations from land preparation to harvesting. Research and/or extension workers must be involved to provide guidance in the management of the plots and expertise for the development of fertilizer recommendations. Research and/or extension workers should:
§ Assist in selecting the locations for omission plots.
§ Provide quality seed and preweighed packets of the required fertilizers.
§ Collect data as required.
§ Monitor and guide activities and ensure that crop management is properly implemented.
Determination of crop yield
1. At crop maturity, harvest the central 5 -m2 area in each treatment plot.
2. Make sure that all the grains in the 5 -m2 areas are collected and the harvested samples are properly placed in labeled bags.
3. Thresh and clean the harvested grain samples.
4. Sun dry the grain sample to constant weight.
5. Convert the sample weight to yield in t ha−1 by using the formula:
Yield (t ha−1) = [(Sample yield in kg)/5] x 10
6. Determine the N, P, K response:
N response = Yield in NPK – Yield in PK
P response = Yield in NPK – Yield in NK
K response = Yield in NPK – Yield in NP
Site Specific Nutrient Management
