SITE-SPECIFIC NUTRIENT MANAGEMENT (SSNM)
Potassium (K) addition plot technique
Attaining high rice yield requires a balanced supply of N, P, K, and micronutrients. However, rice farmers often do not apply sufficient fertilizer K to replace the K removed with harvested plant parts, which can lead to depletion of the K fertility of soil and loss of yield due to insufficient supply of K. As such, farmers need to know whether the supply of plant-available K at a given location is adequate for targeted high yields. The K addition plot technique is a simple tool to assess whether farmers should apply more fertilizer K to attain high yield and profit.
The K addition plot technique involves broadcasting fertilizer K, as muriate of potash or KCl, to a small plot within the farmers’ fields. The grain yield from the K addition plot is compared to the yield from an adjacent plot in farmers’ fields, managed similarly but without adding more K than what is usually used by the farmer. Higher yield in the K addition plot indicates a deficiency of K, and highlights the need to apply additional K fertilizer to achieve high yield.
K addition plots should ideally be placed in fields with a history of little or no K application and a low amount of rice residue incorporation. Lighter textured soils are likely to be K-deficient. Hence, these types of soils, if present, should be included among the selected locations for K addition plots.
Layout and installation
1. Select locations in farmers’ fields with the following characteristics:
a. A relatively representative level of soil fertility and history of previous fertilizer use.
b. Relatively good management of the crop and use of fertilizer N and P by the farmer.
c. No shade or effect from adjacent trees.
2. Immediately after the farmer transplants or direct-seeds rice, measure a 5 x 5 m (25 m2) area in the field for the K addition plot (Fig. 1). Place bamboo or wooden stakes on the four corners of the plot.
3. Surround the plot with a small soil bund with a height sufficient to prevent movement of surface water inside and outside the plot. For transplanted rice, do this about 14 days after transplanting; for direct-seeded rice, do this 10–14 days after sowing rice.
4. Uniformly broadcast KCl (60% K2O) within the plot about 14 days after transplanting or 10-14 days after sowing wet-seeded rice. The added KCl should be enough to supply at least 30 kg K2O ha−1. (A higher rate of K2O can be used at locations where severe K deficiency is suspected.)
5. After 4 days, place channels in the soil bund (i.e., break the soil bund) to allow water to freely flow in and out of the K addition plot. The plot would then receive irrigation water as part of the normal practice of irrigating the farmer’s field.
6. Within 20 days after transplanting and sowing rice, identify a representative 5 x 5 m area in the farmer’s field at least 10 m away from the K addition plot. This area must have the same cropping and fertilizer history as the area where the K addition plot is placed. Mark this area with bamboo or wooden stakes. It will be used for measuring rice yield produced from the farmer’s K management practice.
7. At early panicle initiation, repair the soil bund around the K addition plot with a height sufficient to prevent movement of surface water inside and outside the plot.
8. Uniformly broadcast KCl at 30 kg K2O ha−1 within the K addition plot.
9. After 4 days, place channels in the soil bund (i.e., break the soil bund) to allow water to freely flow in and out of the K addition plot. The plot would then receive irrigation water as part of the normal practice of irrigating the farmer’s field.
10. Other than for K application, manage the rice within the K addition plot exactly as in the adjacent farmer’s field throughout the growing season.
Management of K addition plots
Farmers use their own field management and irrigation operation practices. All the farmer’s field operations and fertilizer applications in the K addition plot must be identical to the adjacent farmer’s field and to the area designated for the measurement of yield in the farmer’s field. The K addition plot will receive an extra application of at least 60 kg K2O ha−1 in addition to the farmer’s K application rate.
Determination of crop yield
1. At crop maturity, harvest the central 5-m2 area in the K addition plot (Fig. 1).
2. Likewise, harvest a representative 5-m2 area in the designated area outside the K addition plot (Fig. 1).
3. Make sure that all the grains in the 5-m2 areas are collected, and harvested samples are properly placed in labeled bags.
4. Thresh and clean the harvested grain samples.
5. Sun dry the grain sample to constant weight.
6. 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
7. Determine the K response:
K response = Grain yield in K addition plot – grain yield in farmer’s K practice
Determination of filled and unfilled grain
1. At crop maturity, take four representative panicles from plants immediately surrounding the central 5-m2 harvest area in the K addition plot and the plot with farmer’s K management (Fig. 1).
2. Strip all spikelets, both filled and unfilled, from the panicles and place in a tray or container. Perform this procedure separately for both K addition plot and farmer’s K management practice.
3. Separate and count the total number of filled (FSpNo) and unfilled spikelets (UFSpNo).
4. Calculate filled spikelet percentage using:
[FSpNo/(FSpNo+UFSpNo)] x 100
Fig. 1. Illustration of the layout for a K addition plot and harvest areas for determination of grain yield and grain filling in a farmer’s field.
Site Specific Nutrient Management
