Fertilizer placement in wetland rice

Paddy soils are characterized by two distinct layers: 1 ) a surface oxidized layer a few millimeters to a centimeter present at the soil interface; and 2) an underlying reduced layer (anaerobic) which is the principal zone of root development. The fate of applied N and its efficient use depend on where it is placed.

Paddy soils

Broadcasting ammonium-based fertilizer in the oxidized layer is an inefficient method of fertilizer application. In this method, nitrogen is lost by a combination of nitrification-denitrification, ammonium volatilization, leaching and surface run-off. As a consequence, a maximum of only 28% of the total applied nitrogen is eventually taken up by the plant.

Broadcasting ammonium-based fertilizer

DECREASING N-FERTILIZER LOSSES IN PADDY SOILS:

N-availability could be increased by applying fertilizer in soils without standing water or by deep placement into the reduced zone. The following are methods by which this could be accomplished:

1. Apply N in split. Broadcast the first split to a puddled field without standing water, thoroughly incorporating it into the soil and introducing water 4 days after transplanting. The final top-dressing of N should be made at 5-7 days before panicle initiation into floodwater not exceeding 5 cm. Thorough incorporation could also be done by using a rotary weeder.

Apply N in split

2. Another method is deep placement or applying fertilizers into the reduced zone. This method limits N losses and assures longer availability of nitrogen for the plant. A maximum of 68% of the total N applied can be obtained by the plant.

Applying fertilizers

Deep placement could be accomplished by manually placing fertilizer 10-15 cm deep from the soil surface and between alternate rows and hills after transplanting or just before or at panicle initiation. Prilled urea or a pinchful of urea may be used for this method. However, no data are available for urea. The mudball could also be used. This technique, developed by the Japanese, consists of covering a certain amount of urea with mud and then placing it 10-15 cm from the soil surface. Although the mudball technique is known to increase plant recovery of N. this has not been widely adapted because it is too labor-intensive.

A less laborious procedure is applying N fertilizer between rows right after transplanting or at panicle initiation and then incorporating it using a rotary weeder.