Soil physical properties and grain yields of lowland rice in response to sawah preparation intensities and soil amendment types

Sawah, an Indo-Malaysian word for rice paddy, is used in West Africa to refer to a lowland rice field that is bunded, puddled and levelled for ease of water control and management. There are insufficient data on lowland soil and rice responses to sawah preparation intensity, defined by relative completeness of these three land preparation activities (bunding+puddling+levelling). This study assessed soil physical properties and rice grain yield under sawah preparation intensities [complete (bunded, puddled, levelled), quasi-complete (bunded, slightly puddled, levelled) and incomplete (bunded, non-puddled/levelled) sawah, and control] and soil amendments [NPK, poultry droppings (PD), rice husk (RH), its ash (RHA) and control] in southeastern Nigeria for 3years. At rice maturity, soil bulk density increased with decreasing intensity of sawah preparation and was lower with than without amendments, being lowest in RH-amended plots. Aggregate sizes were adversely affected in the three sawah plots and by soil amendments, mainly NPK. Mean-weight diameter of aggregates was always highest in complete sawah, while showing lowest and highest values in NPK and PD plots, respectively. Sawah bunding/puddling and amendments positively affected soil water content. Saturated hydraulic conductivity was always highest in complete sawah plus PD and lowest in the control. Rice grain yield did not always increase with sawah preparation intensity but all sawah plots out-yielded the control after the first year. All amended plots always out-yielded the control, with higher values due to PD/RH compared with RHA/NPK in the third year. Soil physical control of rice grain yield revealed strengths (R-2, 0.847-0.420) in decreasing order of conductivity, bulk density/water content, <0.25-mm aggregates and mean-weight diameter. Only soil bulk density related inversely to grain yield up till a critical value of 1.05gcm(-3). Complete sawah or, where improved soil structure after rice is not desired, quasi-complete/incomplete sawah can be combined with organic amendments (PD/RH) to grow lowland rice.