Effect of Rice Straw and Rice Hull Ash Addition on the Efficiency of Urea Broadcast to a Lowland Rice Field

Abstract

Field experiments were conducted during consecutive dry and wet seasons in 1998 under a rice-fallow-rice cropping seqence at the Suphanburi Rice Experimental Station to study the effect of rice straw (C/N = 67) and rice hull ash (C/N = 76) application on the efficiency of urea broadcast to lowland rice field after transplanting at a rate of 70 Kg N ha^-1. The ¹⁵ N balance data at grain maturity for the dry season rice showed that fertilizer ¹⁵N recovered by grain from the urea + straw treatment (4%) was significantly lower than those from urea applied either alone (11%) or in combination with hull ash (11%).However, the amounts of ¹⁵N recovered by the straw and roots were similar to all treatment, ranging from 5 to 8% and 0.26 to 0.40%, respectively. After the harvest of dry season rice, 27-36% of applied N still remained in the soil, locating mainly in the top 0-0.05 m layer below the soil surface. The quantities of unaccounted-for which presumed to be lost as gaseous N emissions from the soil-plant system ranged from 4 to 54 % of the applied N, with no difference among treatment. During both fallow season NO₃-N remained the dominant from of extractable inorganic-N in the soil, The residual fertilizer urea applied to the dry season rice crop could be utilized by the succeeding rice only less than 3% from the original application. In the dry season grain yield responsed to N application was significant (P= 0.05) Organic material sources either straw or hull ash did not significantly change grain yield and N accumulation in rice. In terms of grain yield and N uptake by rice, no significant residual effect of urea applied to rice in the dry season was seen on succeeding rice in the wet season.