As shown in Table 6, the pH significantly increased from 4.9 in the
control to 6.0 at 20 t RHA ha21 both at 4 and 12 WAP. Therefore, use of
20 t RHA ha21 was accepted as the economical optimum level on this
tropical Ultisol (Acid Sands) to achieve optimum pH response. Similarly,
Seripong (1988) obtained the best plant growth with 25 t ha21 burned rice
husk or 114 kg P ha21 on an acid soil in Thailand. He also concluded that
P and RHA improved nodulation, yields, the soil pH, organic matter
content, and cation exchange capacity (CEC) of the Thailand acid soils.
RHA is reported to be very economical source of carbon, and it produces a low-cost alternative to cement in conjunction with lime (Nasima et al. 1996;
Smith 1985).