Author : Sisavanh Xayavong

Summary : The objectives of this study were (1) to estimate the effects of long-term (20 years) yearly application of different types of organic residues on soil chemical properties; and (2) to investigate the effects of combined use of organic residues and chemical fertilizer on soil chemical properties, nutrient availability, and plant growth.

The research was based on a long-term field experiment consisting of two studies: Study 1 used soil samples from the long-term field experiment, while Study 2 was on plant bioassay from a pot experiment based on the use of soils from the 20-year field experiment. In Study 1, there were five plant organic residue treatments with differences in concentrations (g kg-1) of N, lignin (L), polyphenols (PP) and cellulose (CL). The residue treatments were: (1) unamended (control, CT); (2) rice straw (RS) (4.9 N, 26 L, and 8.3 PP); (3) groundnut stover (GN) (20.9 N, 69 L, and
12.3 PP); (4) dipterocarp freshly fallen leaf litter (DP) (6 N, 246 L, and 69.3 PP); and
(5) tamarind freshly fallen leaf and petiole litter (TM) (11.9 N, 161 L, and 38.8 PP). A randomized complete block design (RCBD) with three replications was employed. Soils used were from years (Y) 2, 6, 11, 16, and 20. In Study 2, a two-way factorial arrangement (five-residue quality factors x two-chemical fertilizer factors) in a completely randomized design (CRD) with four replications was used. The residue quality factors were five residue treatments obtained from the soils of the long-term (20 years) experiment, while the chemical fertilizer factors were: no added chemical fertilizer (-F) and added fertilizer (+F) at the recommended rates (70.3 kg ha-1 N,
30.7 kg ha-1 P, and 58.4 kg ha-1 K).

In Study 1, soil organic carbon (SOC) content under the GN, DP, and TM treatments showed an increasing trend during the intermediate period (Y 2, 6, 11, and 16) when compared to the initial content before residue application (Y 0). In comparison, among the residue treatments, TM was associated with the highest SOC contents, which was not significantly different from those of GN. RS was associated with the lowest SOC content throughout the 20-year period. In relation to soil organic nitrogen (SON) content, GN and TM (the leguminous residues) showed trends of increasing SON content with time. These two leguminous residues were also associated with significantly higher SON content (0.33 and 0.30 g kg-1, respectively) than the other residue treatments, at the end of the study (Y 20). SOC accumulation was dependent on the initial ratio of L/N and CL content, while SON was dependent on the initial C/N ratio of the residues. Among the residue quality factors, TM had the highest soil pH as well as highest cation exchange capacity (CEC) in Y 2 and 20. In addition, the highest soil Ca content was under TM throughout the 20-year period.

In general, soil K content was highest in RS during the first 11 years. Meanwhile, CT was associated with the lowest soil K content, and the other treatments were intermediate. For soil Na concentration, after Y 6, all the treatments showed a general trend of decreasing Na content over time. There were trends of decreases in most soil chemical properties studied from Y 6. However, the most notable and uniform decreases in all soil chemical properties were in the period from Y 11 to Y 16, during which soil pH, CEC, and cation concentrations decreased sharply in all the residue treatments. Cation ratio of soil structural stability (CROSS) was calculated to indicate the associations of the soil chemical properties studied. This parameter was recently developed to measure the relative dispersive powers of Na and K, and the flocculating powers of Ca and Mg. It was found to decrease between Y 2 and 20 under GN, DP, and TM, to levels below those of RS. The lower CROSS values were taken to indicate higher aggregate formation relative to the higher CROSS values.

In Study 2, under the sole residue treatments, only high-quality GN produced a significantly higher corn shoot biomass than the control (CT). However, the combination of all the residues with chemical fertilizer significantly increased corn shoot biomass in comparison to CT, with the highest biomass in GN. Similar results were found in plant uptake of N and P. TM did not enhance growth to the same extent as GN due to the lower quality of TM. For the high-K residues GN (25 g kg-1) and RS (14), GN produced a lower concentration and plant uptake of K than RS. For low-K residues, TM (6 g kg-1) and DP (4 g kg-1), TM produced a lower concentration and uptake of K than DP. Both legume residues had high Ca content (21 and 35 for GN and TM g kg-1, respectively). This produced a high Ca/K ratio of 2 and 7 under GN and TM, respectively. These values were higher than the Diagnosis and Recommendation Integrated System (DRIS) norm ratio of 0.3. The antagonistic effects of Ca on K were discussed. Calcium content of the organic residues appeared to play an important role in changes in the soil chemical properties studied, as Ca influenced clay flocculation and aggregate formation. Meanwhile, Ca also played an important role in cation uptake, as it could exert antagonistic effects on the uptake of K. The selection of appropriate quality organic residues with regard to their Ca content is therefore imperative for fertility and plant growth improvement in sandy soils, in addition to the other more well-known chemical constituents, N, L, PP, and CL.