Improvement of Charge Characteristics of Oxisols Using Basalt and Rice Husk Compost for Cocoa Growth

One of the major limiting factors of Oxisols for agricultural crop production is a very low negative charge, leading to deficiency of base cations imparted by leaching. This condition is exacerbated by the low pH and high Al saturation. Basalt is naturally available and well known as parent material of fertile and highly productive soils, whereas rice husk is an agro-waste that can cause serious environmental problems , which on the other hand, is a highly valuable material if used as a soil amendment. The general objective of this study was to assess charge characteristics of Oxisols and restore their productivity by generating surface negative charge and increasing nutrient content using basalt and rice husk compost for cocoa growth. Charge characteristics of three Oxisols were determined using potentiometric titration method. Results indicate that the B horizons of profile derived from basalt (Kuantan Series) had considerably lower pH0 values (4.1-4.9) than those of the profiles derived from serpentinite (5.6-5.8 ) (Sungai Mas Series) and andesite (5.3-6.0) (Segamat Series). These high pH0 values indicate the soils have very low negative charge. The removal of SOM or iron oxides increased or decreased pH0 values, respectively, suggesting that SOM is responsible in lowering pH0 values, while iron oxides in increasing pH0 values. The comparison of total SOC at corresponding application rates (0 to 20 t ha-1) of composted rice husk (CRH) under laboratory, glasshouse and field conditions for the first 12 months showed the magnitude of SOC content was in the order field> laboratory> glasshouse. The magnitude of SOC residual in soil was higher for the field conditions (0.24-0.69% C) than laboratory (0.05-0.23% C) and glasshouse (0.05-0.17% C) although the SOC loss was higher for the field (0.25-0.97 unit) than laboratory (0.04-0.16 unit) and glasshouse (0.04-0.22 unit). This is probably due to the interaction between CRH and Gliricidia litter to preserve organic C in the soils. The type of organic C functional groups was determined using a solid state cross polarization magic angle spinning (CP/MAS) 13C nuclear magnetic resonance (NMR) spectroscopy. The C functional groups were similar when determined under laboratory, glasshouse and field conditions. Under laboratory and glasshouse conditions, O-alkyl C was dominant, followed by acetal with minor proportion of alkyl, methoxyl, aromatic, phenolic and carboxyl C. For field conditions, the organic C functional groups were dominated by O-alkyl and alkyl C, followed by aromatic, carboxyl and acetal C. The three decomposition indices: alkyl/O-alkyl C ratio (A/O-A), hydrophobicity/ hydrophilicity ratio (HB/HI) and aromaticity were related to the decrease in total SOC content and CO2 production. The trends of the three indices in all conditions were similar, i.e., all values increased with increasing duration of CRH application, suggesting the increase of decomposition rate. The changes of decomposition indices within different periods for the first 12 months were very small in all conditions. These findings indicate CRH decomposed slowly; thereby it has a long residence time in soil. Point of zero net charge (PZNC) was determined using ion adsorption method. Charge characteristics obtained at various sampling times within 24 months of basalt application for laboratory and field conditions, and 15 months for glasshouse showed consistent decreases in pH0 and PZNC values with time, but they decreased slowly, suggesting that the basalt was able to generate negative charge. However, the process took place rather slow. The exception was basalt application in the subsoil under laboratory conditions which increased pH0 values. The rates of basalt application for laboratory and glasshouse conditions were 0 to 80 t ha-1, whereas for field conditions they were 0 to 20 t ha-1. Hence, to indicate the generation of negative charge of the three conditions, application rates of 0 to 20 t ha-1 and sampling time of month 12 were used (except for field 24 month was used). The net negative charge generation (NetC) at equilibrium soil pH 5.0 was 1.5, 1.0 and 1.9 cmolc kg-1 for laboratory, glasshouse and field conditions, respectively. The corresponding NetC values under natural soil pH were 1.8, 0.3 and 2.0 cmolc kg-1. Further increase of application periods to 24 months and 15 months for laboratory and glasshouse, respectively increased NetC values by 3.5 and 0.6 cmolc kg-1, respectively. The application of basalt significantly increased Ca, Mg, K, Na and Si of the in situ soil solution. The concentration of base cations at any given incubation period under laboratory conditions is in the order of Na> Mg> Ca> K, suggesting that Na was released the fastest, followed by Mg, Ca and K. Under glasshouse and field conditions, the order of ion concentration is shifted to Mg> Ca> Na> K and Ca>Mg>Na~K, respectively. For composted rice husk (CRH), the duration of application under laboratory, glasshouse and field conditions was similar to basalt. The CRH application of 0 to 20 t ha-1 consistently decreased pH0 and PZNC values in all conditions. In contrast to basalt, the CRH in the subsoil under laboratory conditions decreased pH0 values. The NetC generation at equilibrium soil pH of 5.0 was 0.4, 0.6 and 2.3 cmolc kg-1 for laboratory, glasshouse and field conditions, respectively. The corresponding NetC values under natural soil pH were 0.5, 0.5 and 3.3 cmolc kg-1. The application of CRH significantly increased concentrations of Ca, Mg, K, Na and Si of the in situ soil solution at application rate of 40 to 80 t ha-1 under laboratory and at application rate of > 10 t ha-1 for glasshouse and field conditions. The order of released ions were K> Na> Si> Mg> Ca for laboratory, Si> K> Mg> Ca> Na for glasshouse and Si > Ca > K > Mg > Na for field conditions. In contrast to K, Si, Mg, Na and Ca concentrations, the application of basalt or CRH and their combination significantly decreased concentrations of Mn and Al in all conditions. The individual application of basalt and CRH significantly increased height, stem diameter and dry matter weight (DMW) of cocoa under glasshouse conditions. The cocoa growth significantly increased at application rates of > 10 t ha-1 for basalt and > 20 t ha-1 for CRH. The optimal rates of basalt and CRH under glasshouse conditions were 22 and 20 t ha-1, respectively. The results from glasshouse (for CRH) were in agreement with the results from the field experiment. CRH in combination with basalt significantly increased the height and stem diameter of cocoa under field conditions. The application of 5 t ha-1 is suggested for field application as this rate significantly increased cocoa growth compared to control but was not significantly different with 10 and 20 t ha-1 application rates. Basalt and rice husk could be used to restore chemical properties of Oxisols as they are able to increase soil negative charge, increase nutrient content of in situ soil solution and reduce Al and Mn, which in turn significantly improved cocoa growth.

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