Department of Plant and Soil Sciences

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    Response of macadamia nut yield, quality, and soil health to the application of different rates of macadamia husk compost in a subtropical climatic zone of Vhembe District, South Africa
    (2025-05-16) Maemu, Silence Fhulufhelo; Odhiambo, J. J. O.; Murovhi, N. R.
    Macadamia farmers are currently using macadamia husk compost as a supplement to chemical fertilizers in their respective orchards. Regardless of the good results from the application of macadamia husk compost, there is no consensus on the recommended amount of macadamia husk compost to apply in order to improve the fertility status of the soil as well as to increase crop yield and quality. A three-year study to investigate the value of using compost made from macadamia husks on macadamia trees was initiated in April 2020, on a sandy loam soil classified as Rhodic ferralsols . A Completely Randomized Block Design (CRBD) with four replications was used in this study. The aim of this study was to establish an application guideline that specifies the rates of macadamia husk compost application. However, the applied rates must be able to improve the fertility status of the soil and at the same time not compromise the yield and quality of macadamia nuts. Macadamia husk compost was mixed with pine bark and wood chips at the ratio of 30:35:35, respectively, and applied in August 2020, 2021 and 2022 after each harvest. Twelve months after macadamia husk compost application at the rate of 0 (control) t ha-1, 4 t ha-1, 8 t ha-1 and 12 t ha-1 of data on macadamia nut yield, nut quality, soil properties, and leaf nutrient concentration parameters were collected. The compost application process was repeated again after 24 months (2022). Collected data was subjected to a statistical analysis using GenStat® version 17 (VSN International, UK). The soil pH values of the compost treated soils were significantly (p<0.05) higher than those of the control and increased with increasing rate of compost application. The maximum pH value of 6.47 was observed after applying 12 t ha-1 of compost. This result could be attributed to a high number of exchangeable cations present in the compost. However, the pH values observed in this study were within the acceptable range for the growth and production of macadamia trees. Application of compost significantly increased (p<0.05) the levels of Microbial carbon (MB-C), Microbial Nitrogen (MB-N), Soil Organic carbon (SOC) and Particulate Organic Matter (POM) contents in the study site. Applying 12 t ha-1 of macadamia husk compost to the soil at the study site increased SOC, POM, MB-C and MB-N contents by 149 %, 190 %, 319 % and 50 %, respectively This increment in soil biological properties emphasises the importance of adding compost to the soil with the aim of increasing the levels of organic matter content which is considered as the reservoir for plant nutrient. Exchangeable cations Potasium (K), Calcium (Ca) and Magnesium (Mg) significantly (p<0.05) increased with the compost applications of 8 t ha-1 and 12 t ha-1 compared to when no compost was applied. These increases can be attributed to a direct contribution of these nutrients from the compost itself and/or the increased cation exchange capacity of the soil as a result of compost addition. The incremental application of compost increased the available phosphorus content in the soil. The maximum values (16.93 and 13.22 mg kg- 1) of available phosphorus content were recorded with the application of 8 t ha-1nd 12 t ha-1 of compost, respectively. However, the concentration of available phosphorus in the soil was below 30 mg kg-1 which is the recommended soil norm for macadamia production in South Africa for all soil types. The results of this study found that the control treatment had the maximum bulk density value (1.34 g cm-3). Soil amended with 8 t ha-1 and 12 t ha-1 of compost had the least bulk density values (1.28 and 1.29 g cm-3, respectively). Soil bulk density tends to decrease with an increase in the quantity of compost application and is an indicative of an increase in soil pore space where water is moved through the soil more efficiently by large pores than by smaller ones. Macadamia husk compost application rates showed no (p>0.05) significant difference on the soil agregate stability of the different treatments. On the other hand, macadamia husk compost applied at different rates at the study site was more effective at increasing the infiltration rate of water in the soil. This could be associated with the fact that compost act as a surface mulch thus reducing evaporation. Mulching or covering the soil surface with a layer of plant residue is an effective method of conserving water, because it reduces surface runoff and increases infiltration of water into the soil. The infiltration rate was higher for 12 t ha-1 compared to 0 t ha-1, in a minute there was already an infiltration rate of 41.57 mm h-1 whereas for the control (0 t ha-1) it was 17.71 mm h-1. There was also a significant (p<0.05) effect on the water holding capacity related with an increase in the application rates of the compost with the least percentage at 0 t ha-1 (1.33%) and the maximum at 12 t ha-1was (2.2%). There was a significant (p<0.05) effect of treatments on the concentration of K and Mg in the leaves of macadamia trees. The maximum (0.66%) and minimum (0.58%) leaf K were observed on trees amended with 12 t ha-1 and control, respectively. Application of compost did not have significant effect on the concentration of Nitrogen (N), Phosphorus (P), Calcium (Ca), Cupper (Cu), Manganese (Mn), Iron) Fe and Boron (B) in the leaves of macadamia trees except for Zinc (Zn). The concentration of nutrient elements (N, P, Zn, Cu, and B) in the leaves of macadamia trees under investigation was considered low but not deficient when compared with recommended standard leaf norm, whereas Potasium (K), Calcium, Magnesium (Mg), Mn and Fe were sufficient for the growth and production of macadamia trees. The results of this study have proved beyond reasonable doubt that the application of compost made from macadamia husks can increase yield and quality of nuts, and improve the fertility status (physical, chemical and biological properties) of the soil. In the light of the above, it was recommended that South African Macadamia Assocication Council (SAMAC) through its structures must encourage all macadamia farmers to start using compost made from macadamia husks as a supplement to inorganic fertilizers. In this study, application of 12 t ha-1 was the most appropriate rate of application. However, for meaningful recommendation for a wide range of macadamia growers, further studies need to be conducted over a long period of time in different macadamia growing locations covering different soil types.