This study investigated the effects of vermicompost tea (aqueous extract) on yield and chemical quality of pak choi (Brassica rapa cv Bonsai, Chinensis group) grown in three media (two soils and a peat-perlite medium) under two fertilizer regimes (compost and synthetic fertilizer). The impacts of tea application on the chemical and biological properties of the growth media were also investigated. Vermicompost teas were prepared using various extraction methods (non-aerated, aerated, aerated with additives) with 1:10 (v:v) chicken manure-based vermicompost to water dilution and applied weekly at the rate of 200 mL plant⁻¹ for 4 weeks. Application of vermicompost tea increased plant production, total carotenoids and total glucosinolates in plant tissue. This effect was most prominent under compost fertilization. Total phenolic was lower in vermicompost tea treated plants compared to those treated with only mineral nutrient solution and the water control. Vermicompost tea improved mineral nutrient status of plants and media, and enhanced the biological activity of the media. Variability in yield and chemical quality of plants across treatments was explained largely by variability in tissue N uptake and dry matter accumulation. Dehydrogenase activity and soil respiration of vermicompost tea-treated growth media were approximately 50% higher than untreated media. This study confirmed that vermicompost tea can positively influence plant yield and quality and increase soil biological activity in multiple soil types.

Highly weathered soils in Hawai’i are low in fertility, negatively affecting plant growth. The potential of biochar for improving soil nutrient availability to crops is promising, and prompts this study. Two biochars at 2% (w/w) made of lac tree (Schleichera oleosa (Lour.) Oken) wood and mixed wood (scrapped wood and tree trimmings) with and without vermicompost or thermocompost at 2% (w/w) were added to an Ultisol (Ustic Kanhaplohumult, Leilehua series) and an Oxisol (Rhodic haplustox, Wahiawa series) of Hawai’i. In each soil two additional treatments—lime + compost and un-amended soil—served as the control. Chinese cabbage (Brassica rapa cv. Bonsai) was used as the test plant in two greenhouse plantings, which had a factorial completely randomized design with three replicates per treatment. The results indicated that soil acidity, nutrient in the soils, plant growth and nutrient uptake were improved by the amendments compared to the control. The combined additions of biochar and compost significantly increased pH and EC; reduced exchangeable Al; reduced Mn and Fe in the Oxisol; increased P, K, and Ca content of the soils; and increased Ca, Mg and Fe uptake. Exchangeable aluminum in the Ultisol decreased from 2.5 cmol+/kg to nil; Mehlich-3 extractable P, K, Ca, Mg, Fe, and Mn in the Ultisol increased by 1478%, 2257%, 1457%, 258%, 125% and 72%, respectively compared to the un-amended soil, while the same nutrients increased or decreased in the Oxisol by 180%, 59%, 308%, −14%, and −36%, respectively. Shoot and total cabbage fresh and dry matters increased by 94%, 96%, 107%, and 112%, respectively, as compared to the lime plus compost treatment. Cabbage growth in the Ultisol amended with the lac tree wood biochar and vermicompost was almost twice over the lime and vermicompost treatment. Essential nutrients in the plant tissues, except for N and K, were sufficient for the cabbage growth, suggesting increases in nutrients and reduced soil acidity by the additions of biochar combined with compost were the probable cause. It is recommended that locally produced biochars and composts be used to improve plant nutrient availability in the highly weathered soils.

Previous work has demonstrated the potential of compost tea to enhance plant growth and nutritional status. One factor thought to contribute to variability in the efficacy of compost tea is the amount of compost per unit volume of water. To address these gaps in our understanding, two greenhouse trials and two field trials were conducted to investigate the effects of various extraction ratios on the growth, mineral nitrogen (N), and phytonutrient content of pak choi ( Brassica rapa, Chinensis) and on soil biological properties. In greenhouse experiments, plants were fertilized with a single rate of chicken manure-based thermophilic compost. In field trials, three fertilizer treatments: 1) rendered meat byproduct or Tankage (Island Commodities, Honolulu, HI); 2) soluble fertilizer (16:16:16); and 3) chicken manure-based thermophilic compost were applied. Aerated vermicompost teas were prepared using chicken manure-based vermicompost and water at various ratios. Pak choi plants were treated weekly for 4 weeks with 10%, 5%, 3%, and 1% vermicompost teas in the greenhouse experiments and 10% and 5% teas in the field trials. Applications of vermicompost tea significantly increased plant growth, N content, total carotenoids, and total glucosinolates in plant tissue; this response was greatest in chicken manure-fertilized treatments. Increases in yield and phytonutrient content were associated with increased N uptake. Vermicompost tea also increased soil respiration and dehydrogenase activity over the control (water). Plant growth, phytonutrient content, and microbial activities in soil increased with increasing concentrations of vermicompost tea. Within the range of concentrations evaluated (1%–10%), greatest plant growth response was observed with 5% and 10% vermicompost tea, indicating that the optimal water-to-vermicompost ratio for extraction is lower than 50:1 and is likely in the range of 10:1 to 20:1. The findings suggest that vermicompost tea could be used to improve plant nutrient status and enhance soil biological properties in vegetable production.

Using local resources and minimizing environmental impacts are two important components of sustainable agriculture. Meat and bone meal (MBM), tankage, is a locally produced organic fertilizer. This study was conducted to investigate the response of sweet corn (Zea mays L. var. saccharata Stuart.) and soil water nitrate (NO₃-N) concentration to MBM application at two locations, Waimānalo and Poamoho, on the island of O’ahu. The objectives were to determine effects of six application rates (0, 112, 224, 336, 448 and 672 kg N ha⁻¹) and two application timings (preplant and split application) on: (1) sweet corn growth, yield, and quality, and (2) soil water nitrate concentration within and below the root zone. The split-plot was designed as four replicates randomly arranged in a complete block. Plant growth of roots and shoots, yield, and relative leaf chlorophyll content of sweet corn increased with increasing application rates of MBM in both locations. At Poamoho, yield was 13.6% greater in preplant versus split application. Nitrate-nitrogen losses were reduced by 20% at Waimānalo and 40% at Poamoho when MBM was applied in split applications. These findings suggest that MBM is an effective nitrogen source for sweet corn and a split application of MBM may reduce the potential for pollution.

In Hawaii, there are many small farm holders. Intercropping may increase diversity of farm products, reduce input costs, and increase farm profitability. Two field experiments were conducted in Hawaii to study the effect of three legume species intercropped with sweet corn (Zea mays L.) on the growth, relative chlorophyll content, biomass, and yield of sweet corn. The treatments included corn + soybean (CS), corn + bush bean (CB), corn + cowpea (CC), and corn alone (CO; control). In the first growing season, 30 days after planting, relative chlorophyll content of corn leaves was significantly higher in CC treatment with 53%, 56%, and 68% compared with CB, CS, and CO, respectively. While, in the second growing season, 30 days after planting, relative chlorophyll content of corn leaves was significantly higher in CC treatment with and 31%, 33%, and 66% compared with CB, CS, and CO, respectively. Corn plants in the CC treatment were significantly taller and had greater leaf area than those in other treatments. Fresh and dry weights of corn biomass, corn + legume biomass (with and without pods), and fresh weight of corn ears were all significantly higher for the CC treatment as compared with the other treatments. Mean fresh weight of corn ears under CC treatment increased by 9.9%, 35.1%, and 53.5% compared with CB, CS, and CO, respectively. The results suggested that lower competition and/or the contribution of fixed N in the CC treatment contributed to the better growth of corn.