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.

Use of Organic Fertilizers to Enhance Soil Fertility, Plant Growth, and Yield in a Tropical Environment | InTechOpen, Published on: 2016-06-30. Authors: Amjad A. Ahmad, Theodore J.K. Radovich, Hue V. Nguyen, et

Spatial variability of soil physical and hydrological properties within or among agricultural fields could be intrinsically induced due to geologic and pedologic soil forming factors, but some of the variability may be induced by anthropogenic activities such as tillage practices. No-tillage has been gaining ground as a successful conservation practice, and quantifying spatial variability of soil physical properties induced by no-tillage practices is a prerequisite for making appropriate site-specific agricultural management decisions and/or reformulating some management practices. In particular, there remains very limited information on the spatial variability of soil physical properties under long-term no-tillage corn and tropical soil conditions. Therefore, the main objective of this study was to quantify the spatial variability of some selected soil physical properties (soil surface temperature (ST), volumetric water content (θ_v), soil resistance (TIP), total porosity (θ_t), bulk density (ρ_b), organic carbon, and saturated hydraulic conductivity (K_sat)) using classical and geostatistical methods. The study site was a 2 ha field cropped no-tillage sweet corn for nearly 10 years on Oahu, Hawaii. The field was divided into 10 × 10 and 20 × 20 m grids. Soil samples were collected at each grid for measuring ρ_b, θ_t, and soil organic carbon (SOC) in the laboratory following standard methods. Saturated hydraulic conductivity, TIP at 10 and 20 cm depths, soil surface temperature, and θ_v were also measured. Porosity and ρ_b have low and low to moderate variability, respectively based on the relative ranking of the magnitude of variability drawn from the coefficient of variation. Variability of the SOC, TIP, and K_sat ranges from moderate to high. Based on the best-fitted semivariogram model for finer grid data, 9.8 m and 142.2 m are the cut off beyond which the measured parameter does not show any spatial correlation for SOC, and TIP at 10 cm depth, respectively. Bulk density shows the highest spatial dependence (range = 226.8 m) among all measured properties. Spatial distribution of the soil properties based on kriging shows a high level of variability even though the sampled field is relatively small.

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.

Mizuna (Brassica rapa var. japonica), a member of family Brassicaceae, is a leafy vegetable having phenolic and other compounds beneficial to human health, such as natural antioxidants (Khanam et al. 2012). In October 2020, a field of mizuna (variety: Early) on Oahu island was observed having 20-30% diseased plants. Four randomly selected infected mizuna plants, showing the symptoms of wilt and stem rot (Figure 1A-D), were collected and isolations were made to determine the pathogen. Small sections of infected stems were cut, surface sterilized with 0.6% sodium hypochlorite solution for 30 sec, followed by three consecutive rinses in distilled water. The tissues were macerated in a sterile 1.5 ml centrifuge tube containing 100 μl sterile water-macerated tissues were streaked onto crystal violet pectate medium (CVP) (Hélias et al. 2011) and incubated at 26 ± 2°C for 48 h. Isolated bacterial colonies that formed pits on the CVP plates were re-streaked onto dextrose peptone agar: Peptone (10 g/L), Dextrose (5 g/L) and Agar (17 g/L) (DPA-without tetrazolium chloride; Norman and Alvarez 1989) to obtain purified colonies for DNA isolation using DNeasy Blood and Tissue Kit (Qiagen, Germantown, MA). The two housekeeping genes (dnaA and gapA) were amplified and sequenced following the protocols used by Dobhal et al. (2020) and Boluk et al. (2020), for identity confirmation and phylogenetic analysis. Cleaned PCR products were sent to the GENEWIZ facility (Genewiz, La Jolla, CA) for sequencing of sense and antisense strands. The obtained sequences were aligned, manually edited, and consensus sequences were analyzed with BLASTn using the NCBI GenBank nucleotide and genome databases for identity confirmation. The BLASTn results demonstrated 100% query coverage of all four strains (PL248-PL251); and showed 100% identity of PL248 and PL249, and 99% identity of PL250 and PL251 with Pectobacterium brasiliense. All the sequences were submitted to the NCBI GenBank database under the following accession numbers: dnaA gene MW560271 - MW560274 (PL248 - PL251); and gapA gene MW560275 - MW560278 (PL248 - PL251). Pathogenicity was assessed by artificially inoculating 100 µl bacterial suspension of each strain (PL248 - 1.12x 10⁸ CFU/ml; PL249 - 1.32x 10⁸ CFU/ml; PL 250 - 1.2x 10⁸ CFU/ml and PL251 - 1.15x 10⁸ CFU/ml) onto four-week-old mizuna (variety: Leafy Asian Greens) plants in three replicates, using sterile pipette tips, which was stabbed into stem halfway and wrapped with parafilm. The inoculated plants were well maintained under controlled greenhouse conditions. As negative controls, three plants were inoculated with 100 µl distilled water. Soft rot and wilt symptoms (Figure 1E-H) were observed 24 hours post inoculation. No symptoms were observed on control plants (Figure 1F). All four strains were re-isolated from the inoculated plants and confirmed as P. brasiliense based on resequencing of the dnaA region and 100% homology with the sequences of original strain. In the phylogenetic tree (Figure 2), based on two housekeeping genes (dnaA and gapA), the bacterial strains from mizuna grouped with other P. brasiliense retrieved from the NCBI GenBank database. To our knowledge, this is the first report of P. brasiliense infecting mizuna plants in Hawaii or in the USA and is important because this species is one of the most aggressive pectolytic pathogens in the genus Pectobacterium. Understanding the diversity of different pectolytic phytopathogens is essential to formulating risk mitigation strategies as P. brasiliense could potentially pose a threat to additional vegetable crops, especially the crucifers vegetables (Arizala et al. 2019; Klair et al, 2021).