The effect of giant kelp (Macrocystis spp.) organic matter amendment on growth and yield of Tomato (Solanum lycopersicum, var. Roma VFN) in loamy sandy soils of North Central Namibia

Abstract Soils in the semi-arid north-central Namibia have low capacity to support tomato production, hence posing a threat to the livelihoods of small-scale vegetable farmers. A field experiment was conducted during 2022 to 2023 autumn season to evaluate the effect of cultivated giant kelp (Macrocystis spp.) fragments incorporated into soil, on the growth and yield of tomato (Solanum lycopersicum) variety Roma VFN under daily and alternate-day irrigation. The experiment was arranged in a split-plot design with four replications. Two irrigation frequencies were the main plots while soil amendments were subplots (control, cattle manure (200 g/plant), NPK fertilizer (180 kg N/ha), kelp fragments low dose (K1, 100 g/plant), and kelp fragments high dose (K2, 200 g/plant). Soil treatment was highly significant (p<0.001) in all growth parameters (number of leaves/plant, plant height and SPAD) and yield components (fruit yield/plant and fruit number/plant (p<0.05 and 0.01)). Moreover, the combination of soil amendment and irrigation frequency was significant (p<0.01) only on fruit yield/plant. This indicates that the effect of soil amendment on fruit yield is dependent on the irrigation frequency used. Growth and yield parameters were the highest in K2 and K1, especially K2 under alternate – day irrigation. Rapid degradation of kelp in the soil may have supplied bioavailable nutrients, phytohormones and other constituents that are not available in other treatments, to promote growth and yield of tomato. The study is the first to demonstrate the potential of cultivated kelp to enhance the growth and yield of tomato with water saving alternate-day irrigation schedule in the nutrient deficient soil of northern Namibia during warm winter season.