Insect Frass as a Novel Organic Soil Fertilizer for the Cultivation of Spinach ( Spinacia oleracea ): Effects on Soil Properties, Plant Physiological Parameters, and Nutrient Status

Soils naturally low in organic matter need to be amended with recycled organic materials that would boost soil health. In this work, we tested insect frass, the excrements of the yellow mealworm, Tenebrio molitor , as a novel organic soil amendment in a spinach cultivation. In a 60-day pot experiment with spinach ( Spinacia oleracea ), we added frass at rates of 0, 0.25, 0.5, and 1%, as well as an inorganic nitrogen-phosphorus-potassium (NPK) fertilizer as a positive control. We found that organic matter increased significantly from 2.7% in the unamended control to 3.2% in the treatment of 1% frass, showing that frass has a valuable role in boosting soil carbon (C). Also with increasing frass rates, N uptake by plant was enhanced, due to the fact that nitrogen use efficiency (NUE) remained unchanged. These findings exhibit the fact that added N was utilized satisfactorily. However, frass effectiveness was not successful in the case of added P and K: the likely reason is the already very high P and K available contents in the unamended soil. Spinach growth was significantly favored by the amendment of frass in a rate-dependent manner: the higher biomass accumulation was found under 1% frass—six-fold higher than NC, while even at 0.25% the yield was significantly higher. Plants at this treatment had the highest chlorophyll a + b content throughout the course of the experiment and also exhibited the highest photosynthetic efficiency and performance of the plant photosynthetic apparatus under frass treatments. The index of total photosynthetic efficiency (PI total ) also fared better in the frass-added treatments upwards from 0.25%. We conclude that frass is a highly valuable soil conditioner, given the amble organic C and N that adds to soil, resulting in boosted growth of spinach. As this report is a preliminary study, we suggest that future works should expand frass testing to (a) more plants, where a series of important plant physiology features must be evaluated, and (b) the role of frass in trace element availability.