Biogas slurry contains high levels of organic matter and essential nutrients, which enhance soil fertility and structure. Although it can reduce the uptake of heavy metals by plants, improper application may result in soil contamination. This research evaluated the effects of applying semi-solid biogas slurry (SSBS) at 72 or 96 m3/ha, either on its own or combined with a foliar spray of liquid biogas slurry (LBS) at 48 L/ha or a recommended NPK fertilizer on spinach growth. Results showed that all SSBS treatments, whether individually or combined with NPK or LBS, improved growth traits such as plant height and total leaf area. They also increased chlorophyll content (chlorophyll a, b, and total), vitamin C, total phenolic compounds in leaves, and concentrations of nitrogen, phosphorus, potassium, iron, and zinc, along with enhanced nitrate reductase activity. Furthermore, these treatments boosted leafy yield and lowered levels of lead (Pb), cadmium (Cd), nitrate, and oxalate content in spinach leaves. The most pronounced positive effects on growth and physiological-biochemical parameters occurred when SSBS was applied at 96 m3/ha along with either the recommended NPK dose or LBS. These findings indicate that biogas slurry could serve as a sustainable alternative to chemical fertilizers in agriculture.