Real-time soil fertility monitoring can help evaluate soil quality in time for soil improvement, which in turn improves agricultural yields. This paper presents a self-powered and distributed soil monitoring network for smart farms. Each monitoring node is powered by a battery $(5000\ mAh)$ and a small photovoltaic panel to continuously charge the battery, which enables the monitoring nodes to be widely distributed in the farm and continuously monitor soil data with soil fertility parameters, such as nitrogen, phosphorus, potassium, pH, moisture and temperature. All monitoring data can be transferred to a remote data server in the cloud via a 4G/5G network and MQTT protocol. This allows users to analyse the soil monitoring data via an APP to support further operational decisions, such as watering or fertilising. To minimise power consumption and maximise the duration of the power supply, a power management circuit is designed to support the sleep mode, which can turn off the sensors and 4G module during the sleeping period and turn them back on for data monitoring and transmission. Experimental results show that even without charging from the photovoltaic panel, the battery can support the continuous operation of the monitoring node for about eight days, which means that the designed system can be widely used in wild farms for long-term soil monitoring.