A soil moisture sensor comprises a processor 1 and a moisture sensing area 5. The sensing area contains a plurality of individual sensors 2, 3 and also an averaging sensor 7 to give a value over the length of the sensor. The sensors may be arranged in a vertical array so that when in use they may detect moisture levels at various discrete levels in the soil strata (Figure 2, item 10). The device may be mains 4 or battery powered. The sensors may be capacitive in nature and the array may be protected against abrasion and corrosion. The device may be equipped with a processing unit (19, Fig 2) that analyses the results from each sensor. The processing unit may calculate average values or may determine aggregate or weighted moisture levels, where the value may be biased to reflect moisture values proximate to the root structure of the plant. The sensor may send signals to a remote monitoring station which may be integrated with an irrigation system. The device is intended to reduce overwatering but also aims to ensure that the root structure is adequately watered.