Soil Moisture

Soil moisture is the single most important variable for irrigation scheduling. It is measured three ways: (1) gravimetric — dry a sample in an oven and weigh (accurate but slow); (2) volumetric sensors (TDR, capacitance probes) — cheap, real-time, commonly installed at 20, 40, and 60 cm depths; (3) remote sensing — satellite microwave (SMAP, Sentinel-1) estimates surface moisture at 10-km resolution, useful for regional monitoring.

Soil moisture varies by depth and by soil texture. After heavy rain, topsoil is wet and drains to field capacity in 24–48 hours while deeper layers hold water longer. Plant roots draw water from progressively deeper layers as surface soil dries. Monitoring three depths gives a complete picture: 20 cm for seedling and shallow-rooted crops, 40 cm for mid-depth crops like corn in tasseling, and 60 cm for deep-rooted crops like alfalfa.

Irrigation triggers based on soil moisture: irrigate when readings drop to 50–70% of plant-available water. Below 40%, crops enter visible stress; yield reductions begin at mild deficit. Above field capacity risks waterlogging and root disease. Modern farm platforms combine real-time soil moisture sensor data with weather forecasts and crop-stage evapotranspiration to generate daily irrigation recommendations that balance water efficiency with yield.