Electromagnetic (EM) Mapping
The recent advent of the on-the-go mapping systems using electromagnetic sensors and accurate GPS logging provides a new method to map and identify contrasting and variable soil areas. The soil areas may require different management, for example when cultivating, fertilizing or planning irrigation schedules.
At Precision Water Works (PWW), we are equipped with the most up-to-date, ground based sensor now available for soil mapping. Our machine measures the apparent electrical conductivity of the soil, and provides one mean (average) weighted value from one foot to three feet in depth. Finer textured soils have a higher electrical conductivity (EC) value than coarse textured soils, due to better conducting pathways. Similarly, wetter soils have higher EC values than drier soils. It is this interaction of soil EC, soil texture and soil moisture that enables soil EC to be used as a surrogate measure of soil available water-holding capacity.
A map that accurately identifies the boundaries of contrasting soil areas has enormous potential as a very valuable aid to day-to-day farm management decisions. The maps can also be uploaded to computer-controlled equipment for precise guidance. For example, a map of the amount of plant-available water presented in the soil on any one day could be used to direct a centre-pivot irrigator with variable rate nozzle control or even speed control – different amounts of water could be applied to different parts of the field optimizing its use.
Soil EM maps delineate areas of contrasting soils. Targeted soil sampling of these areas is used to develop relationships between soil EC and soil properties, such as the soil available water holding capacity. The lower the EM values the lighter the soil. The higher the EM values the heavier the soil.
Soil available water-holding capacity (AWC) is the amount of plant-available water that a soil can hold – i.e. the amount of water held in the soil between Field Capacity and Wilting Point. Field capacity is the amount of soil moisture or water content held in the soil after excess water has drained away and the rate of downward movement has materially decreased, which usually takes place within 2–3 days after a rain or irrigation in pervious soils of uniform structure and texture. Wilting Point can be estimated as the field moisture content at the end of a significant dry period, when larger pores have completely drained and soil water is mainly sorbed onto clay surfaces and in very small pores.