In order to effectively accomplish this real-time control, the sensors must respond almost instantaneously to changes in the soil. For example, a bulk fertilizer spreader truck may operate at field speeds of 25 miles per hour. This means that 37 feet will have passed beneath the truck if the lag time of the system is one second. The LED's shine red light onto the soil surface and the photodiode measures the amount of reflection. This reflection signal is related to the amount of organic matter in the soil.
Moisture can also affect the sensor but as long as the soil is moist, the effects are small.
If these efforts succeed, site-specific farming will become even more economical possibly even automatic.