A wide variety of sensors, such as the following, are available for use in agriculture:  Airflow is a measurement of the air permeability of the soil.  Acoustic: Determines the decibel level of the sounds made by pests.  Chemical: Determines the concentration of a certain chemical, such as ammonium, potassium, or nitrate, or determines the circumstances present, such as the pH level or the existence of a particular ion.  Electromagnetic: The capacity of the soil to conduct electrical charge is measured, and this measurement may be used to identify features like as the amount of water present, the amount of organic matter, or the degree to which the soil is saturated.  Electrochemical: Determines the levels of nutrients present in the soil.  Humidity: Determines the amount of moisture present in the air, for example in a greenhouse.  Soil Moisture: Determines the level of moisture present in the soil.