Sensors for Soil Hard Pan Determination - A Review

The compaction of soil or sub-soil due to repeated ploughing at same depth for number of years makes the soils an impermeable layer called soil hardpan. The introduction of heavy machinery is also responsible for creating soil hard pan. Compacted soils restrict root growth, which in turn limits crop yield. The soil compaction sensor systems are based on soil strength sensors, fluid permeability sensors, water content sensors, or their combinations. This paper reviews different technologies developed and available in India and around the world for soil hard pan determination. The cone penetrometer (ASAE S313.3, 2006) is most commonly used for measurement of soil hard pan in India. Repeatability and difficulties in sampling in hard or dry soils are the primary drawbacks of this type of penetrometer. The second most popular instrument is penetrologger to measure the resistance to penetration of the soil and to record the results for further processing. This method is very accurate and has a large measuring range. Though it has several advantages over cone penetrometer, still it cannot be operated at constant pressure. It is an inefficient method when large data are needed in a short time since variation in moisture content in soil affects the soil cone index reading. A hydraulic operated cone penetrometer is also used to measure soil hard pan. It is more accurate method and gives precise reading but this type of machine has limited use for indoor testing. Different types of prototype sensors were developed for direct or indirect measurement of soil compaction. The on-the-go soil compaction sensor either of tip based or tyne based may be adopted for Indian conditions. These type of sensor will make the data acquisition fast, accurate, precise and will suit for all conditions.