An Evaluation of a Nuclear Density Gauge for Measuring Infield Compaction in Soils of the South African Sugar Industry

neutron and gamma-ray sources whichenable bulk density and moisture to be determined simultaneously. This instrument has a single probe which provides for both gamma-ray scattering and gamma-ray transmission depending on the required depth of measurement. Surface measurements (0 to 100mm integrated) are made with the backscatter technique (Figure 2) and the measurements between 50 and 300 mm are made using the gamma-ray transmission mode (Figure 3). A gamma-ray attenuation method for determining bulk density to a depth of 300 mm in the field is described. Tests which were conducted on plots in a compaction trial showed that the method was as accurate as the standard core sampling procedure and up to five times faster. The results also indicated that the compacted layer was situated at a depth between 150 and 200 mm whichwasconsistent with measurementsobtained with the hand penetrometer. Regression analysis indicated that plant performance, when measured in terms of sucrose yield, was negatively correlated with bulk density values obtained at a depth of200 mm by both the gamma-ray and undisturbed core methods. The simplicity of operation and ruggedness of the equipment makes it useful for survey and advisory work. Experimental procedure Introduction Because infield operations have been mechanized considerably during the past two decades, compaction of the soil and its effect on crop growth has received increasing attention by many agricultural researchers. Some examples of researchconducted in South Africa include the influence of soil strength on plant growth (Bennie and Laker'; Maller') and the influence of texture (van der Watt"; Moolman-), moisture holding capacity and organic matter content (Maud') and crust formation (Weber and van Rooyen"; Hutson') on soil compactibility. In the sugar industry. field investigations have indicated that sugarcane growing in soils of the Longlands and Westleigh forms is more prone to severedamage due to soil compaction (Swinford and Boevey") than cane growingin clay loam soilsof the Hutton form (Johnston and Wood'). In the past. methods based on some form of penetrometer measurements have been used to assess the degreeof compaction in the field. Penetrometers have the advantage of simulating the action of plant roots in a soil. by measuring soil strength/shearing resistance, but a disadvantage is that reliable results can only be obtained when the moisture content of the soil is close to field capacity. Measurements of the bulk density of the soil, by means of undisturbed cores. can be used to assess soil compaction. This method is more reliable but unfortunately very time consuming, particularly when large areas are involved and separate samples have to be taken to depth. The gamma-ray radiation bulk density meter which was pioneered by Vomocil'? and developed further by Soane", offers a direct means of determining bulk density in the field. It is as accurate as the core sampling method but up to five times faster. This technique is widely employed by civil engineers in foundation and road construction work, but its use by agriculturalists has been limited in this country. These considerations led to a closer investigation of the merits of using a nuclear bulk density meter for assessing compaction problems in the field.