Relationship among compaction, moisture and penetration resistance in horticultural soil. Carlos Gracia 1 , Estela Alemany 1 , Inmaculada Bautista 2 1 Unidad de Mecanización y Tecnología Agraria, Universidad Politécnica de Valencia. 2 Unidad de Edafología y Climatología. Grupo REFOREST, Universidad Politécnica de Valencia, Camino de vera s/n, 46022 Valencia, Spain. cgracia@dmta.upv.es ABSTRACT Penetration mechanical resistance in agricultural soil is recognized as a limiting parameter on root development. This makes it a necessary variable for an adequate management of the soil. However, in field measurements by means of a cone penetrometer become hard to achieve. In case of stony soils or soils with previous crop residues these measures also turn to be erroneous because of circumstantial interpositions into the vertical path of the cone. Therefore, it is useful to relate penetration resistance to other variables such as texture, organic matter, bulk density or moisture content which determination is more precise. This study sets the relationship among penetration resistance, bulk density and moisture content in a sandy clay loam horticultural soil containing a high percentage of small stones, as those found in Eastern Spain. However, this stony characteristic does not seem to advise against its use to grow vegetables. Penetration resistance was measured periodically by a cone penetrometer from laboratory soil samples. Samples got rid of stones, were enclosed in cylindrical containers and led to different levels of compaction. Compacted samples were moistened up to field capacity by capillary rise. Measures were taken as the soil dries by forced evaporation and up to the wilting point. Finally, the obtained measures led to a functional relationship between penetration resistance, moisture content and bulk density. From this, it is possible to generate tables for in field application. Keywords: compaction, moisture penetration resistance, penetrometer. 1. Introduction The capacity of plant roots to penetrate the ground is limited with increasing soil resistance (Mason et al., 1988). In many species, the growth of roots is prevented completely, from a specific resistance of 2.5 MPa (Taylor, 1971). The inability of roots to penetrate compacted soil is well documented in the literature (Kirkegaardet al., 1992; Venezia et al., 1995; Lakers, 2001). According to (Bengough et al., 2001), soil penetration resistance (SR) is, “the force required to push the cone into the soil divided by the cross-sectional area of its base.” Initially, the SR could be measured by a cone penetrometer (Bengough et al., 2001) considering the pressure exerted by the probe over the resistance to friction, and adhesion strength due to soil-metal (Farrell and Greacen, 1996). Although this method is widely employed, sometimes the use of a cone penetrometer turns out to be tough and defective. This is enhanced in high presence of stones or previous crop residues. Alternatively, it is proposed indirect measures of resistance to penetration, starting from their dependence on certain parameters of the soil. This will take into account, texture, organic matter content, bulk density, moisture content, among others. It comes to determining then, the functional relationship of