Acta Horticulturae · 2008 · 30 citations · 0 references
AgricultureAgronomyPlant SciencesSoil Organic MatterSoil PhysicsSoil-water RelationsGrowing MediaPlant Hydraulic FunctionPorosity AnalysisPorosity CharacterizationSoil Hydraulic PropertiesWood MaterialsGrowing Media PropertiesPlant Growth AnalysisPlant Growth SubstratesPore VolumeSubstrate Bulk DensitySubstrate PorosityWater Retention CapacityWater Retention Properties
The variety of materials and mixes of materials used in horticulture as growing media is large. Peat products, wood products, composts, mineral fibres, mineral particles, synthetic foams, synthetic fibres and organic fibres; all these products are more or less suitable growing media. Selected physical aspects related to plant growth requirements, are used to discuss the suitability of growing media for plant growth. Aspects mentioned include bulk density, total pore volume, structural stability, root resistance, water retention, rewetting, hydrophobicity, water transport, and oxygen transport. Methods to measure these aspects and experience with the results are also discussed in this review. In conclusion, the physical properties used to assess the suitability of substrates for growth share a large dependence on pore architecture. This made it possible to relate the formulas for water retention, hydraulic conductivity and diffusivity to each other. It seems possible to extend this unification to resistance to rooting and rewetting. Bulk density, total pore space and structural stability exercise their influence on plant growth mainly through the key characteristics for growth assessment: resistance to rooting, water retention, rewetting and water and oxygen transport. Better definitions and possibly new methods to characterise rewetting, hydrophobicity and refreshment rate are advocated. When assessing the influence of water and oxygen transport, individual layers and gradients towards the roots are of importance because of the very fast transport rate changes with water or air filled pore space. INTRODUCTION The variety of materials and mixes of materials used in horticulture as growing media is large. Peat products, wood products, composts, mineral fibres, mineral particles, synthetic foams, synthetic fibres and organic fibres; all these products are more or less suitable as growing media (Table 1). The aim of this review is to discuss those physical characteristics of different growing media which are important for plant growth. The choice of the physical parameters to be discussed is inspired by some of the basic functions of the roots, anchorage, the uptake of water and the uptake of mineral nutrients (Kramer and Boyer, 1995). Anchorage in itself is provided by all rooting media. But there are distinct differences in root growth between various rooting media. These are thought to be represented by root resistance, bulk density and total pore space. These properties are not always stable in time. Therefore structural stability is included in the review. The uptake of water is related to the force with which the rooting medium holds the water, the water retention. The uptake of water is also determined by the dynamic water transport towards the roots. To accurately describe water transport in rooting media it is necessary to consider some changes in water retention in time caused by either hysteresis or hydrophobicity. These changes are thought to be characterised by rewetting and hydrophobicity measurements. The uptake of minerals is questionably – not regarded as a physical aspect of rooting media. The passive transport of nutrients to the roots is quite Proc. IS on Growing Media Ed.: J.-C. Michel Acta Hort. 779, ISHS 2008 240 similar to the dynamics of oxygen transport towards the roots, and oxygen transport is definitively considered a physical aspect of rooting media. Oxygen is necessary for proper root growth and the uptake of some nutrients. This choice of physical aspects is of course limited. Furthermore the aspects are interrelated. A low water tension may facilitate growth by promoting transport of water and nutrients, but at very low water tensions the amount of air-filled pores will become too low for sufficient oxygen transport. Higher water tensions may be used by growers e.g., to create denser pot plants. Imperfect rewetting capacity of a substrate will amplify any unevenness in crop water supply and uptake. In practice growers will use more frequent irrigation cycles on this type of medium. These physical parameters can be described by means of various methods. Some of these methods are long in use and have been proven satisfactorily, while others may show clear disadvantages especially for some materials. Many of the methods are derived from soil science (Klute et al., 1986), while new methods are still developing and need to be tested (Raviv et al., 1999; Blok and Cassamassimo, 2001; Wever et al., 2004; Naasz et al., 2005). CURRENT METHODS FOR SELECTED PHYSICAL ASPECTS Sample Preparation Measurements of physical aspects are usually markedly influenced by both, the selection and exact preparation of bulk samples, and the consequent sub sampling into individual samples. It is therefore important to apply strict rules for sampling and subsampling (EN 12579, 1999; EN 13040, 1999). Furthermore, different groups of rooting media require different sampling and preparation methods, as do different storage systems like heaps, production batches or bags. For example small coherent materials like preshaped plugs for rooting will be used as such albeit after bulk sampling from different production batches. Larger coherent units like poly urethane slabs are cut after bulk sampling from different production batches. Loose soft materials like peat and compost are easily compressed by their own weight and moisture content. They may first have to be bulk sampled from various places and at standard depths from heaps. Then they will be brought to a standard moisture level, entered in a controlled way into sample holders and subjected to a standard pressure. Bulk Density, Total Pore Space, Root Resistance and Structural Stability Bulk density (kg.m) is defined as the ratio of mass of dry solids to the bulk volume of the soil (Blake and Hartge, 1986) (Table 1). Bulk density is measured by weighing a known volume of rooting medium after heating to 105°C. Non-rigid materials like e.g., peat and composts need to be weighed under carefully defined pressure and filling circumstances. Transport vibrations may alter bulk density considerably. Various norms have been put down (EN 12579, 1998). The total pore space (%v/v, TPS), is usually calculated from the total volume minus the volume occupied by the solids. To accurately assess suitability for plant growth, the total pore space may have to be corrected for the amount of non connected pores. There is no fixed method to measure the amount of non connected pores but a fair estimate is usually found by grinding the material to a powder and measuring its density thereafter. The resistance to rooting (kPa) can be measured with a cone penetrometer. The cone penetrometer is supposed to represent a root growing through the medium. The force needed to insert the cone in the medium is measured. Ideally, the penetrometer reading is then correlated to measured pressures of growing roots, but more often it is used on itself. Before the reading becomes stable, some compression of the medium takes place. A larger cone angle and a larger cone diameter increase the distance over which compression takes place before a stable reading is found (Bullens, 2001).