Crítica a la presencia de jugadores norteamericanos

Research reports have shown that interactions between clay and OM exist in the soil. Clay and silt are involved in the formation of organo-mineral complexes and adsorb a large amount of OM and nitrogen in the soil. The report of Reuter (1994) shows that in one of their experiments, a soil made up of 6% silt and clay, and 94% sand held 66% of its organic matter and 70% nitrogen in the smaller silt and clay fractions.

1.1.9.1 Protection of organic matter by clay

Clay interacts with and bonds to organic matter via different mechanisms. In the presence of polyvalent cations such as Ca and Mg, clay particles can bind organic matter on its surface with the cation acting as a bridge holding the two together to form a micro-aggregate (Bonneau and Souchier, 1982 in Theng et al., 1986; Römkens and Dolfing, 1998). The reaction can be represented as:

Clay- + Ca2+ + RCOO - à Clay-Ca-RCOO

The micro-aggregates can then combine to form macroaggregates (Don and Schulze, 2008: Kaiser and Zech, 2000; Leinweber et al., 1993; Reuter, 1994).

Clay could also form complexes with organic matter by absorbing the latter into its crystal layer. Theng et al. (1986) proved that smectitic clay under acidic soil conditions has the capacity to intercalate compounds of organic carbon in this way. Their report showed that polymethylene humic substances tended to accumulate within the clay interlayer crystals of two low pH soils in New Zealand, that were rich in smectites and OM, resulting in an increase in the thickness of the clay layer.

The texture of a soil determines its ability to protect soil organic carbon. A soil with 40% clay has been shown to have higher soil organic carbon and soil microbial biomass carbon than a sandy soil with 15% clay (Franzluebbers et al., 1996), attributed the differences to the increase in input of carbon from biomass in the fine-textured soil as a result of its high fertility compared to sandy soils. Alternatively, this could also result from the ability of clay to protect organic matter against microbial

mineralization (Hassink et al., 1993 in Franzluebbers et al., 1996). The binding of organic matter to clay interlayer surfaces and within aggregates protects the organic matter from decomposition and thereby enhances C sequestration (Fujii et al., 2011; Lutzow et al., 2006; Neff and Asner, 2001; Dixon, 1991).

It has also been observed that soil microbial carbon constitutes a larger fraction of soil organic carbon found in clay, and that the proportion of soil microbial carbon in soil organic carbon increases with the increase in clay content of the soil. Availability of soil water in the well-aggregated soil, as well as the ability of clay to prevent soil fauna from feeding on soil microbes also supports their proliferation in fine-textured soils (Franzluebbers et al., 1996).

One major difference between refined clay mineral and crude clay found in the soil is their CEC. The CEC of clay fractions found in the soil are usually higher compared to corresponding clay from clay deposit, and this has been attributed to the formation of organo-mineral complexes. When clay interacts with organic matter, the properties of the two fractions could combine, producing an additive or synergic effect. That is, the inherent high CEC of the clay together with the high CEC of OM produces the higher CEC (Leinweber et al., 1993).

1.1.9.2 Benefit of co-application of compost and clay on sandy soil

Evidence from research has shown that both clay and OM have the capacity to improve properties and suitability of sandy soils for crop production. However, depending on the type of sandy soil, the climate and the amendment application method, there are negative effects associated with each of the amendments. One of the major benefits of applying the two together is their ability to mitigate the limitations of each other. Clay is expected to offset the non-wetting property of some humic substances, whereas OM should be able to prevent crusting or hard setting of clay, a feature commonly found in sandy soils ameliorated with clay (Djajadi et al., 2012). The other expected benefit of co-application of clay and OM is that they should have a synergistic effect on sandy soil properties. Kramer (1983) reported that the benefits of amending sandy soils with organic matter alone do not persist for as long in the absence of enough soil clay. Having established that both clay and OM properties (high water holding capacity, high CEC, increase in and protection of soil organic

matter) can improve physical and chemical properties of a sandy soil, it is therefore hypothesized (Djajadi et al., 2012) that their combined effects would be significant in reducing water percolation and nutrient leaching, and in improving crop yield.

The combination of clay and OM additions may increase the acceptability of using amendments by the farmers. A clay-OM mixture may reduce the amount of OM farmers require to ameliorate the fertility of sandy soil if it was used alone. The bulky nature of most OM has served as a barrier for its use in agriculture, but when combined with clay, smaller amounts of OM would be required.

Investigating the effect of combined clay and OM on properties of sandy soils has been attempted by some researchers. Djajadi et al., (2012) reported increased aggregate stability, reduced soil respiration, but decreased soil strength when a sandy soil in Australia was amended with lucerne hay applied at 0, 0.4 and 0.8% and kaolinite at 0, 2, 5, and 10% (w/w) in an incubation experiment for 42 days. Others have also reported a reduction in soil respiration when kaolinitic clay and OM is co- applied (Nguyen and Marschner, 2013; Shanmugam et al., 2014; Shanmugam & Abbott, 2014). The combined application of clay and OM can also reduce leaching. Nguyen and Marschner (2013) reported reduction in N and P concentration when a sandy soil amended with compost (27.3 g/kg) and fine subsoil (34% clay) at 5 and 20% (w/w) was leached with reversed osmotic (RO) water 23 days after incubation, but the amendments have no effect on N and P availability in the amended soil. The findings of Shanmugam et al. (2014) further show a reduction in the rate of N released when a kaolin amended biosolids was added to a sandy soil up to 2 weeks at 50 t/ha and up to 4 weeks at higher rates. Mekuria et al. (2014) demonstrated the effectiveness of bentonite (10 t/ha), composted manure and clay (10 t/ha), compost (4 t/ha), rice husk biochar (10 t/ha), biochar compost and combinations of these to increase maize yield in a two-year field experiment. in the Laos Peoples Democratic Republic. The composted manure/clay mixture was made from local clay obtained from pond dredge and cow manure and tested at two sites. Amendments were incorporated to the depth of 15 cm, 15 days before planting maize in the first year and any residual effect was examined in the second year. The result showed that amendments were able to increase maize yield in the two years compared to the control, with the yield of the first year being higher than the second year. The treatments improved soil chemical conditions as well (Mekuria et al., 2014).

The above review shows that few experiments carried out on the use of combined clay and OM as sandy soil amendment are short-term laboratory or glasshouse investigations involving only one type of clay (Djajadi et al., 2012; Nguyen and Marschner, 2013; Shanmugam et al., 2014; Shanmugam & Abbott, 2014). Since the properties of clay and OM differ from one type to another, different responses are expected, depending on the choice of OM and clay type and application rate. Duration of interaction between the two amendments and the introduction of plants could also make significant changes to the observed results. Any benefits achieved in short-term experiments could differ where more time is allowed for interactions between the amendments. Thus, long-term investigation under field conditions would be necessary to validate these observations.

Mekuria et al. (2014) aimed at deriving locally available material for improving soil fertility in their experiment and did not use sandy soil per se. However, one of the soils used in their experiment had 67.7% sand, the other soil 47.7% sand with a clay content of the two soils at 27 and 25%, respectively. These clay contents are high to consider them problematic within the context of the definition of problematic sandy soil (Hartemink and Hunting, 2005; WRB, 2006). The work was conducted in a tropical monsoon climatic region in Asia, where alternating wet and dry seasons mean moisture deficient and excess are limiting factors to crop growth. This is not the case in the United Kingdom (location of the present study), where rainfall is fairly equally distributed all year round. Thus, one could expect differences in responses of plant and soil properties to soil amendments in the two regions. It is also noteworthy that Mekuria et al. (2014) do not report on soil physical and biological properties.