Recycling of organic wastes via their incorporation in cultivated lands is known to alter soil structural stability. Aggregate stability tests are commonly used to express quantitatively the susceptibility of soil structural stability to deformation. The objective of this study was to investigate the effects of biosolids addition, namely composted manure (MC) and activated sludge (AS), and spiking of the soils with orthophosphate (OP), phytic acid (PA) or humic acid (HA), on soil aggregate stability of semi-arid loamy sand, loam and clay soils before and after subjecting the soils to six rain storms (each 30 mm rain with a break of 3–4 days). Aggregate stability was determined from water-retention curves at high matric potential. The effects of the applied amendments on pre- and post-rain aggregate stability were inconsistent and soil-dependent. For the pre-rain state, all of the tested amendments improved aggregate stability relative to the control. For the post-rain condition, aggregate stability was lower in the MC, OP and PA treatments and higher in the AS and HA treatments than in the control. The coarse-textured loam and loamy sand soils were more affected by the soil amendments than the clay soil. For the pre-rain state, addition of organic matter significantly improved macro-porosity and hence the stability of apparent macro-aggregate (>250 μm). Our results indicate a possible advantage for separation of aggregates into macro- and micro-aggregates for more precise evaluation and understanding of the effects organic amendments might have on aggregate stability.
The overall objective of the present study was to determine the loading limits of composts that should be applied annually to irrigated wheat. We conducted a container experiment in a greenhouse during four years. It included eight treatments: sewage sludge compost (SSC) and cattle manure compost (CMC), each applied annually to a sandy soil, at rates equivalent to 3, 6, and 12 kg m(-2), and two controls, one fertilized and one unfertilized. Total dry matter (DM), grain production, and the amount of N, P, and K taken up by plants increased with increasing compost rate. Nitrogen uptake by the plants of the fertilized control was much higher than by the plants of the highest compost rate. Phosphorus and K uptake by the plants amended with the highest compost rate was much higher than by the fertilized control plants. Inorganic N quantity in the soil increased with increasing compost rate and with successive applications. The net N mineralization during the first year of wheat growth was very low, less than 3.5% of the applied organic N under all compost application rates. The contribution of the organic N mineralization increased during the second and third years. Most of the N increase in the compost treatment was found in the upper layer of 0 to 15 cm, whereas in the fertilized treatment N accumulated from the surface to the bottom of the container, 0 to 55 cm. The successive application of high rates of composts resulted in P and K accumulation in the soil profile.
The mechanisms by which roots affect the release of phosphate from soil surfaces into the solution is not clear. This study was undertaken to investigate the competition of several agents, reported as root exudates, with P on adsorption sites on kaolinite and montmorillonite. The agents studied were acetate, bicarbonate, citrate and oxalate, and the amino acids phenylalanine and alpha‐amino isobutyric acid. The adsorption or desorption of P on the clays in the presence of the noted agents at various fixed pHs and known ionic strength, and the effect of slurry density on P adsorption, were determined. Citrate, bicarbonate and oxalate decreased, and acetate and the amino acids increased P adsorption to the clays. A decrease in the solution‐to‐clay ratio from 100 to 10 L/kg resulted in enhanced P adsorption. The unifying principle that explains these findings is that P adsorption on a clay depends on the extent of the depression of the negative potential of the clay platelets. Bicarbonate and citrate anions at pH > 8 compete with P for active Al sites on the clay. The increased P adsorption in the presence of amino acids is suggested to stem from the fact that H‐bonding of the amino groups causes a decrease in interlamellar spacing, and produces quasicrystals. The quasicrystals reduce the spillover of negative charge from the surface to the edges and allow closer approach of the phosphate ion to the Al site.
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