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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 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.

Application of organic wastes to cultivated lands can replace mineral fertilizers but may also alter soil physical properties and enhance pollution potential. The objective of this study was to investigate the effects of biosolids [composted manure (MC) and activated sludge (AS)] and specific biosolid component [orthophosphate (OP), phytic acid (PA) and humic acid (HA)] application on soils differing in texture [loamy-sand (Ramat-HaKovesh, RH), loam (Gilat, GL) and clay (Bet-Dagan, BD)], infiltration rate, runoff volume and soil sediment loss. The soils were packed in erosion boxes (400 × 200 × 40 mm) and subjected to six consecutive simulated rainstorms, each of 186 mm deionized water. The results showed that runoff volume and sediment loss from untreated soils increased with increasing clay contents. In treated soils, the response to AS application differed from the response to other amendments; in the BD clay and GL loam, it was the only amendment that caused a decrease in sediment removed by runoff. In the RH loamy-sand, all amendments reduced the final infiltration rate, but only AS and HA increased the measured runoff. It is proposed that the difference in the response of the soils to the amendments is associated with the soil's ability to attenuate changes in the negative charge on the clay edges following the increase in the specific adsorption of charged anions, thus controlling clay swelling and maintaining aggregate integrity. The effects of amending soils with a source of organic matter in order to control runoff and soil erosion are not straight forward and depend on soil and amendment properties

Trickle irrigation has a special advantage in sandy soils where the accurate control of water and ions in the plant's root volume is critical. The purpose of the present work was to investigate the simultaneous migration of water and nutrients from a drip source in the field, to study the plant's response to various moisture and concentration distributions in the soil and to define optimal conditions for maximum yields. Tomato plants (Lycopersicum esculentum Mill.) grown in a field of fine sand received various daily rates of water, N, and P applied through a trickle irrigation system. Highest fruit yield (about 110 tons/ha) was obtained, when the daily average water content in the soil root volume was about 5% (w/w), the N concentration in the soil solution 140 ± 40 ppm N, and the N uptake rate about 100 mg N plant−1day−1. The estimated daily water consumption by the plants varied between 0.4 and 1.0 liter plant−1day−1, depending on the plants leaf area and climatic conditions. The estimated water quantity used to produce 1 g dry matter was similar in all the treatments — about 250 ± 40 g H2O/g d.m.

Pseudomonas cepacia is known as a rock phosphate (RP) solubilizer in bioreactors and in soils. The objectives of this study were to determine the production rates of gluconic acid (GA, pKd 3.41) and 2‐ketogluconic acid (KGA, pKd 2.66) by the bacteria in the presence of clay minerals which prevail in soils, and the resulting rate and extent of orthophosphate (OP) release into the suspension solutions. Suspensions (1:40) of RP, RP + Ca–kaolinite (CaKL), RP + Ca–montmorillonite (CaMT), and RP + K–montmorillonite (KMT) were inoculated with P. cepacia E37. The electrical conductivity (EC) and pH, and the OP, glucose, GA, KGA, Ca, and Al concentrations were determined in the suspension solutions as functions of time. In a given clay system, the rate‐limiting step in RP dissolution was the rate of GA release by the E37. This acid lowered the pH of all the clay suspensions to 2.7 to 2.8, which resulted in a pronounced increase in the OP concentration in solution, Cp. As glucose was depleted from the system, the KGA concentration increased with a concomitant lowering in pH to ≈2.5. At this pH, a sharp decline in Cp occurred, which was attributed to Al release by the alumosilicates, and formation of a new, stable Al–P or Fe–P solid phase. The E37 glucose oxidation efficiency (GOE) was considerably inhibited in CaKL as compared with CaMT or KMT. The GA and KGA adsorption by the clays or their Ca complexation did not play a role in the E37‐mediated RP solubilization.

Information about responses of plants in general, and of tomato (Lycopersicum esculentum Mill.) in particular, to various trickle‐irrigated soil volumes is meager. This information is essential in planning water rates and geometries and in scheduling drip irrigation systems. Our objective was to study root development and distribution in a sandy soil in the field under three trickle irrigation regimes; yields, dry matter production, water and N content in the root zone and N uptake by the tomato were all evaluated in relation to root development. Irrigation frequency increases from one to three irrigations per day produced a significant increase in dry matter yield, but decreased fresh fruit yield. A seasonal application of 620 mm compared to 320 mm, both applied as one irrigation per day, did not increase dry fruit yield, but increased fresh fruit yield by a factor of 1.46. The main factor that contributed to the different response of the plants to the irrigation treatments, was the root weight and distribution in the soil during the growth period. In the once‐a‐day irrigation treatments, the root weight declined at the time of fruit filling, which caused a delay in production, while when irrigating three times a day, that decrease was not observed, and fruit dry matter production occurred at a higher rate. This response produced a greater final dry matter yield. In all the treatments the rapid fruit filling period was followed by a 55‐day period of slow dry matter production, which, in turn, was followed by an enhanced production rate.

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