Threshold Of Soil Water Content Research Articles

Using plant water status to define threshold values for irrigation management of vegetable crops using soil moisture sensors

Thresholds of soil matric potential (SMP) and available soil water content (AWC) required to prevent water limitations between irrigations were determined for bell pepper, melon, and spring and winter tomato grown in Mediterranean-type greenhouses on the south-eastern coast of Spain. Thresholds were identified by measuring the divergence of leaf water potential of un-watered plants from that of well-watered plants. Soil matric potential thresholds were −58 kPa for pepper, −35 kPa for melon, and −38 to −58 kPa for tomato. In general, SMP thresholds were more negative under lower evaporative demand conditions such as during autumn and winter months. Available soil water content thresholds, for a given crop and drying cycle, differed appreciably depending on soil depth and the method used to calculate the values. For the four crops studied, AWC thresholds calculated at 0–40 cm were 13–15% higher than those calculated at 0–20 cm. Each AWC threshold for 0–20 cm depth was 21–29% lower when AWC was based on laboratory rather than field determinations of field capacity and permanent wilting point. For a given method of calculating AWC, AWC threshold values were similar for different crops and drying cycles, suggesting limited sensitivity of the AWC approach. Using the manufacturer's calibration, the capacitance sensor used for SWC measurements overestimated SWC by an average of 36%. An in situ calibration provided generally good agreement with the actual SWC between 0.15 and 0.22 cm 3 cm −3; however, for higher SWC values, the in situ calibration underestimated SWC. The results of this study demonstrated the uncertainty of using recommended fixed AWC threshold values for irrigation management, using SWC sensors, because of issues related to the definition of rooting depth, measurement of FC and PWP, sensor calibration, and sensor accuracy across the relevant range of water contents. These data suggest that SMP thresholds are much more reliable than AWC thresholds for scheduling irrigations in greenhouse-grown vegetable crops. Technical issues regarding on-farm measurement of SMP and SWC are discussed.

Evolution mechanism of non-hydraulic root-to-shoot signal during the anti-drought genetic breeding of spring wheat

The objectives of this study were to: (1) characterize the evolutional tendency of the non-hydraulic root-sourced signal (NRS) from wheat wild relatives to its modern hexaploid species, and (2) test whether species sensitivity to the NRS was allied with their drought tolerance profiles. The NRS was judged to begin when there was a significant lowering of stomatal conductance without change in leaf relative water content (RWC). The lethal soil water content (LSWC) was operationally characterized as the soil water content (SWC) at the drying lethal point of wheat plants. The threshold of soil water content (TSWC) at which the NRS was triggered, and the LSWC differed amongst six wheat species. For “ MO1” and “ MO4” representing ‘diploid’ species, the TSWC and the LSWC were initiated successively at about 51% FWC (field water capacity) and about 30% FWC, respectively. Conversely, “ Plateau 602” and “ Longchun 8139-2” (modern hexaploid species) exhibited the TSWC and the LSWC between about 68% FWC and less than 14% FWC, a much wider threshold range (TR). Increasing TSWC was significantly correlated with decreasing LSWC ( r = 0.9464 **). The widened TR from the TSWC to the LSWC was also significantly correlated with longer survival days (SD) and higher maintenance ratio of grain yield (MRGY), respectively ( r = 0.9411 ** and 0.8068 *, respectively). Meanwhile, those species having higher TSWC had the least reduction ratio of stomatal conductance under the decreasing soil moisture from −0.2 to −1 MPa. This suggests that advances in yield performance and drought tolerance would be made evolutionally by targeted selection for an earlier onset of NRS.

The influence of grazing on vegetation, soil properties and stream discharge in a small Dartmoor catchment, southwest England, UK

AbstractIntensive grazing has been responsible for environmental degradation of moorland landscapes over the last 50 years. Anecdotal evidence on Dartmoor, UK, for example, has suggested that it is particularly associated with a greater frequency of large floods, lower baseflow and a greater occurrence of erosion. A study was therefore conducted at Holne Moor, a small catchment on east Dartmoor, to investigate the impacts of grazing animals on hillslope hydrology and stream discharge. The vegetation in the study area ranged from heather (Calluna vulgaris), in areas with low grazing intensity, to short grass with occasional erosion scars visible associated with larger sheep numbers. The physical characteristics of the topsoil beneath heather species were very different compared with short grass. This was not due so much to the direct physical impact of grazing animals, but indirectly to the chemical and biological differences brought about by changes in vegetation response and soil structure. The experiment at Holne Moor indicated that soil water content on the hillslope was linked directly to the stream's rainfall–runoff response via the soil water pathways. In relatively dry conditions, the soil water content is mainly determined by local factors such as vegetation. In wet periods, non‐local influences such as slope and contributing area are more important. A soil water content threshold of around field capacity separated the two conditions. A comparison between soil physical properties and soil moisture under different vegetation types indicated that this ‘runoff’ threshold between dry and wet states is lower in heavily grazed areas. It is argued that more intense grazing causes conditions suited to the increased delivery of soil water to rapid flowpaths such as sheep tracks and other rapid routes which connect the source of this hillslope water to the contributing areas during large floods. Any alteration of vegetation and soil properties due to management causes a substantial increase in ‘active’ source areas and therefore discharge during large storm events even before any erosion occurs. Copyright © 2006 John Wiley & Sons, Ltd.

An assessment of the vulnerability of soil structure to destabilisation during tillage. Part I. A laboratory test

A simple laboratory apparatus, consisting of a falling weight, was developed to impose different specific mechanical energy inputs to soil aggregates. Disruption or damage to the soil aggregates was quantified in terms of the amount of mechanically-dispersible clay (particle size 0.7 μm or less), measured by turbidimetry. Natural aggregates, collected from the field were used to determine the sensitivity of three soils to damage as a function of specific energy input and soil water content. Results show that the amount of mechanically-dispersible clay is a function of both soil water content and specific energy input. Below a certain soil water content threshold, approximating to the plastic limit, even relatively large specific energy inputs (in tillage terms) have little effect on clay dispersion. However, as soil water content increases above the plastic limit, the soil becomes increasingly sensitive to mechanical disruption. For soil water contents between the plastic and liquid limits, the amount of clay dispersed is proportional to mechanical energy input. As soil becomes wetter, the rate of increase in dispersion becomes greater. A simple empirical model was developed to predict the amount of dispersible clay from soil water content and mechanical energy input. The laboratory method provides a convenient technique for rapid assessment of soil to mechanical damage, particularly that caused during tillage.