Omission Trials Research Articles

Identification of Nutrients Limiting Rice Growth in Soils of Northeast Thailand under Water-Limiting and Non-Limiting Conditions

SummaryGlasshouse nutrient omission trials are useful in identifying nutrient limitations for plant growth in soils under the same environmental conditions. Soils of low fertility are commonly used for production of rainfed lowland rice (Oryza sativa L.), and the crop often encounters water stress. Nutrient requirements may be modified when standing water disappears from the field. Two experiments with rice seedlings were conducted in a glasshouse at Ubon Rice Research Center, Thailand, to identify the nutrients which limit rice growth in soils of Northeast Thailand, and to determine whether nutrient limitations are affected by water availability. In Experiment 1, rice was grown on two soils (Roi et and Ubon series) under well-watered and water-limiting conditions, and 15 nutrient treatments were imposed. In Experiment 2, six soils from Northeast Thailand were examined using the same 15 nutrient treatments. The nutrients which clearly limited the growth of rice plants in soils of Northeast Thailand were nitrogen (N) and phosphorus (P). In some cases, potassium (K) and sulfur (S) also limited growth, and in one soil zinc (Zn) and boron (B) also limited growth. A shortage of N was the most important limitation for plant growth in all soils except one in which P was more important. The low supply of P decreased plant height and leaf area development during early growth ; low N supply had a greater effect later during growth. The omission of P had a larger detrimental effect on growth when water supply was limited. In the Roi et soil, the omission of S had a large effect on leaf area and total dry matter production only under water stress conditions, but this was not found in the other soils examined. These results from glasshouse studies show that the nutrients limiting rice growth depend on soil type and water availability in soils of Northeast Thailand.

The Timing of Auditory and Visual Signals by Pigeons: Differential Sensitivity to Intertrial Interval Duration

In Experiment 1, pigeons were trained to discriminate short (2-s) and long (8-s) durations of a light to yellow/blue comparisons and the same durations of a tone to red/green comparisons. Following training with a constant intertrial interval (ITI) of 10 s, the ITI was varied within sessions (4, 8, 16, or 32 s). On trials with the light, signal duration alone influenced choice responding. The preceding ITI duration had no effect. On trials with the tone, both the signal duration and the preceding ITI duration affected choice responding. Subsequent delay testing in the context of variable ITIs generally replicated the different response biases previously reported with tone vs light signals. In Experiment 2, signal omission testing resulted in a bias to respond “short” regardless of whether the comparisons associated with light signals or the comparisons associated with tone signals were presented. However, on signal omission trials the preceding ITI duration continued to affect choice responding when comparisons associated with the tone signals were presented, but not when comparisons associated with the light signals were presented. These findings regarding stimulus properties and ITI effects can be explained by the information processing model of timing.

Fluoxetine Alters the Effects of Cocaine on Vigilance Task Performance of Rats

The potential modulating influence of fluoxetine (FLU) on the effects of cocaine (COC) on vigilance task performance was explored. Rats were trained in a discrete-trial task in which pressing one of two levers was reinforced by food, with the correct lever indicated by the position of a briefly illuminated light. Tests with FLU (2.5, 5.0, and 10.0 mg/kg) revealed no significant effects of FLU on mean choice accuracy, log choice reaction time, or number of omission trials; however, 10.0 mg/kg FLU did produce a small but reliable increase in mean log food retrieval latency. The effects of COC (1.25, 2.5 and 10.0 mg/kg) were then assessed following pretreatment with saline or FLU (2.5 and 10.0 mg/kg). COC increased mean choice accuracy, with maximal performance enhancement occurring with 2.5 mg/kg COC; mean log choice reaction time was decreased by all three doses of COC, although there was considerable within-subject variability at the 10.0 mg/kg dose of COC. Pretreatment with FLU reversed both of these performance-enhancing effects of COC in a dose-dependent fashion. Log food retrieval latency was not significantly affected by the lower doses of COC and was significantly increased by the 10.0 mg/kg dose of COC. FLU and COC appeared to interact in an additive fashion with this measure. The number of omission trials per session was significantly increased by 10.0 mg/kg COC—an effect that was not significantly altered by FLU pretreatment. These results support previous studies indicating that acute FLU exposure can reduce many of COC's properties that may contribute to COC's efficacy as a positive reinforcer.

Fertilizer requirements for maintenance of a perennial ryegrass (Lolium perenne L.)/white clover (Trifolium repens L.) pasture growing on a humus iron podzol in N.E. Scotland

AbstractRyegrass/white clover pastures were reseeded on heather moor in north‐cast Scotland in 1978/79. By 1981, despite moderate fertilizer applications, the pastures had deteriorated and dry matter (DM) production was low. After preliminary soil and herbage analysis an omission trial was carried out in 1982/83 at two sites (A and B) to identify the factors which limited production.At site A depressions in growth occurred in the absence of N, P and K. Perennial ryegrass was severely N deficient with concentrations in leaves of less than 22 g N kg−1 in spring and summer even after application of 120 kg N ha−1 annually. Rates of nitrogen fixation were high in spring but rapidly declined in June and July as the soil moisture tension increased. Application of N fertilizer also reduced the N2 fixation rate. Deficiencies of F and K occurred despite apparently high levels of extractable F in the soil. Uptake of these nutrients was inhibited in the dry soil during the summer. White clover was more susceptible to drought than perennial ryegrass, probably because it rooted at a shallower depth.Growth at site B was limited by acidity and lack of N and K. The soil pH was 4 8 (s.d. = 0middot;75) at 1‐5 cm depth. Application of 2 5 t lime ha−1 in spring 1982 had not altered the pH by autumn 1983.It was concluded that methods of incorporating lime into the soil, together with ways of increasing the rates of N2 fixation by white clover and transfer to grass, should be investigated further. Regular small applications of N and K may be necessary to sustain DM production at the level required by the farming system.

Electrodermal orienting to stimulus omission

The study examined whether complete omission of a tone stimulus following its repeated presentation would elicit an electrodermal orienting response and whether the factors of stimulus significance and subject lability influenced the probability and magnitude of any response to omission. A statistically significant increase in response on omission trials over that in control periods without stimulation preceding omission was observed. The omission effect was more likely to occur with labile subjects, those showing high levels of nonspecific electrodermal activity, than with stabiles; stimulus significance was not found to exert any systematic effect. Contrary to expectation, a measure of EEG alpha activity included in the study failed to show the omission effect. The failure was attributed to problems of measurement. Incidental findings of the study refute an arousal interpretation of electrodermal lability.

Grazing studies on the Guadalcanal Plains, Solomon Islands. 1. Climate, soils, and soil fertility assessment

SummaryThe Guadalcanal Plains encompass approximately 460 km2 with both forest and grassland phases. Mean annual rainfall is 2160 mm, with a drier season from June to October, but soil moisture deficits seldom occur. Soils are freely to imperfectly drained brown loam to clays (Ustropepts). Soil analyses show available P concentrations of 15–17 μg/g, adequate K, Cu, Zn and Mn, and high Ca and Mg, but sulphate sulphur is low. A nutrient omission trial gave significant negative responses to the omission of P, K, S and Mo. However, responses to P applications were not obtained in the field with Centrosema pubescens, but significant responses to K and S were found under a cut and remove system. Yield response in the grasses Brachiaria decumbens and B. mutica to applied N was markedly limited in the absence of applied S, even though B. mutica was able to maintain S concentrations in herbage at 2–3 times those of B. decumbens. Without applied S, S contents and N:S ratios in B. decumbens were inadequate for beef cattle, but adequate in B. mutica except at high N inputs (300 kg/ha/year). Application of 40 kg S/ha to the legume C. pubescens increased yields by approximately 50%, reduced incidence of Cercospera leaf spot, and gave increased N concentrations. For establishment of sown mixed pastures on the Guadalcanal Plains we recommend the basal application of 20 kg P/ha, 50 kg K/ha, 30 kg S/ha and 0·15 kg Mo/ha, with 30 kg S/ha applied every 2 years.

Stimulus omission and recovery of the electrodermal and digital vasoconstrictive components of the orienting response

Three experiments are reported which investigated Sokolov's (1968) hypothesis that, after a number of stimulus presentations, complete omission of a stimulus leads to increased responsiveness of the orienting response (OR). The skin conductance response (SCR) and ringer pulse volume (FPV) response components of the OR were studied. In experiment 1 ( N = 60), the effect of number of pre-omission training trials on response recovery was investigated, while experiment 2 ( N = 120) investigated the effects of stimulus intensity (70 or 90 dB) and interstimulus interval (12 or 21 sec) on recovery to stimulus omission following a fixed number of training trials. In experiment 3 ( N = 40), an attempt was made to control for possible below-zero habituation effects by training each subject to a habituation criterion before stimulus omission. All experiments employed a 1000 Hz tone of 3 sec duration which was presented at a constant interstimulus interval. Although recovery of the SCR did occur under some conditions, the results were largely negative. Manipulation of the number of training trials, training stimulus intensity and interstimulus interval had little effect on response recovery. A consistent finding, however, was that subjects who displayed SCR recovery also displayed significantly more spontaneous fluctuations in skin conductance during the pre-stimulus period and required significantly more training trials to reach the criterion of habituation than did subjects displaying no recovery. Moreover, the SCRs displayed by ‘labile’ subjects on omission trials were significantly larger than those displayed on either the last training trial or during a control interval just prior to stimulus omission.