Journal of Agricultural Science and Technology
Volume:5 Issue: 1, Apr 2003

To achieve a reasonable level of precision in tractor-based field operations, a tractor operator has to guide accurately, monitor and control both the tractor and the attached implement. Since guidance is the most time consuming task among the others, researchers have attempted to automate the guidance task. However, the use of automatic guidance and control in agricultural applications is not always appropriate. Transportation of the vehicle on a public road is an example of this. Some researchers, therefore have focused on Vision-Aided methods to give some guidance aid to the driver rather than on eliminating the driver. To investigate the accuracy of such methods, a Vision-Aided tractor guidance belt-type simulator was developed. An experimental prototype of the simulator was constructed. To evaluate the prototype, a completely randomized factorial experiment was conducted with forward speed, heading angle, and camera tilt angle being the major factors under investigation. The simulator performed satisfactorily at 5 and 7km/h and mean deviations of 1.14 and 2.31cm were obtained respectively.

A new once-through air solar collector was modeled and tested. In this solar heater a transpired absorber was used. The cover of the collector was double glazed and consists of many slats and assembled in such a way that it formed a stair step fashion and made many slots through which inlet air was sucked into the collector. The sucked air is believed to recover part of the sort wavelength radiation absorbed by the glass sheets. Furthermore, the long wavelength emission from the transpired absorber was trapped by the double glazing cover and could also be captured by the air thus reducing total heat loss. A mathematical model was developed to predict the effects of variations in the input parameters on the collector thermal efficiency. The theoretical results showed that the thermal performance of the collector was sensitive to air flow rate, ambient temperature, solar irradiance, absorber emissivity variations, Slat length and slot height. The collector was tested under a solar simulator over a wide range of air flow rates. The experimental results were in good agreement with the theoretical values. An absorbing efficiency as high as 82% could be obtained. Since the air heater was once-through, it is very suitable for grain drying purposes.

Current assimilation, as a source of carbon for grain filling in cereals, may be limited for normal grain filling under Mediterranean climate. In such conditions reserves accumulated before anthesis play an important role in grain growth, but the extent of their contribution depends on prevailing environmental conditions. The experiment described here was carried out to determine the effects of different levels of water stress on dry matter and nitrogen accumulation and their remobilization from the shoot to the grain. The pot experiment was conducted in the glasshouse using two wheat cultivars (Sun 92A and Vasco) differing in yield and protein content. The plants were subjected to water stress at 10 days after anthesis (daa). The following treatments were established: not watered (severe stress), medium stress (-2 Mpa water potentional of the Flag leaf), mild stress (-1 Mpa water optional of the Flag leaf), divided root and control. Results indicated that the dry matter and N content of vegetative organs and its concentration were greater at 24 daa than at maturity under all conditions. Water stress during the post anthesis period was unfavorable to a high assimilation rate, so yield was determined to a great extent by the availability of water and the behavior of the cultivars related to shoot reserve remobilization during the grain filling period.

The effects of nitrogen application (Urea, 46% N), fruit thinning and ringing on flower bud formation of “Golden Delicious” apple trees were studied in two independent experiments in 1992 and 1993. In the first experiment, the effects of nitrogen application and fruit thinning at levels of 0, 25, 50 and 75% on flower bud formation were studied in full-bearing trees. In the second experiment, the effects of nitrogen application and ringing on flower bud formation were studied in poor-bearing trees. The results showed that full-bearing fertilized and unfertilized apple trees produced almost the same flower buds, but the thinning treatments at levels of 50 and 75% produced higher flower bud formation as compared with controls (P< 0. 0 1). For poor-bearing trees, both ringing and application of nitrogen together or alone showed an increase in flower bud formation in the following year compared to the controls (P< 0. 0 5).

The forms, amounts and distribution of phosphorus (P) were determined in 28 calcareous soils from South-Western Australia to evaluate soil P status and their contribution to soil P tests. The soils under study exhibited wide ranges in content of total P (TP), organic P (org.-P), inorganic P (Pi) fractions and of P extracted by soil tests for plant available P. The values of TP ranged from 33 to 392 mg/kg with a mean of 176 mg P/kg soil. Org.-P constituted a considerable portion of TP (mean 35%). The mean ratio of organic C/ org.-P was 287. In general, the distribution of Pi fraction was Ca2-P, 15; Ca8-P, 27; Al-P, 26; Fe-P, 14; occluded-P (O-P), 20, Ca10-P, 16 mg P/kg soil, accounting respectively for 13%, 23%, 22%, 12%, 17%, and 13% of total Pi. The pattern of distribution of the Pi fractions, however, differed between virgin and cultivated soils. For virgin soils, the average relative abundance of Pi in the secondary Ca-P [(Ca2-P)+(Ca8-P)], nonoccluded Fe- and Al-P [(Al-P)+(Fe-P)] and acid extractable P (primary Ca phosphate (Ca10-P)) was 2.5:2:1 and in cultivated soils was 2:2.5:1, respectively. Statistical analysis indicates that the Ca2-P, Al-P, Fe-P and Ca10-P fractions made significant contributions to available P as estimated by the Olsen and Colwell methods. Stepwise regression showed that most of variation in Olsen-P (90%) and Colwell-P (82%) was accounted for by Ca2-P. Scanning electron microscopy (SEM) showed that P was uniformly distributed in the soil matrix to the limit of the spatial resolution and sensitivity of this technique. There were no local concentrations of P or spatial associations of other elements with P.

Gypsiferous soils occur in the arid and semi-arid regions of Iran and are found in different geomorphic units. Few studies are available on the genesis and classification of these soils. In the present study, fifteen pedons from an area extending from Jaafarabad Mountain to the central piedmont plain are studied. The objective is to classify the gypsiferous soils studied according to the USDA Soil Taxonomy (1990, 1994, 1996, 1998, and 1999) and WRB (FAO, 1974, 1988- FAO, ISSS, ISRIC 1994, 1998) systems. For this purpose, the designation, amount of gypsum, depth and the thickness of gypsic horizons will be taken into account. The study area includes different geomorphic units such as gravelly fan, quaternary gravelly alluviums and piedmont plain. The 1994 version of the USDA Soil Taxonomy allows for more characteristics of the soils under study to be defined as compared with the previous versions, while the more recent versions (1996, 1998, and 1999) have remained unchanged in this regard. FAO (1974, 1988) and its successor WRB (1994, 1998) show continual and immense progress allowing for greater differentiation of various soils. It will be shown that the WRB 1998 version offers greater possibilities for more detailed characteristics to be included in the classification system; hence, its higher efficiency in comparison to the USDA system.

The properties of soils affected by salinity and the processes involving degradation of soil structure are relatively well-known. However, the effects of saline and sodic conditions on the physical and mechanical properties of soils have been studied to a lesser extent. In this research, the effects of electrical conductivity (EC) and sodium adsorption ratio (SAR) of solutions were assessed on soils containing various amounts of organic matter under laboratory conditions. The soils used were collected from the Dasht-e-Naz of Sari region in the north of Iran and contained the same clay type, predominantly Illite. The major difference between the soils was their organic matter content. The treatments consisted of applying solutions with definite EC and SAR (two levels of EC: 0.5 and 4 dS/m and three levels of SAR: 0, 5 and 15). With an increase of SAR, the wet aggregate stability (WAS) decreased and the amount of dispersible clay (DC) increased. At a similar SAR, higher EC values were associated with lower DC. The order of average aggregate stability of soils were: virgin soils > soils under continuous cultivation of Festuca > soils under intensive cultivation with regular crop rotation > soils under continuous cultivation of Agropyron. The analysis of variance showed significant differences (at 1%) between soil types, soil sampling depths, EC and SAR.

Phosphorus (P) fertilizer recommendations for calcareous-sandy soils low in organic matter need further investigation. Therefore, the objectives of this study were to evaluate the effects of P and manure on corn (Zea mays L.) growth and chemical compositions and P recommendations for calcareous sandy soils with low organic matter under greenhouse conditions. Treatments consisted of a factorial arrangement of P levels (0, 25, 50, and 100 mg kg-1 soil as KH2PO4), manure rates (0, 10, 20 and 30 g dried sheep manure per kg soil) and three soils (Soil 1, sandy loam, initial P 10.8 mg kg-1; Soil 2, sandy loam, initial P 7.6 mg kg-1; and Soil 3, loamy sand, initial P 5.5 mg kg-1) in a completely randomized design with four replications. Results showed that P application in Soil 1 decreased corn dry matter. However, application of 25 or 50 mg P kg -1 soil increased corn yield significantly in Soils 2 and 3, respectively. Maximum corn yield was obtained when 30 g kg -1 manure was added to sandy loam soils and 20 g kg-1 to loamy sand soil. Application of P and manure significantly increased plant P concentration and uptake in all three soils. Zinc concentration in plants treated with Phosphorus was higher than in the control in soils 1 and 2. Such a trend was not observed in soil 3, but manure application increased it. Iron concentration in plants treated with P increased in soils 1 and 2 but was decreased in soil 3; however, manure application increased it in all soils. Plant Mn concentration and uptake responses to P and manure application was not consistent. Applied P, in general, increased plant Mn Concentration in soils 1 and 2, but had no effect on plants in Soil 3. Manure effect on plant Mn concentration was not consistent. It seems that addition of manure to sandy soils can improve soil productivity and increase corn yield. Due to the low P buffering capacity of sandy soils, application of high rates of P can increase P concentration to an undesirable level in soil solution. This may depress plant growth and also availability of some micronutrients like Fe and Zn to corn plants. Therefore, P fertilizer recommendations for sandy soils should be based on the soil test P level. Manure application is recommended for sandy soils, due to its positive effects on nutrient uptake and plant growth. Prior to any phosphorus fertilizer recommendations for sandy soils the results of this experiment should be verified under field conditions and measuring P concentration in soil solution at different stages of plant growth is highly recommended.