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Currently used conventional tillage systems for small-scale vegetable production in the region of Saltillo, Coahuila, Mexico require a considerable amount of hand labor, energy and materials for all activities. Seedbed preparation can require up to 60% of the total production cost in some systems in Mexico. Further, soil is degraded and eroded due to the system. Conservation tillage may reduce costs and prevent soil degradation, but appropriate tools, such as, no-tillage seeders for small-scale farmers are not available. This papers reports on the design and construction of a prototype of a no-tillage seeder for small-scale conservation tillage using a 2-wheeled tractor. Three main functions received particular attention: opening of the soil, placing seed and/or fertilizer and closing the slot. Because of its vapor conservation and good seedling emergence, tools to create T-shaped slots were chosen, with adapted depth control and closing and covering devices. A systematic design process was applied in order to reach the required decisions. Function diagrams were defined from where morphologic charts guided the selection of the configuration of the seeder. A preliminary evaluation included testing of two furrow opener disc types (notched and fluted), and four crop residue levels, 0, 30, 60 and 100%, with respect to performance of the seeder. An evaluation showed that with low cover amounts a consistent and firm seed cover was obtained, but emergence quality decreased due to insufficient residue cover. The notched disc had a better performance than the fluted disc. The inverted T-shape in the soil was not always sustained due to technical flaws. It was possible to build a prototype under 2000 dollars with basic tools in a local workshop. Further research will focus on the biological performance and improvement of the mechanical components and performance.

In the tropical area of Mexico, when and how to carry out tillage is a qualitative decision. There is no quantified information about the interaction between a chosen process of cultivation, soil type and weather, which dictate the tool and power requirements. Waste of energy and soil degradation by erosion and compaction, and lack of timeliness are recognized problems caused by inadequate tillage management in the tropical area of Mexico.In this thesis, the workable range of soils was quantified in the field, and the limits of that range, represented in the laboratory by tests to soil samples. This information is the basic part of a soil workability based procedure which is developed as a decision support tool for selecting specific tillage practices. The methodology was conceived for application in the tropical area of Mexico. For this reason a description of the area under study in relation to the present agricultural production system, including tillage related problems, was presented in Chapter 2 .In Chapter 3, The field and the laboratory studies to quantify the workable range of soils in the area, were treated. The methodology and results from those studies were presented and discussed, considering the technological result of the tillage operation and the input of energy at a range of soil moisture contents. Empirical relationships between initial moisture content and the technological result of tillage showed that results from Loam and Clay soils changed from optimum to sub-optimum at soil water potentials that rather coincide with the plastic limit in relatively moist soil. As soil was drying out , the implement result changed from optimum to sub-optimum at soil water potentials well below to that for the shrinkage limit, so the actual field work was smaller than the theoretical friable range of the soils. The minimum input of specific energy to obtain optimum results was close to the soil water potential, where results changed to sub-optimum as the soils where drying out. Observations in a Sandy soil indicate that required technological result is attained at almost any moisture content.The thresholds that delimit the optimum and sub-optimum results in the field are represented by means of laboratory tests, air permeability test (for a wet limit) and a Drop test (for the dry limit). The methodology and results from laboratory experiments are also presented, and any links and discrepancies in representing field results were discussed. The thresholds established by the results of the laboratory tests agree well with the findings in the field. This allow that worability limits in terms of soil water potential can be established for other soils in the area by means the mentioned laboratory tests.This was the main part of this research since it was the development of the methodology to characterize the workable range of soils for the area under study.Once the workable range of a soil is known, whether this status is present on time (during the workable period within the cropping calendar) depends on the interactions between soil characteristics and weather. In Chapter 4, the necessary data used to model the soil-water balance in order to obtain the relevant information for tillage planning and operation, was discussed. The possibility of use PTF's for deriving the hydraulic characteristics of soils from limited textural data normally available in the area under study was explored, as they are required by deterministic soil-water models. It was found that PTF's developed in The Netherlands estimate well the hydraulic characteristics for a Loam soil of the area under study.The soil workability and the soil-water balance are the 'tools' to calculate the workable periods, but in this area the data available to determine both 'tools' is available at different spatial scales. The possibility of extending the methodology applied at the experimental field level to other scales and the use of workability information for operational decisions and planning was discussed in Chapter 5.To decide objectively which tillage practice will be chosen and when tillage should be carried out, a great deal of information has to be analyzed. To take advantage of the soil workability methodology developed, it shall be used within an automated procedure wherein the user need only supply its specific circumstances and receive as output the relevant information to utilize as support for planning decisions. In chapter 6 an initial version of such an automated procedure was presented.In Chapter 7 From results and discussions and having as reference the posed hypothesis, general conclusions were stated regarding the development and use of the methodology on soil workability to support the planning and operation of tillage systems in the area under study.