Alleviation of soil physical constraints in direct-seeding systems in Uruguay
Martino, Daniel L.
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One of the major constraints to adoption of zero tillage in Uruguay is soil compaction, which results from a combination of factors including a wet climate, fine textured soils with low water infiltration capacities, and traffic by machinery and cattle: These conditions would be particularly detrimental during the transition from tillage-based systems. The objectives of this thesis were: (1) to assess the extent of soil physical constraints for crop production with zero tillage in Uruguay; (2) to develop analytical methods for describing soil quality from the standpoint of physical structure; (3) to evaluate the effectiveness of soil loosening by the Paraplow in increasing the viability of direct-seeding systems; and (4) to study the interactions of crop sequences with subsoiling by Paraplow on soil properties and crop behaviour. The evidence obtained supports the conclusion that soils impose certain restrictions for the development of crops with direct seeding, chiefly low infiltration capacity, low oxygendiffision rates, and high soil strength. Subsoiling was effective in relieving these constraints over all the volume of the upper 0.45 m of soil, with a maximum effect at 0.2-0.3 m depth. Paraplowing induced yield increases of between 14 (wheat) and 102% (corn), associated with improved crop emergence, a more thorough soil exploration by roots, superior weed control, higher tiller survival and reduced floret abortion. The effects of paraplowing on soil physical properties rapidly declined with time after subsoiling, but lasted for up to 25 months. Effects on crop productivity had similar residuality. The additional yield advantage of paraplowing before each crop, compared with one pass in two years was very small, considering the extra energy cost involved. Sunflower as the first crop in the cropping sequence resulted in reduced infiltration capacity of the soil in the subsequent cropping seasons, as compared with rotations that started with corn. This was reflected in a 4% reduction in wheat and barley productivity, mainly due to lower kennel weight. Wheat tolerated adverse soil physical conditions better than barley. A cone penetrometer was extensively used to assess soil physical quality. Penetration resistance (PR) measured at a certain soil depth was not independent of PR values in soil layers located up to 180 mm above. However, 77% of this effect was restricted to a distance of 45 mm. Autocorrelation was highest where PR decreased with depth, and it was concluded that only in this case PR values should be corrected. The relationship between soil moisture and PR was described by an exponential model, and was affected by soil management practices, as well as by the soil depth considered. The empirical coefficient b, which described the rate of change of PRwith moisture at low moisture contents, varied between $-$0.003 and $-$1.10 among the 14 situations analysed. The lower limit of available water, defined as the soil moisture content at which PR equals 2 MPa, was also shown to vary widely with tillage practices.