Volumen 20, Número 1 (Junio, 2002)

Water conservation is essential for successful dryland crop production in semiarid regions. Improved water (also soil) conservation has been achieved in many cases by using conservation tillage, including no-tillage, which is the ultimate type of conservation tillage. The purpose of this article is to review and discuss the principles and practices of conservation tillage and the results of some studies relative to their effect on conserving water for improving crop yields in dryland regions such as those in southern South America. Water and soil conservation is possible by using tillage methods that reduce the amount and/or rate of runoff and improve water infiltration in soil. This is possible even when crop residues are not retained on the soil surface, which is covered by the broad definition of conservation tillage. Improved water and soil conservation, however, usually occurs when crop residues are retained on the soil surface, as covered by the operational definition of conservation tillage. Water and soil conservation usually improves with increasing amounts of residues retained, and results of numerous studies involving conservation tillage, including no- tillage, show that increased water conservation increases the yield of dryland crops. Improved soil conservation resulting from use of conservation tillage helps protect soils for sustained crop productivity.

Phytoliths are hydrated silica bodies that are included in plant tissues. Along with another bioliths and volcanic ashes, phytoliths are the main low-density mineral fraction in soils of the Pampa region. The aim of this work was to determine the impact of low-density minerals in the silt fraction on physical properties related with structural stability and porosity. Mixtures with different low-density minerals and clay content were constructed to determine their tensile strength and swelling – shrinkage capacity. The general trend of tensile strength curves showed that the highest the water content, the lowest the tensile strength. This variable showed significant differences at all water contents but at higher water potential. Mixtures with low percentage of light silt fraction showed the highest tensile strength. Near saturation mixtures didn’t show a well defined crash point. Overall, tensile strength was higher with high clay content, low water content and without low-density minerals. Shrinkage curves were obtained for the same mixtures used for tensile strength tests varying the low-density minerals content. In this case, two types of clay were used: illite and swelling clay. Mixtures with illite behave very rigid. Low-density minerals level didn’t modify the matrix volumetric behaviour. The swelling clay curves showed a normal shrinkage behaviour and a higher slope than the illite curves. Thus, clay type appears to be the main factor affecting volume change with water content. For both clay types, mixtures with higher low-density minerals content showed the highest specific volume values. As a conclusion, the surface layer of the Pergamino soil has a high low-density minerals content, which involves negative effects on its tensile strength and therefore on structural stability.

Several studies have reported modifications in soil chemical and physical parameters under rotations with legume inclusion. This modification had the potential of improving soil fertility and grain production. The objective of this research was to evaluate changes in the distribution and quality of soil organic fractions during a 3 year wheat (Triticum aestivum) – 3 year red clover (Trifolium pratense) rotation. Soil samples at 0 – 0.15 m depth were taken in five of the six year rotation. Organic fractions, physically (particulate organic carbon, COP) and chemically (humic and fulvic acids, AH and AF, respectively) separated were determined and chemically and spectrometrically characterised. The quantity and distribution of the organic fractions were different along the rotation. The more labile soil organic matter fractions, COP and AF, were higher during legume phase and diminished during wheat phase. This decrease of labile relative to recalcitrant organic matter resulted in significative differences in COP:total organic carbon (COT) and AH:AF ratios between wheat and clover phase. The COP:COT ratios were lower than 19 under wheat and higher than 21 under clover. The AH presented higher nitrogen and lower sulphur contents during clover phase, while the opposite was observed during wheat phase. This compositional changes were clearly reflected by the N:S and C:S atomic ratios. Total acidity in AH was slightly increased during the legume phase, as a consequence of the significative increase in –OH phenolic groups. Infrared and fluorescence spectra of the AH from soils under wheat and clover were similar among them, probably due to the short rotation period that produce only minor changes in the AH molecular structure. Legume inclusion in the rotation increased labile organic matter fractions and modified the composition of the recalcitrant ones. Both changes would improve soil fertility and grain production.

Corn (Zea mays L.) yields increased in the last years, and this increase has been associated with higher nutrient requirements. Nevertheless, nitrogen fertilization and the tillage system can produce changes in some soil characteristics and in the growth of crop roots that can improve the availability of other nutrients. The objectives of this work were to evaluate the effect of nitrogen fertilization and the tillage system on the concentration of macro and micronutrient in corn plants. Four experiments were conducted during the 1997/98 growing seasons. Three were conducted under conventional tillage (LC) in Balcarce, Paraná and Rafaela, and the other under LC and no-till (SD) in Balcarce. The treatments were N rate: 0, 70, 140 and 210 kg ha-1. Another treatment consisting of 140 kg N ha-1 plus sulfur, magnesium and zinc (140v) was also included. Macro and micronutrients were determined in leaf samples collected at flowering (opposite to- and below the ear). The results were interpreted according to critical values (NC) and sufficiency ranges (RSN). Nitrogen availability and tillage system affected significantly nitrogen accumulation, dry matter and grain yield. The higher nitrogen availability in Paraná and Rafaela generated luxury consumption of nitrogen, whereas in Balcarce low nitrogen availability caused high nitrogen use efficiency. The addition of nitrogen increased sulfur and manganese leaf concentration in all experiments but did not affect calcium and magnesium concentrations. In Balcarce, this treatment also increased phosphorus, potassium, iron, copper and zinc concentration. The application of magnesium, sulfur and zinc at sowing did not improve corn yield. Under SD there were lower leaf nitrogen, potassium and iron concentration at flowering. The RSN at flowering, indicated that potassium, magnesium, sulfur, copper and zinc were the most limiting nutrients. However, grain yields were close to the potential yield for this locations.

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Specific objectives of this work were: a) to determine the fresh leaf sap (JF) nitrate dynamics during the growing season in garlic cv. “Fuego INTA” , b) to evaluate the relationship between nitrate concentration in JF and different N fertilizer rates, and c) to establish sufficiency concentration nitrate for fertirrigated garlic based on JF analysis associated with maximum yield. Two field studies were conducted on loamy sand soil (Tipic Torrifluvent) in Mendoza, Argentina with drip irrigation. During the first year urea, UAN (30N, liquid fertilizer) and ammonium nitrate at rate of 150 y 300 kg N ha-1 were used. The following year, urea and UAN at rate of 150, 300 and 450 kg N ha-1 were applied Nitrate in garlic leaves was determined with a portable nitrate electrode, the Cardy Meter. Results showed that there were no differences between nitrogen fertilizers. However the total yield was adjusted to nitrate rates by a quadratic model (P>0,001) with R2=0,79 and 0,83 for two-year trials, respectively. Nitrate concentration in JF increased since September until October and then decreased as the crop matured. Nitrate levels were correlated with higher rates of N fertilizers. The determined thresholds of nitrate concentration in the JF to achieve maximum yields were 1,5; 1,3; 1,7 and 0,8 g L-1 for 1-Set., 15-Set., 1-Oct., and 15-Oct., respectively. During the vegetative / bulbing initiation stage (178 days after emergency) the nitrate concentration sufficiency on JF associated with maximum yield was 1,7 g NO3 L-1.

Two chemical methods were analyzed as alternative for estimating available phosphorus in agricultural soils, considering the Bray and Kurtz I test as the reference. The ammonium lactate method is not useful for estimating available phosphorus from soils with pH levels higher than 7. However, the NaHCO3 method is recommended for a wide range of pH values from 5.5 to 8.5. Significant correlations were obtained by comparing the three methods studied. The Bray and Kurtz test exhibit the lowest dispersion of values and also the lowest soil phosphorus availability index. In general terms both two alternative methodologies estimate 4 – 5 mg kg-1 more available phosphorus than Bray and Kurtz I, even in the case of low levels of phosphorus sufficiency. The methodologies recommended here are expected to be useful for testing values and also giving accuracy to soil analysis interpretation.