Search

Sampling field soils to estimate soil water content and soil water storage (S) is difficult due to the spatial variability of these variables, which demands a large number of sampling points. Also, the methodology employed in most cases is invasive and destructive, so that sampling in the same positions at different times is impossible. However, neutron moderation, time domain reflectrometry, and, more recently, frequency domain reflectrometry methodologies allow measurements at the same points over long time intervals. This study evaluates a set of neutron probe data, collected at 15 positions placed randomly along a coffee crop contour line, over 2 years at 14-day intervals. The temporal stability of S was again demonstrated, so that wetter or dryer locations remain so over time, and the definition of such positions in the field reduces the number of sampling points in future S evaluations under similar conditions. An analysis was made to determine the minimum number of sampling points to obtain the average S of the field within a chosen level of significance. Classical statistical analysis indicated that the 15 measurement positions could be reduced to four or even to one position to obtain a reliable field S average. State--time analysis showed S estimations depend more on previous measurements of rainfall P (52%) than on evapotranspiration ET (28%) and S (20%). The analysis also showed that ET was not realistically estimated from previous measurements of S; it was more dependent on previous measurements of ET (59%) than on P (30%) and S (9%). This statistical procedure showed great advantages over classical multiple regressions. Future studies of this type should be carried out at regularly spaced observation points in a grid, in order to allow a 2-D and 3-D state-space-time analysis. Additional keywords: multiple regression, neutron probe, sampling number, state-space, state-time., Introduction Soil water storage (S) in agricultural soil profiles is an important parameter for a rational management of any crop, besides giving information on environmental aspects of the water cycle. [...]

Modeling techniques applied to agriculture can be useful to define research priorities and understanding the basic interactions of the soil-plant-atmosphere system. Using a model to estimate the importance and the effect of certain parameters, a researcher can notice which factors can be most useful. The modeler should define his objectives before beginning his work and construct a model that fulfills the proposed objectives.

O presente trabalho teve por objetivo identificar os estádios da cultura de feijão que apresentam a menor sensibilidade ao estresse de água no solo, nos quais a irrigação pode ser omitida sem um significativo decréscimo na evapotranspiração, para melhor definir práticas de irrigação em situação de água escassa. O experimento foi realizado em um solo franco arenoso (Typic Haplustoll), em Tumbaco-Pichincha, Equador, entre julho e novembro de 1994, região caracterizada por um clima temperado e seco (temperatura média anual de 16ºC, umidade relativa média do ar 74% e chuva de 123 mm durante o peláodo do experimento). Sete regimes de irrigação foram utilizados, incluindo irrigação ótima durante todo o ciclo, irrigação deficiente durante todo o ciclo, irrigação tradicional da região, deficiência lúdrica no período vegetativo, na floração e formação de vagens, no enchimento de vagens e na maturidade. As parcelas experimentais foram de 33,6 m2 (8 linhas de 7 metros de comprimento, distantes entre si 0,6 m) com uma população de 120.000 plantas/ha. Os tratamentos de irrigação foram aplicados depois da germinação uniforme das sementes e estabelecimento da cultura. A umidade do solo foi monitorada com sonda de nêutrons, até a profundidade de 50 em, 24 horas antes e depois de cada irrigação. A evapotranspiração atual da cultura foi estimada pela técnica do balanço hídrico. Da análise dos resultados obtidos, concluiu-se que a deficiência de água no solo, durante a fase de enchimento da vagem, afetou a produtividade do feijoeiro. Nesta fase a eficiência de uso de água pela planta (Ec = 0,46 kg/m3) foi a mais baixa. Deficiência de irrigação no estádio vegetativo não reduziu a produtividade, permitindo uma economia de água de 30%.

In tropical agroecosystems, limited N availability remains a major impediment to increasing yield. A [sup.15]N-recovery experiment was conducted in 13 diverse tropical agroecosystems. The objectives were to determine the total recovery of one single [sup.15]N application of inorganic or organic N during three to six growing seasons and to establish whether the losses of N are governed by universal principles. Between 7 and 58% (average of 21%) of crop N uptake during the first growing season was derived from fertilizer. On average, 79% of crop N was derived from the soil. When [sup.15]N-labeled residues were applied, in the first growing season 4% of crop N was derived from the residues. Average recoveries of [sup.15]N-labeled fertilizer and residue in crops after the first growing season were 33 and 7%, respectively. corresponding recoveries in the soil were 38 and 71%. An additional 6% of the fertilizer and 9.1% of the residue was recovered by crops during subsequent growing seasons. There were no significant differences in total [sup.15]N recovery, (average 54%) between N from fertilizer and N front residue. After five growing seasons, more residue N (40%) than fertilizer N (18%) was recovered in the soil, better sustaining the soil organic matter N content. Long-term total recoveries of [sup.15]N- labeled fertilizer or residue in the crop and soil were similar. Soil N remained the primary source of N for crops. As higher rainfall and temperature tend to cause higher [sup.15]N losses, management practices to improve N use efficiency and reduce losses in wet tropical regions will remain a challenge. Abbreviations: FUE, fertilizer use efficiency; NUE, nitrogen use efficiency; SOM, soil organic matter. doi:10.2136/sssaj2009.0192

Among the methods used to measure soil bulk density, the following have been prominent: paraffin sealed clod (PS), volumetric ring (VR), and the modern methods like gamma ray computed tomography (GCT) and the neutron/gamma surface gauge (SG). The objective of this work was to compare soil bulk density values obtained through these methods, with the aim of assisting researchers on the choice of the more appropriate method. For this, a 200-m spatial transect was chosen in an experimental area cultivated with coffee, belonging to ESALQ/USP, Piracicaba, SP, Brazil. The SG readings were first taken in the field and thereafter soil samples were collected at 8 different points, spaced at 25 m, for the other methods. The lowest values of soil bulk density were obtained for the SG method (average 1.468 g/[cm.sup.3]) and the highest for the PS (average 1.685 g/[cm.sup.3]), which was similar to the GCT method (average 1.684 g/[cm.sup.3]). The average soil bulk density for the VR method, which has been used in soil science as a standard method, was 1.544 g/[cm.sup.3]. The Tukey test indicates that the PS and GCT methods do not differ significantly (P > 0.05). They do differ in comparison with VR and SG, which also do not differ among themselves. Additional keywords: soil bulk density, computed tomography method, surface gauge method, volumetric ring method, paraffin sealed clods method., Introduction Soil bulk density is traditionally used as an indicator of soil structure (Gantzer and Anderson 2002) and soil compaction (Martinez and Zinck 2004; Reichardt and Timm 2004). Soil structural [...]

The description of soil water dynamics using the Darcy-Buckingham approach involves the determination and use of soil hydraulic conductivity K v. soil water content θ functions. Many of the methods developed for the measurement of K are based on simplifying assumptions, such as the unit gradient and the choice of fixed models for the K(θ) relation. The need of quick, simple, and inexpensive methods to measure K(θ) in the field using a large number of replicates has also led soil physicists to develop simple methods. This paper presents a procedure that makes use of parameters of equations used to explain the internal water drainage process, and that naturally leads to the exponential character of the K(θ) relation. Results show that the parameterised equation represents a more rigorous estimation of K(θ), compared with the methods that assume unit gradient. Additional keywords: unit gradient, internal drainage, soil water storage., Introduction The Darcy-Buckingham approach for the description of soil water dynamics involves the determination and use of soil hydraulic conductivity K v. soil water content θ functions. One of the [...]

The change in management practices of the sugarcane crop in Brazil, from the traditional trash burning before harvest to the new practice that leaves harvest residues on the field after harvest, can lead to alterations in the water regime and also in soil compaction levels. In this study a neutron-gamma surface gauge was used to monitor spatial and temporal variabilities of soil moisture and density in a experimental sugarcane area submitted to 3 harvest management practices: (i) mulched crop with harvest residues, (ii) crop with bare inter-row, and (iii) crop with ash residues from trash burning before harvest. Variability of soil water content and bulk density was studied using geostatistical tools and analysis of variance was used to compare averages. Autocorrelations and semivariograms indicate a spatial dependence of soil water contents, which were higher in the presence of trash residues left on the soil surface after harvest. The average difference between treatments (i) and (ii) was about of 15%, indicating the beneficial mulching effect with respect to soil water retention. Differences between (ii) and (iii) were much smaller. Additional keywords: gamma-ray attenuation, surface neutron-gamma probe, sugarcane, mulch., Introduction Soil water content and bulk density are parameters commonly used to characterise and to study the physical behaviour of soils in agronomic systems. They have a fundamental influence on [...]

A stochastic approach was used to determine the underlying process causing different spatial yield variability of ryegrass and alfalfa. The effective concentration of soil nitrogen correlated with local nitrogen uptake and symbiotic nitrogen fixation by bacteria. The model identified effective soil nitrogen, nitrogen fixation and crop yield as first-order autoregressive processes moving through the experimental transects.

Fractal scaling has been applied to soils, both for void and solid phases, as an approach to characterize the porous arrangement, attempting to relate particle-size distribution to soil water retention and soil water dynamic properties. One important point of such an analysis is the assumption that the void space geometry of soils reflects its solid phase geometry, taking into account that soil pores are lined by the full range of particles, and that their fractal dimension, which expresses their tortuosity, could be evaluated by the fractal scaling of particle-size distribution. Other authors already concluded that although fractal scaling plays an important role in soil water retention and porosity, particle-size distribution alone is not sufficient to evaluate the fractal structure of porosity. It is also recommended to examine the relationship between fractal properties of solids and of voids, and in some special cases, look for an equivalence of both fractal dimensions. In the present paper data of 42 soil samples were analyzed in order to compare fractal dimensions of pore-size distribution, evaluated by soil water retention curves (SWRC) of soils, with fractal dimensions of soil particle-size distributions (PSD), taking the hydraulic conductivity as a standard variable for the comparison, due to its relation to tortuosity. A new procedure is proposed to evaluate the fractal dimension of pore-size distribution. Results indicate a better correlation between fractal dimensions of pore-size distribution and the hydraulic conductivity for this set of soils, showing that for most of the soils analyzed there is no equivalence of both fractal dimensions. For most of these soils the fractal dimension of particle-size distribution does not indicate properly the pore trace tortuosity. A better equivalence of both fractal dimensions was found for sandy soils.

Abstract The method of Libardi to estimate soil hydraulic conductivity in the field, during the redistribution of soil water, is discussed and improved. It is shown that if the saturated [...]

Surface gamma-neutron gauges are extensively used for the determination of soil physical properties such as density. Substantial differences are sometimes observed between density values provided by these gauges when using factory-supplied calibrations and gravimetric density measurements. These differences can be as high as 16%. In this paper it is shown that, using homogeneous materials with different known densities covering a wider range, it is possible to find a better calibration equation. This calibration provides density values in agreement with gravimetric measurements. Additional keywords: gamma-ray attenuation, backscatter, soil bulk density., Introduction Measurements of bulk density and water content are widely used to characterise soil physical conditions so that the behavior of agronomic systems can be better understood. Among several available [...]