Finn Plauborg

The Effects of Drought and Nitrogen on Light Interception, Growth and Yield of Winter Oilseed Rape

Abstract The effects of drought and nitrogen on growth of winter oilseed rape were investigated during 1991–93 on a coarse sandy soil. Drought in the flowering phase or in the pod-filling phase or throughout the seasons was partly combined with three nitrogen levels of 200, 100 and 0 kg N ha−1. During the dry years 1992 and 1993, at 200 kg N ha−1, seed yield, pod number and seeds per pod were strongly decreased by drought, whereas the seed weight increased after drought during flowering. The straw yield was less affected by drought. Lower nitrogen levels reduced seed yield in the fully irrigated treatments; in the unirrigated treatments, however, the yield was slightly higher at the 0 N and 100 N level than at 200 N in both 1992 and 1993. Nitrogen and drought increased the nitrogen concentration of the seeds and decreased the oil concentration. The interception of photosynthetically active radiation (PAR) was estimated from spectral reflectance ratio measurements. The dry-matter: radiation quotient (∊t) f...

Evaporation from bare soil in a temperate humid climate—measurement using micro-lysimeters and time domain reflectometry

Abstract In humid areas soil evaporation is an important factor in relation to soil tillage and timing of irrigation early in the growing season. Direct measurements of evaporation from a loamy sand were made using micro-lysimeters. The accuracy of daily measurement was about ± 0.5 mm H 2 O day −1 . The micro-lysimeter method was not valid in periods with high precipitation. The use of time domain reflectometry (TDR) for measuring soil water content was investigated using a manual interpretation of the trace. The precision of changes in soil water content calculated from daily measurements with TDR was about 1.3 mm H 2 O, when using probes of 50 cm length. However, improved precision may be obtained by the use of an automatic interpretation of the trace. Estimates of daily evaporation from bare soil calculated from the water balance equation and measurements of soil water content with TDR were compared with measurements with micro-lysimeters. The TDR technique was suitable for estimating bare soil evaporation when the soil water content was integrated over a 0–50 cm soil profile and drainage had ceased at the lower depths of the profile. Evaporation during a 13 day drying period in spring, just after the soil had been fully rewetted, was about 26 mm. During a 23 day drying period later in the season the evaporation from the bare soil was about 30 mm. In both periods the accumulated evaporation was rather high and equivalent to about 65% and 50% of the accumulated potential evapotranspiration in the first and second drying period, respectively, even though the soil water content in the 0–50 cm profile was well below field capacity at the beginning of the second drying period.

Simple model for 10 cm soil temperature in different soils with short grass

Information on soil temperature, which e.g. governs soil nitrogen turnover and emergence of crops, is not routinely available from meteorological stations and is rather costly to obtain at field level from automated measurements or calculations with physically based models. The present study presents empirical and simple models for soil temperature at 10 cm depth in grass-covered soils. The global model is based on 10 years data from 13 meteorological stations at different soils in Denmark. The root mean square error of the model was 0.97 °C, and 98% of the variation in data was explained. The model was not improved by inclusion of the discrete factors, soils or stations. The predictions from station specific models were compared to the global model, and the potential for using the global model for other crops was briefly discussed.

Pesticide leaching through sandy and loamy fields - long-term lessons learnt from the Danish Pesticide Leaching Assessment Programme.

The European Union authorization procedure for pesticides includes an assessment of the leaching risk posed by pesticides and their degradation products (DP) with the aim of avoiding any unacceptable influence on groundwater. Twelve-years results of the Danish Pesticide Leaching Assessment Programme reveal shortcomings to the procedure by having assessed leaching into groundwater of 43 pesticides applied in accordance with current regulations on agricultural fields, and 47 of their DP. Three types of leaching scenario were not fully captured by the procedure: long-term leaching of DP of pesticides applied on potato crops cultivated in sand, leaching of strongly sorbing pesticides after autumn application on loam, and leaching of various pesticides and their DP following early summer application on loam. Rapid preferential transport that bypasses the retardation of the plow layer primarily in autumn, but also during early summer, seems to dominate leaching in a number of those scenarios.

Dual permeability soil water dynamics and water uptake by roots in irrigated potato fields

Water movement and uptake by roots in a drip-irrigated potato field was studied by combining field experiments, outputs of numerical simulations and summary results of an EU project (www.fertorganic.org). Detailed measurements of soil suction and weather conditions in the Bohemo-Moravian highland made it possible to derive improved estimates of some parameters for the dual permeability model S1D_DUAL. A reasonably good agreement between the measured and the estimated soil hydraulic properties was obtained. The measured root zone depths were near to those obtained by inverse simulation with S1D _DUAL and to a boundary curve approximation. The measured and S1D _DUAL-simulated soil water pressure heads were comparable with those achieved by simulations with the Daisy model. During dry spells, the measured pressure heads tended to be higher than the simulated ones. In general, the former oscillated between the simulated values for soil matrix and those for the preferential flow (PF) domain. Irrigation facilitated deep seepage after rain events. We conclude that several parallel soil moisture sensors are needed for adequate irrigation control. The sensors cannot detect the time when the irrigation should be stopped.

Screening tomato genotypes for adaptation to high temperature in West Africa

Tomato is an important vegetable widely grown in the tropics due to its nutritional value and financial benefits for farmers. In Ghana, there is an undersupply caused by production ceasing entirely from October to May due to high temperatures. Heat stress has been reported to cause excessive flower drop leading to drastic reduction in yield; however, genotypic differences in heat tolerance exist in tomato. A greenhouse experiment was conducted to screen 19 different tomato genotypes for their tolerance to heat stress. The genotypes were selected because they were the commercial varieties widely available to farmers. The average day and night temperatures recorded were 33.8°C and 25.9°C, respectively. Cultivars were evaluated for heat adaptation traits such as flower drop and number of fruits. There were highly significant differences between the genotypes for numbers of fruits per plant, ranging from 1 to 27. Fruit yield per plant ranged from 26.7 to 571.8 g. The locally developed open pollinated cultivar ‘Nkansah’ had the highest fruit number and fruit yield per plant, but produced the smallest sized fruits of only 25.2 g. Cultivars with large size fruits were ‘Queen’, ‘DV 2962’ and ‘Wosowoso’ with a combined average of 85.7 g. The first two principal components (PCs) accounted for 79.7% of total variation. The first PC had positive weights for number of flowers per truss, number of fruits per plant, total number of flowers, yield per plant and number of trusses. PC2 explained 11% of the total variability among genotypes and had positive weights for all traits except number of days to flowering and weight per fruit. The highly contrasting material on traits such as flower and fruit production under high temperatures can be used for further research to elucidate the physiological responses conferring adaptation to heat stress.

Using ground-based spectral reflectance sensors and photography to estimate shoot N concentration and dry matter of potato

Abstract Two years experiments were set up to evaluate the performance of different vegetation indices (VI) to estimate shoot N concentration (N c ) and shoot dry matter (DM) for a potato crop grown under different nitrogen (N) treatments. Possibilities to improve the performance of VI using normalization by leaf area index (LAI) or camera-derived ground cover fraction (GC) were also investigated. Results indicated that N c was significantly correlated to RRE (Near-infrared divided by red edge reflectance) and RRE/GC with a coefficient of determination (R 2 ) of 0.62 and 0.78, respectively, indicating that inclusion of auxiliary parameter GC together with RRE substantially improved the correlation as compared to using only RRE. However, no significant correlation between N c and RVI (Ratio Vegetation Index, near-infrared divided by red reflectance) or NDVI (Normalized Difference Vegetation Index) was found. However, DM was highly correlated to RVI and NDVI. Moreover, DM showed significant relationship (R 2  = 0.86) with GC, highlighting its versatile usefulness in estimating agronomic variables DM and N c , which are the core variables to assess N status of crops for a better N application.

Carbon Dioxide in Arable Soil Profiles: A Comparison of Automated and Manual Measuring Systems

Carbon dioxide (CO2) concentrations in arable soil profiles are influenced by autotrophic and heterotrophic respiration as well as soil physical properties that regulate gas transport. Although different methods have been used to assess dynamics of soil CO2 concentrations, our understanding of the comparability of results obtained using different methods is limited. We therefore aimed to compare the dynamics in soil CO2 concentrations obtained from an automated system (GMP343 sensors) to those from a manually operated measurement system (i.e., soil gas sampled using stainless steel needles and rods). In a winter wheat field in Denmark, soil CO2 concentrations were measured from 29 November 2011 to 14 June 2012 at upslope and footslope positions of a short catena (25 m). Carbon dioxide was measured at 20- and 40-cm soil depths (i.e., within and below the nominal plow layer) using the two measurement systems. Within the measurement range for the GMP343 sensors (0–20,000 ppm), mean results from the two systems were similar within the plow layer at the upslope (P = 0.060) and footslope (P = 0.139) position, and also below the plow layer at the upslope position (P = 0.795). However, results from the two systems deviated for the soil from the footslope position below the plow layer (P = 0.001). These results were partly attributed to larger variation in soil parameters below than within the nominal plow layer. The data suggested that generally the application of either system may be adequate; however, differences may occur in response to soil spatial variability. A better coverage of spatial variability is more easily addressed using manually operated systems, whereas temporal variability can be covered using the automated system. Depending on the aim of the study, the two systems may be used in combination to enhance both spatial and temporal data coverage.

Sub-optimal model-based deficit irrigation scheduling with realistic weather forecasts

This paper analyses the performance of sub-optimal irrigation schedules obtained daily by solving a multi-objective optimization problem with updated weather measurements and forecasts. The approach was tested using five crops at four European locations with contrasting weather conditions. Four- and 6-day Global Forecast System (GFS) forecasts were used at all locations, and comparison with a down-scaled locally tuned model was conducted at one location. Accurate GFS temperature forecasts were observed at all four locations, but the accuracy of the potential evapotranspiration calculated from the GFS forecasts was not as consistent. Precipitations forecasts were very poor at all locations. In Greece, the down-scaled locally tuned forecasts were only marginally better than the GFS ones. In most cases, recomputing the sub-optimal irrigation schedule daily greatly reduced the impact of the imperfect weather forecasts on the final results. Using 4- or 6-day actual forecasts did not yield results appreciably better than those obtained using only historical averages as surrogate forecasts. The main consequence of the imperfect forecasts was that the final yield differed from the target one, but the (yield, irrigation) combination remained close to optimal, unless the target yield was set too high and water availability was not the main factor limiting crop development.