M. Ângelo Rodrigues

Establishment of continuous critical levels for indices of plant and presidedress soil nitrogen status in the potato crop

Abstract Critical levels for six plant and presidedress soil nitrogen (N) indices were established for the growing season, and their relative accuracy to diagnose the need for supplemental sidedress N was also compared. Field trials were conducted from 1996 to 1998 with irrigated potato in northeastern Portugal. Fertilizer treatments included several preplant and sidedress N rates. Petiole nitrate concentrations (determined by a standard laboratory method and with the portable RQflex reflectometer), leaf N content, leaf greenness (SPAD-502 chlorophyll meter), presidedress soil NO3-N and presidedress soil inorganic-N (NO3-N + NH4-N) were selected as N indicators. The Cate-Nelson graphical method and an analytical procedure using Mitscherlich type curves were used to determine critical levels. In both cases, a yield reduction of 10% was accepted. The accuracy of the diagnostics was estimated from the Cate-Nelson graphical method, quantifying the point percentage that appears in negative quadrants (the error rate). The graphical method yielded lower critical levels, appearing as the most conservative sidedress N recommendation basis. The critical levels decreased linearly between 15 and 45 days after emergence (DAE). The linear equations achieved provide continuous critical levels for the growing season and are shown below: Petiole NO3-N (g kg−1, dry wt basis) = − 0.737 DAE + 36.879(r 2 = 0.92); Petiole NO3 (g kg−1, from fresh tissue) = − 0.182 DAE + 9.417 (r 2 = 0.69); Leaf N (g kg−1, dry wt basis) = − 0.453 DAE + 61.028 (r 2 = 0.91); Chlorophyll-SPAD (SPAD units) = − 0.463 DAE + 64.400 (r 2 = 0.93); Soil NO3-N (mg kg−1) = − 1.096 DAE + 49.279 (r 2 = 0.92); and Soil inorganic-N (mg kg−1) = − 1.245 DAE + 56.599(r 2 = 0.92). The N indicators with lower error rate were the presidedress soil NO3-N and presidedress soil inorganic-N (both showing an error rate of 8.3%), followed by petiole nitrate concentration [determined in laboratory (12.0%) and with RQflex reflectometer (12.5%), leaf N content (13.0%), and leaf greenness (14.6%)]. Error rates were similar throughout the growing season, meaning that it is possible to get information about the need for supplementary N in the very early growth stages.

Olive Yields and Tree Nutritional Status during a Four-Year Period without Nitrogen and Boron Fertilization

Nitrogen (N) and boron (B) are mobile elements in soil. Therefore, the application of these nutrients is typically performed annually, as a single dose, or even split into several fractions in the case of N. In olive (Olea europaea L.), however, controversial literature has suggested that yearly application of N may not be required. In the case of B, some authors indicated that one single application is sufficient for 3 or 4 years. Thus, the effects of these elements on olive yield, leaf N and B concentrations, as well as soil available N and B were investigated during a field trial performed in an olive orchard located in northeast Portugal, in which N and B were not applied for four consecutive growing seasons. Fertilizer treatments consisted of the following: the control, which was a complete fertilization plan where N and B were included (N + B treatment); –N treatment, with N excluded from the fertilization plan; and –B treatment, with B excluded. Available soil N and B were estimated from a pot experiment with Italian ryegrass (Lolium multiflorum L.) and from chemical laboratory extractions. Olive yield decreased significantly in the –N treatment in comparison to the control. A slight yield reduction in the –B treatment in comparison to the control was also observed. Leaf N and B concentrations decreased significantly in the –N and –B treatments, respectively, in comparison to the N + B treatment. Soil available N and B at the end of the experiment were significantly lower in the –N and –B treatments, respectively, in comparison to the N + B control. The results showed a continuous decrease in olive yield and leaf N and B concentrations, which reflected the reduction in soil-available N and B in the treatments lacking the respective nutrient. Therefore, it seems prudent to recommend adjustments to the rates of N and B every year to prevent reduction in tree crop performance and improve nutrient-use efficiency.

Response of stevia to nitrogen fertilization and harvesting regime in northeastern Portugal

ABSTRACT The adaptation of stevia to the growing conditions of NE Portugal is assessed, including the tolerance of this species to cold temperatures, and the potential to produce biomass when grown as an annual crop and when subjected to various nitrogen (N) rates and two harvesting regimes. Almost all the plants died during the winter of 2014 (minimum temperatures peaked at −8.0°C), making it necessary to replant the crop the following spring. With the best cutting regime (double cut) and N rate (150 kg N ha−1), 1514.4 and 2390.0 kg ha−1 of dry leaves were produced, respectively, in 2014 and 2015. Leaf chlorophyll concentrations estimated by the SPAD (Soil and Plant Analysis Development)-502 chlorophyll meter and a NDVI (Normalized Difference Vegetation Index) carried out by the Field Scout CM 1000 spectroradiometer showed significant differences among N rates, proving to be good indicators of plant N nutritional status. Based on the leaf analysis, provisional sufficiency ranges for N are proposed, namely 25–35 g kg−1 for mid-summer and 15–25 g kg−1 for early autumn. The fluorescence of chlorophyll a and the transient fluorescence intensity performed by the OS-30p+ fluorometer failed to show any stress induced by no-N control treatments in comparison to N-treated plants.

Influence of Sampling Date on Soil Nitrogen Availability Indices

In spite of the great effort that has been devoted to the search for a chemical laboratory index to predict nitrogen (N) mineralization capability of soils, the results have not yet been fully satisfactory. A continued effort is still needed to increase the knowledge of the sources of variation that influence potentially available soil N. The time of sampling has received little attention, taking into account its potential to influence N-mineralization patterns. In this work, soil samples from three different agrosystems, consisting of a double-crop sequence of small grains and maize, an intensively grazed pasture, and a rainfed olive orchard, were collected at different dates. Several chemical extractions were performed, and the results were correlated with N uptake by turnip (Brassica campestris, L.) grown in a pot experiment. Kjeldahl N was the chemical test that best correlated (R 2 = 0.621) with N uptake by turnip. Kjeldahl N showed great versatility relative to the origin of the soil samples. However, it was not very sensitive to the time of sampling. It did not detect changes occurring in the soil over a short period of time. Soil inorganic N showed the second highest coefficient of correlation (R 2 = 0.483) with N uptake by turnip. In contrast to that observed with Kjeldahl N, soil inorganic N appeared as an index that can vary greatly over the short term. The hot saline potassium chloride (KCl) extractions gave generally fair results. The poorest, however, were obtained with the ultraviolet absorption of extracts of 0.01 mol L−1 sodium bicarbonate (NaHCO3) measured at 250- and 260-nm wavelengths.

Urban agriculture in Bragança, Northeast Portugal: assessing the nutrient dynamic in the soil and plants, and their contamination with trace metals

Abstract Urban agriculture is increasing in both developing and developed countries. In spite of its multiple benefits, urban agriculture can cause unwanted aspects such as environmental damage and health risks to consumers. This work was carried out in a social garden project developed by the Polytechnic Institute of Bragança, Portugal. It checked if cropping practices were environmentally sound and monitored the quality of the edible vegetables, in particular their concentration of trace metals. Bragança is a small city of 25,000 inhabitants located in Northeastern Portugal. In this study the plant nutritional status and the inorganic nitrogen (N) levels in the soil as indirect indicators of the risk of N loss to the environment; and the concentrations of trace metals in the soil and plant tissues as a measure of the exposure of gardeners to trace metals were studied. The results showed the existence of low leaf N concentrations, often close to the deficiency range of the respective species, and also low inorganic N levels in the soil during the winter, suggesting reduced risks of nitrate leaching and denitrification. Trace metal concentrations were low in the soils when compared with the limits set by national and international legislation. As a result, the content of trace metals in edible plant parts was also low. The positive results found in this urban agriculture project might be due to its organic farming basis and to the absence of major sources of pollution in the vicinity of the gardens. This information is useful to local gardeners, since they can trust the products they take home, but also to other urban agriculture projects in even more populated cities.

A comparison of a pasture ley with a maize monoculture on the soil fertility and nutrient release in the succeeding crop

ABSTRACT Specialization within agriculture has been a key factor in increasing farm income. The production systems have become increasingly simple, since farmers only grow a small number of crops which have a favourable market price. However, monocultural systems require increasing use of agrochemicals leading to unsustainable environmental costs. In this work, the soil fertility of two plots in a crop rotation previously grown for 5 years as pasture or maize monoculture was evaluated. In the pasture, the upper 0–20 cm soil layer sequestered 17.4 Mg organic C ha−1 and accumulated 403 kg N ha−1 more than under maize monoculture. Analytical data from pot experiments showed that soil samples from the pasture plot released significantly more mineral N than soil samples from the maize monoculture. Maize dry matter (DM) yields in 2012 and 2013 were 15.3 and 10.0 Mg ha−1 in the pasture plot and 8.8 and 8.4 Mg ha−1 in the maize monoculture plot. Nitrogen recoveries by maize were 175.4 and 68.0 kg ha−1 in the pasture and 78.3 and 50.3 kg ha−1 in the maize monoculture plot. The pool of organic matter accumulated during the pasture phase immobilized important nutrients which benefited the succeeding crop as the organic substrate was mineralized.

Management of Nitrogen-Rich Legume Cover Crops as Mulch in Traditional Olive Orchards

A low-input agricultural system needs a natural source of nitrogen (N). Legume species can fix great amounts of N that can be subsequently used by a nonlegume crop. In this study three legume cover crops were grown in traditional olive orchards in northeastern Portugal from October 2009 to May 2010, and the aboveground biomass was mechanically destroyed and left on the ground as a mulch. In the following growing season, from October 2010 to May 2011, two nitrophilic plant species were grown in circular microplots of 154 mm surrounded by polyvinyl chloride rings to assess the soil N availability. The N fixed by the legume cover crops, estimated by the difference technique, was shown to vary from 79.7 to 187.5 kg N ha−1. The nitrophilic plant species identified a small peak of soil available N in the autumn of 2010, probably resulting from the mineralization of the root system of the legume species. In the next spring, the increase of soil-available N in the plots where the legume cover crops had been grown, in comparison to the control plot, was residual. The great amounts of N present in the mulched materials seem to disappear without having entered the soil. Mulching with high-N content biomass may be troublesome due to the high risk of N losses probably by ammonia (NH3) volatilization.

Sufficiency ranges for lemon balm and nutrient removals in aboveground phytomass

ABSTRACT A set of fertilizer experiments were conducted during three growing seasons with the aim of establishing sufficiency ranges and crop nutrient removals for Melissa officinalis L. Critical nutrient concentrations were determined by the Cate–Nelson method or by removing 10% of extreme high and low values, respectively if a positive response to a given nutrient was recorded or not. Sufficiency ranges for macro, micronutrients, and SPAD-readings were set as: 27.0–40.0 g N kg−1; 0.8–2.7 g P kg−1 (May–August); 1.5–3.8 g P kg−1 (September–November); 10.0–25.0 g K kg−1 (May–August); 18.0–32.0 g K kg−1 (September–November); 5.0–25.0 g Ca kg−1; 3.5–8.5 g Mg kg−1; 18–125 mg B kg−1; 5–25 mg Cu kg−1; 75–500 mg Fe kg−1; 20–300 mg Zn kg−1; 30–250 g Mn kg−1; 30–45 SPAD-units. These results will allow laboratories to use plant analysis as an important tool in improving the fertilizer recommendations for this species.

Assessment of the nitrogen fertilization effect on bioactive compounds of frozen fresh and dried samples of Stevia rebaudiana Bertoni

The present study aims to assess the effect of different nitrogen (N) rates on the chemical composition and antioxidant properties of stevia frozen fresh and dried leaves, and to define the best growing conditions to maximize the levels of bioactive compounds. In general, processing affects more significantly the tocopherol and sugar contents than N fertilization. The most abundant sugars were xylose, arabinose + fructose and sucrose, presenting dried samples with higher contents than frozen fresh ones, while the latter better retained tocopherols than dry samples. Regarding phenolic compounds, greater levels were found in dried samples and in those fertilized with 25 kg N ha-1. Leaves from plants fertilized with 25 and 50 kg N ha-1 also evidenced higher antioxidant activity, which seemed to be influenced by the phenolic composition. In general, N fertilization provides an improvement in the chemical composition and bioactive potential of stevia leaves.

Sufficiency ranges and crop nutrient removals for peppermint (Mentha X piperita L.) established from field and pot fertilizer experiments

ABSTRACT Peppermint is an important aromatic and medicinal plant used across the world in pharmaceutical, cosmetic and food industries. However, there is a lack of agronomic research on this crop which hinders the implementation of best agricultural practice at farm level. Plant analysis, for instance, cannot be used as a tool to implement a suitable fertilizer recommendation program, since sufficiency ranges and crop nutrient removals have not yet been established. Thus, the main objectives of the present work were to assess the response of peppermint to varying nitrogen (N), phosphorus (P), potassium (K) and boron (B) rates, to establish sufficiency ranges from macro, micronutrients and SPAD-readings and to estimate crop nutrient removals in the aboveground biomass. Field trials and pot experiments were conducted from 2013 to 2015 in a wide range of conditions involving 12 N, P, K or B fertilizer trials and a total of 48 cuts of biomass. Nitrogen fertilization increased dry matter yield of peppermint on the vast majority of sampling dates. In contrast, P, K, or B did not produce a significant effect on dry matter yield in any of the experiments. The sufficiency ranges set for macronutrients N, P, K, Ca and Mg are respectively 32.0 – 42.0, 1.2 – 4.5, 10.0 – 30.0, 7.0 – 23.0, and 4.0 – 10.0 g kg−1. Those for micronutrients B, copper (Cu), iron (Fe), zinc (Zn), and manganese (Mn) are respectively 20 – 200, 5 – 25, 100 – 600, 25 – 300, and 30 – 200 mg kg−1. Sufficiency range for SPAD-readings is 45 – 50 SPAD units. All these ranges were established for the commercial harvesting date. The amounts of N, P, K, calcium (Ca), and magnesium (Mg) removed in aboveground biomass are respectively 22.7, 1.6, 26.4, 16.4 and 4.8 kg Mg−1 of dry biomass.