Monitoring of soil organic carbon over 10 years in a Mediterranean silvo-pastoral system: potential evaluation for differential management

Soil organic carbon (SOC) plays a vital role in determining the susceptibility to land degradation. The recommended procedure for the recovery of the characteristic poor soils of the Southern region of Portugal is the installation of grazed permanent pastures and increase of soil fertility. The objectives of this study were: (i) to identify the spatial and temporal patterns of soil nutrients at four points in time over a 10-year period in a perennial pastureland; (ii) to test new tools for survey of the spatial variability of soil nutrients; (iii) to evaluate the potential for differential organic management. A 6 ha permanent bio-diverse pasture field, grazed by sheep and improved by annual application of super phosphate fertilizer, was installed on a shallow soil in Mediterranean conditions. Spatial variability and temporal stability of topsoil macronutrients (phosphorus, nitrogen and potassium), SOC and pH were measured. The results indicate that SOC and pH have great potential for implementing differential management. In the case of SOC, the management classes map shows that over 80 % of the area has temporal stability, while more than 50 % of the area has low levels of SOC (<10 g kg^?1), justifying the potential for differential application of C-rich organic soil amendments. The geospatial measurements of apparent soil electrical conductivity (EC_a) and NDVI index showed significant correlation between these parameters and soil properties, revealing the potential of these tools for producing detailed soil maps, decisive for understanding the changes in soil properties under sustainable management systems.     

Apparent electrical conductivity in dry versus wet soil conditions in a shallow soil

The general objective of this study was to evaluate the stability of patterns of apparent soil electrical conductivity (EC_a) in dry versus wet soil conditions in a shallow soil typically used for pastures in Mediterranean conditions of the southern region of Portugal. A 6 ha experimental field of permanent bio-diverse pasture was divided into 76 squares of 28 × 28 m. The soil electrical conductivity was measured using a Dualem 1S sensor under dry conditions (June 2007) and under wet conditions during the rainy season (March 2010). Soil samples, geo-referenced with GPS, were collected in a depth range of 0–0.30 m. The soil was characterized in terms of bedrock depth, moisture content, texture, pH, organic matter content, and macronutrients (nitrogen, phosphorus, and potassium). Pasture samples, also geo-referenced with GPS, were collected to measure the pasture dry matter yield. The statistical analysis of apparent electrical conductivity between dry and wet soil conditions resulted in a linear significant correlation coefficient (R = 0.88). The results also showed a significant correlation between apparent electrical conductivity and the relative field elevation (R = ?0.64 and R = ?0.66), the pasture dry matter yield (R = 0.42 and R = 0.48), the bedrock depth (R = 0.40 and R = 0.27), the pH (R = 0.50 and R = 0.49), the silt (R = 0.27 and R = 0.38) and soil moisture content (R = 0.48 and R = 0.45), in dry and wet conditions, respectively. A multi-variate regression was carried out using the following soil parameters that showed significant correlation with EC_a and that did not present multi-collinearity: pH, bedrock depth, silt and moisture content. The results showed, in dry and wet conditions, that the analysis was significant (R = 0.75 and R = 0.84, respectively). Overall, these results indicate the temporal stability of EC_a patterns under different soil moisture contents, which is relevant with respect to the time when a field should be surveyed and is important for using the electrical conductivity sensor, as a decision support tool for management zones in precision agriculture.     

Development of an adaptive surface irrigation system

Cablegation is a simple system for automating surface irrigation in small- and medium-sized fields using a gated pipe. In this work, a Programmable Logic Control, PLC, was used to develop an adaptive cablegation system capable of establishing the infiltration equation in real time and then adjusting the irrigation times to the infiltration rate and field geometry. A controlling program was developed for the on-field determination of the infiltration equation, simulation of advance in each furrow, and the optimization and management of the irrigation event. The equipment was tested in three experimental stations, including a Luvissol field organized in contour terraces with furrows of various lengths. The results demonstrate the capability of the system to adapt the application times to the different furrow lengths and the gradual decrease in the soil infiltration and to recommend an application depth that optimizes the Application Efficiency. Various improvements were made to this solar-powered cablegation, resulting in a reliable surface irrigation system capable of unsupervised operation.     

Spatial variability and temporal stability of apparent soil electrical conductivity in a Mediterranean pasture

The general objectives of this study were to evaluate (i) the specificity of the spatial and temporal dynamics of apparent soil electrical conductivity (EC_a) measured by a electromagnetic induction (EMI) sensor, over 7 years, in variable conditions (of soil moisture content (SMC), soil vegetation cover and grazing management) and, consequently, (ii) the potential for implementing site-specific management (SSM). The DUALEM 1S sensor was used to measure the EC_a in a 6 ha pasture experimental field four times between June 2007 and February of 2013. Soil spatial variability was characterized by 76 samples, geo-referenced with the global positioning system (GPS). The soil was characterized in terms of texture, moisture content, pH, organic matter content, nitrogen, phosphorus and potassium. This study shows a significant temporal stability of the EC_a patterns under several conditions, behavior that is an excellent indicator of reliability of this tool to survey spatial soil variability and to delineate potential site-specific management zones (SSMZ). Significant correlations were obtained in this work between the EC_a and relative field elevation, pH, silt and soil moisture content. These results open perspectives for using the EMI sensor as an indicator of SMC in irrigation management and of needs of limestone correction in Mediterranean pastures. However, it is interesting to extend the findings to other types of soil to verify the origin of the lack of correlation between the EC_a data measured by DUALEM sensor and properties such as the clay, organic matter or phosphorus soil content, fundamental parameters for establishment of pasture SSM projects.     

Evaluation of Variable Rate Technology for Fertilizer Application in Permanent Pastures

This paper describes the field tests carried out to evaluate the new Vicon RS-EDW centrifugal fertilizer spreader, to be used in Precision Agriculture. The results of initial dynamic calibration tests showed a significant and systematic difference between the application rates of the two discs （about 20%）. This difference had to be corrected by the manufacturer that carried out the necessary changes to the electric actuators. The fertilizer spreader had good distribution homogeneity, considering different working velocities and different positions in the parcel. The pattern distribution curve for 18% super phosphate fertilizer led to an effective working width of 28 meters, with a coefficient of variation of 15%. The longitudinal test, under experimental working conditions led to a machine delay time of 6-7 seconds. The results show an actual fertilization application density between 74%-90% of that determined for each location. These results confirm that the spreader can be used to spread fertilizer differentially in the field.     

Spatial and temporal stability of soil phosphate concentration and pasture dry matter yield

Several methods are described that could be used by a farm manager to define the spatial and temporal stability within a field from a series of yield maps. A quantitative analysis of soil phosphate concentration and pasture dry matter yield data over 4 years (2004–2007) were investigated to identify the spatial and temporal stability in a 6 ha pasture field. The data were combined into two maps that characterize the spatial and temporal variation recorded over the 4 years. The two maps were then combined to create a single map with five management classes, each with different characteristics that can have an impact on the way the field is managed. These categories are: high yielding and stable, high yielding and moderately stable, low yielding and stable, low yielding and moderately stable and unstable. The unstable class represents 83 and 93% of the total area with regard to soil phosphate concentration and pasture dry matter yield, respectively. Results from this study show that the significant temporal stability found cancels out over time, leaving a relatively homogenous map of spatial variation. The implication of the findings is that each pasture field should be managed according to the current year’s conditions. These results also justify a further study that evaluates the soil phosphorous dynamics under Mediterranean conditions.     

Soil phosphorus retention in a Mediterranean pasture subjected to differential management

The objective of this study was to test a simple input‐output model to estimate soil phosphorus (P) retention over 12 years in a Mediterranean pasture. A 6‐ha permanent pasture field in Southern Portugal was subjected to different management systems in terms of grazing and fertilizer application. The soil and pasture were monitored and samples were geo‐referenced with RTK GPS. The model was developed to explain the effect of P fertilizer, based on the premise that P added through fertilizer and amounts removed in harvested plant products are the main fluxes affecting available P status, and was used to estimate the soil P retention. The results show that P fertilizer addition to Mediterranean grazed pastures on shallow soils influences several P dynamics under different grassland management systems. A positive linear relationship between the P budgets and the changes in measured P was demonstrated over the study periods. This work highlights the difficulty associated with the creation of recommendation maps for differentiated P fertilizer application in pastoral systems and indicates that before any accurate decision making, the historic knowledge of changes in the concentrations of P in the soil should be considered, as well as a prediction of the residual effects of applied nutrient. The importance of this knowledge increases when the aim is to use techniques and technologies for variable‐rate fertilizer application and implementation of site‐specific management strategies in the context of precision agriculture.     

Calibration of a capacitance probe for measurement and mapping of dry matter yield in Mediterranean pastures

The main objective of this study was to calibrate a commercial capacitance probe for measuring pasture dry matter yields under Mediterranean conditions. The standard method of assessing pasture biomass is based on cutting all the forage within a specified area and requires great effort and expense to collect enough samples to accurately represent a pasture. The field tests were carried out in 2007, 2008 and 2009 on different dates (phenological stages), and on five dairy farms, representing typical pastures in the region (grasses; legumes; and bio-diverse flora, mixture of grasses, legumes and others species). The linear regression techniques used in 2007 to relate the weight of the herbage (direct measurements) to the meter reading of capacitance (indirect measurements) led to high regression coefficients in grasses (R² = 0.90; P < 0.01) and heterogeneous botanical composition (R² = 0.87; P < 0.001) and moderate regression coefficient in legumes species (R² = 0.48; P < 0.05). The validation of the calibration equations in 2008 and 2009 in two sites showed RSME values of 130 kg ha⁻¹ in heterogeneous botanical composition and 456 kg ha⁻¹ in legumes. The results indicated that the capacitance probe together with a GPS receiver might support site-specific management of pastures which would be useful in large areas.     

Mapping soil and pasture variability with an electromagnetic induction sensor

The general objective of this study was to test a non-contact electromagnetic induction probe to evaluate the soil and pasture variability in a precision agriculture project. Assessment of the variability of soil and vegetation in a permanent pasture is the basis for management of variable rate fertilization, which is the main instrument used by farmers for improvements in permanent pasture in Alentejo, Portugal. The traditional process of sampling and evaluation of the soil is very demanding in terms of time, labour and reagents, and can derail a project of precision agriculture. This paper describes the major steps followed by the authors to simplify the methodology of soil evaluation in a permanent pasture based on measuring the apparent soil electrical conductivity. Tests were carried out in a parcel of approximately 6 ha, which was subdivided into 28 m × 28 m squares. The soil samples and the evaluation of apparent electrical conductivity were geo-referenced with a global positioning system. The geospatial data were processed by ArcGIS software and the statistical analysis resulted in significant correlation coefficient values between apparent electrical conductivity and altitude, soil pH and pasture dry matter yield.     

Vermicompost and Manure Compost Reduce Water-Deficit Stress in Pot Marigold (Calendula officinalis L. cv. Candyman Orange)

This study was carried out to evaluate the impact of irrigation regime and potting media on morpho-physiological and biochemical characteristics of pot marigold. The experiment was arranged factorially based on a completely randomized design. The first factor was irrigation regime in three levels of 80, 60 and 40% available water content and the second factor was potting media in five levels of 20% vermicompost, 30% vermicompost, 20% manure compost, 30% manure compost and control (sand and soil in equal proportions). Morpho-physiological traits (plant height, stem diameter, number of flowering stem, root diameter, root length, root dry weight, aerial dry weight, total dry weight, relative water content, ionic stability and water use efficiency) and biochemical traits (malondialdehyde content, catalase and peroxidase activity, chlorophyll and carotenoid contents) were measured. Morpho-physiological parameters, chlorophyll and carotenoid decreased under water deficit, while increased with application of vermicompost and manure compost. Also, lipid peroxidation, catalase and peroxidase activity enhanced under water deficit, while decreased with application of vermicompost and manure compost. In other words, the application of vermicompost and manure compost in potting media reduced the harmful effects of water deficit. Total dry mass and water use efficiency were about 3-fold higher in plants grown in 30% vermicompost or 30% manure compost substrate compared to those in control plants. The results suggest that the application of 30% manure compost could be recommended as suitable potting media due to reducing the negative effects of water shortages, helping to nourish the plant, cheapness and accessibility compared with 30% vermicompost.