Nitrogen

Nitrogen-fixing bacteria such as cyanobacteria have the capability to fix atmospheric nitrogen at ambient temperature and pressure, and intensive cultivation of cyanobacteria for fertilizer could lead to its use as an “environmentally friendly” replacement or supplement for nitrogen (N) fertilizer derived from the Haber–Bosch process. Prior research has focused on the use of N-fixing bacteria as a soil inoculum, and while this can improve crop yields, yield improvements are generally attributed to plant-growth-promoting substances produced by the bacteria, rather than to biological N fixation. The intensive cultivation of cyanobacteria in raceways or bioreactors can result in a fertilizer that provides N and organic carbon, as well as potentially similar growth-promoting substances observed in prior research work. On-farm or local production of cyanobacterial fertilizer could also circumvent infrastructure limitations, economic and geopolitical issues, and challenges in distribution and transport related to Haber–Bosch-derived N fertilizers. The use of cyanobacterial N fertilizer could have many agronomic and environmental advantages over N fertilizer derived from the Haber–Bosch process, but study of cyanobacteria as a replacement for other N fertilizers remains very limited. Scientific and practical challenges remain for this promising but as-yet unproven approach to maintaining or improving soil N fertility. Full article

Although NH₄⁺ fertilization is known to acidify rhizosphere and enhance nutrient uptake, the effects on a nutrient-sufficient acidic soil amended with lime are not demonstrated. Thus, the influence of NH₄⁺ fertilization of an unlimed and limed (3 g calcium carbonate per kg soil) acidic soil on the nutrient uptake and growth of maize was studied in comparison to NH₄NO₃ fertilization. The pH of limed rhizosphere soil was about two units higher than that of the unlimed soil. The maize plants were grown in pots under greenhouse conditions for about two months. The results showed that the pH of the NH₄⁺-fertilized unlimed and limed soil was 0.54 and 0.15 units lower than the NH₄NO₃-fertilized soil. Liming negatively affected shoot and root dry matter production, whereas the NH₄⁺-fertilized plants produced higher dry matter than the NH₄NO₃-fertilized plants, with significant difference of 28% in the limed soil only. Liming decreased Fe concentration in rhizosphere soil from 99 to 69 mg kg⁻¹ and decreased plant-available Mn the most (71%), whereas the NH₄⁺-fertilized unlimed and limed soil had 48% and 21% higher Mn concentration than the respective NH₄NO₃-fertilized soils. Similarly limed rhizosphere soil had 50% lower plant-available Zn concentration than the unlimed soil, and the NH₄⁺-fertilized soil had an 8% higher Zn concentration than the NH₄NO₃-fertilized unlimed soil. The liming negatively affected P, K, Mn, and Zn concentrations and contents in maize shoot to a lower degree in the NH₄⁺-fertilized soil, whereas the positive effect of NH₄⁺ on the nutrient concentration and contents was vigorous in the unlimed soil than the limed soil. It is concluded that NH₄⁺ fertilization could be beneficial in enhancing nutrient uptake and growth of maize in both acidic and alkaline soils, despite the higher inherent plant-available concentrations of the nutrient in soil. Full article

In higher plants, hydrogen sulfide (H₂S) is a recognized signaling molecule that performs multiple regulatory functions. The enzyme L-cysteine desulfhydrase (LCD) catalyzes the conversion of L-cysteine (L-Cys) to pyruvate and ammonium with the concomitant generation of H₂S, and it is considered one of the main sources of H₂S in plants. Using non-denaturing polyacrylamide gel electrophoresis (PAGE) in combination with a specific assay for LCD activity, this study aims to identify the potential LCD isozymes in wild-type Arabidopsis thaliana seedlings of 16 days old grown under in vitro conditions, and to evaluate the potential impact of nitric oxide (NO) and H₂S on these LCD isozymes. For this purpose, an Atnoa1 mutant characterized to have a low endogenous NO content as well as the exogenous application of H₂S were used. Five LCD isozymes were detected, with LCD IV being the isozyme that has the highest activity. However, the LCD V activity was the only one that was positively modulated in the Atnoa1 mutants and by exogenous H₂S. To our knowledge, this is the first report showing the different LCD isozymes present in Arabidopsis seedlings and how their activity is affected by NO and H₂S content. Full article

The utilization of compost to enhance plant productivity and symbiotic nitrogen fixation (SNF) has been recognized as an effective alternative to synthetic nitrogen fertilizers. This environmentally sustainable method is readily accessible to farmers. This study investigated the effect of olive pomace (“alperujo”, AL) compost on the nodulation and SNF of soybeans (Glycine max L.) and their natural symbiont (Bradyrhizobium diazoefficiens). For that, soybean plants were subjected to several doses of AL compost under controlled greenhouse conditions. At the end of the experiment, the dry weight of plant biomass (aerial part and roots), the number and fresh weight of nodules, and nitrogen and leghaemoglobin contents were analyzed. The application of AL compost significantly improved soybean growth, as demonstrated by an increase in both plant biomass and height. Furthermore, nodular leghaemoglobin content and nitrogen content were found to be enhanced by the addition of AL compost (7 and 40%, respectively), indicating an increase in nodule effectiveness and symbiotic efficiency. Our results provide clear evidence of the synergetic effect of AL compost on the soybean-B. diazoefficiens association, probably due to AL-compost improved soybean roots development, and rhizospheric organic matter and nutrients assimilation by rhizobia. Full article

Soybean plants can fix atmospheric N₂ in the root nodule, a symbiotic organ with rhizobia. The primary forms of N transported from N₂ fixation are ureides, allantoate, and allantoin, supplemented with asparagine. The nitrate absorbed in the roots is transported to the shoots in the forms of NO₃⁻ and asparagine with a little portion of ureides. The concentrations of N-metabolites were analyzed by capillary electrophoresis after supplying various concentrations of urea, precursors of ureides, and allopurinol, an inhibitor of xanthine dehydrogenase, to investigate the ureide synthesis pathway in the roots. When the non-nodulated soybean plants were treated with 0–5 mM of urea, the concentrations of asparagine and glutamine in the xylem sap and the roots increased remarkably. In addition, allantoate concentration increased with the urea concentrations becoming higher. Allopurinol inhibited the accumulation of allantoate but did not affect the asparagine and glutamine accumulation in roots, stems, leaves, and xylem sap, supporting that allantoate is synthesized by purine degradation in roots the same as in the nodules. When ureide precursors were supplied to the nodulated soybean plants, the concentrations of asparagine and glutamine in the xylem sap and roots increased, suggesting that the ureide precursors were absorbed and assimilated to amides in the roots. Full article

Mixed-species grassland containing legumes were suggested to increase yield compared to monocultures. Furthermore, some legumes were suggested to be able to sustain growth, even under drought conditions. The first aim of the current study was to measure if multispecies grassland with legumes is also more productive when their N input due to symbiotic N₂ fixation is taken into account. Our second aim was to determine the benefit of grass–legume mixtures in terms of dry matter production under naturally occurring drought conditions. Mixed-species grasslands, consisting of monocultures and variable mixtures of (a) Trifolium pratense, (b) Trifolium. repens, (c) Lolium perenne, and (d) a mixture of drought-tolerant grasses (GSWT based), were assessed for their dry matter production over two years with contrasting weather patterns. The legume–grass seeding mixtures received either a fixed (180 kg N ha⁻¹) or adapted N-fertilizer application (0–180 kg N ha⁻¹), with the latter taking the assumed symbiotic N₂ fixation by legumes into account. Mixed-species grassland showed improved yield compared to monocultures both in comparably humid and drought-affected years. The benefits of multispecies grass–legume mixtures were considerably more obvious under a fixed but still measurable under an adapted N-fertilizer regime. The species diversity effect appears to be significantly dependent on the additional N supply enabled by legumes’ symbiotic N₂-fixation. Legumes and drought-tolerant grasses yielded equally well under drought conditions, although legumes showed major advantages during moderate drought and humid conditions. White and red clover, although both legumes, differed significantly in their persistence under elevated-N and their dry matter production under low-N fertilizer application, but were equal in their tolerance towards drought. Full article

Anthropogenic nitrogen (N) emissions can have considerable effects on terrestrial ecosystems, with chronic N deposition leading to changes in plant species composition. The Athabasca Oil Sands Region (AOSR) represents a large point source of N emissions, which has prompted concern for surrounding habitats. The objective of this study was to determine the relative importance of N deposition as a driver of plant species community composition against bioclimatic and soil chemical variables. Further, we sought to identify community thresholds in plant species composition across a N deposition gradient. This assessment was performed for 46 Jack pine (Pinus banksiana Lamb.)-dominant forest sites surrounding the AOSR spanning Alberta and Saskatchewan. In total, 35 environmental variables were evaluated using redundancy analysis (RDA), followed by gradient forest analysis applied to plant species abundance data. Soil chemical variables accounted for just over 26% of the total explainable variation in the dataset, followed by bioclimatic variables (19%) and deposition variables (5%), but joint effects between variables also explained a significant portion of the total variation (p < 0.001). Total deposited nitrogen (TDN), and sulphur (TDS) along with bioclimatic and soil chemical variables, were identified as important variables in gradient forest analysis. A single, definitive threshold across TDN was identified at approximately 5.6 kg N ha⁻¹ yr⁻¹ (while a TDS threshold was found at 14.4 kg S ha⁻¹ yr⁻¹). The TDN threshold range was associated primarily with changepoints for several vascular species (Pyrola asarifolia, Pyrola chlorantha, Cornus canadensis, and Arctostaphylos uva-ursi) and bryophyte and lichen species (Pleurozium schreberi, Vulpicida pinastri, and Dicranum polysetum). These results suggest that across Jack pine-dominant forests surrounding the AOSR, the biodiversity-based empirical critical load of nutrient N is 5.6 kg N ha⁻¹ yr⁻¹. Full article

Biological N₂ fixation, a major pathway for new nitrogen (N) input to terrestrial ecosystems, largely determines the dynamics of ecosystem structure and functions under global change. Nevertheless, the responses of N₂ fixation to multiple global change factors remain poorly understood. Here, saplings of two N₂-fixing plant species, Alnus cremastogyne and Cajanus cajan, were grown at rural and urban sites, respectively, with the latter representing an environment with changes in multiple factors occurring simultaneously. Symbiotic N₂ fixation per unit of nodule was significantly higher at the urban site than the rural site for A. cremastogyne, but the rates were comparable between the two sites for C. cajan. The nodule investments were significantly lower at the urban site relative to the rural site for both species. Symbiotic N₂ fixation per plant increased by 31.2 times for A. cremastogyne, while that decreased by 88.2% for C. cajan at the urban site compared to the rural site. Asymbiotic N₂ fixation rate in soil decreased by 46.2% at the urban site relative to the rural site. The decrease in symbiotic N₂ fixation per plant for C. cajan and asymbiotic N₂ fixation in soil was probably attributed to higher N deposition under the urban conditions, while the increase in symbiotic N₂ fixation per plant for A. cremastogyne was probably related to the higher levels of temperature, atmospheric CO₂, and phosphorus deposition at the urban site. The responses of N₂ fixation to multiple global change factors and the underlying mechanisms may be divergent either between symbiotic and asymbiotic forms or among N₂-fixing plant species. While causative evidence is urgently needed, we argue that these differences should be considered in Earth system models to improve the prediction of N₂ fixation under global change. Full article

A century after the discovery of rhizobia, the first Beta-proteobacteria species (beta-rhizobia) were isolated from legume nodules in South Africa and South America. Since then, numerous species belonging to the Burkholderiaceae family have been isolated. The presence of a highly branching lineage of nodulation genes in beta-rhizobia suggests a long symbiotic history. In this review, we focus on the beta-rhizobial genus Paraburkholderia, which includes two main groups: the South American mimosoid-nodulating Paraburkholderia and the South African predominantly papilionoid-nodulating Paraburkholderia. Here, we discuss the latest knowledge on Paraburkholderia nitrogen-fixing symbionts in each step of the symbiosis, from their survival in the soil, through the first contact with the legumes until the formation of an efficient nitrogen-fixing symbiosis in root nodules. Special attention is given to the strain P. phymatum STM815^T that exhibits extraordinary features, such as the ability to: (i) enter into symbiosis with more than 50 legume species, including the agriculturally important common bean, (ii) outcompete other rhizobial species for nodulation of several legumes, and (iii) endure stressful soil conditions (e.g., high salt concentration and low pH) and high temperatures. Full article

Lucerne is regarded as the best legume crop for forage to be cultivated in South Africa. It is commonly used to produce good quality hay. It also plays an important role in soil conservation, regeneration, and crop rotation systems as it supplies substantial amounts of nitrogen to succeeding crops through symbiotic N₂ fixation, which makes it the preferable choice for intensive forage production systems. Fertilizer in liquid inoculant formulations has demonstrated to contribute growth and yield increase for leguminous crops. Therefore, the aim of this paper was to determine the effects of Sinorhizobium meliloti liquid formulation inoculation on the growth, yield, and nitrogen content in lucerne. The strain RF14 (Sinorhizobium meliloti) was collected from the Agricultural Research Council at Roodeplaat (Plant Health and Protection located (East), Pretoria (South Africa). The liquid inoculant contained 6.5 × 10⁹ viable cells mL⁻¹. According to the Kooen–Gieger climatic classification, the experiments were conducted on two different climatic zones. The first site was in Bronkhorspruit (Blesbokfontein farm) in the Gauteng province and the second was in Hartbeesfontein (Rietfontein Farm) in the Northwest province. The results showed that lucerne inoculation with liquid inoculant formulation of Sinorhizobium meliloti significantly increased nodule number, size, growth, and yield in both bioclimatic zones. The significantly increased were compared to the negative control. The Sinorhizobium meliloti inoculant increased nitrogen accumulation in all inoculated treatments compared to the control. The finding of this research provides important information on the impact of rhizobium microbial inoculant application in the improvement of soil fertility through nodule formation. In addition, seed vigor improvement was translated in overall growth and yield increase in lucerne plants. Full article

Chronic elevated nitrogen (N) deposition can have adverse effects on terrestrial ecosystems. For large areas of northern Canada distant from emissions sources, long-range atmospheric transport of N may impact plant species diversity, even at low deposition levels. The objective of this study was to establish plant species community thresholds for N deposition under multiple environmental gradients using gradient forest analysis. Plant species abundance data for 297 Jack pine (Pinus banksiana Lamb.)-dominant forest plots across Alberta and Saskatchewan, Canada, were evaluated against 43 bioclimatic and deposition variables. Bioclimatic variables were overwhelmingly the most important drivers of community thresholds. Nonetheless, dry N oxide (DNO) and dry N dioxide deposition inferred a total deposited N (TDN) community threshold of 1.4–2.1 kg N ha⁻¹ yr⁻¹. This range was predominantly associated with changes in several lichen species, including Cladina mitis, Vulpicida pinastri, Evernia mesomorpha and Lecanora circumborealis, some of which are known bioindicators of N deposition. A secondary DNO threshold appeared to be driving changes in several vascular species and was equivalent to 2.45–3.15 kg N ha⁻¹ yr⁻¹ on the TDN gradient. These results suggest that in low deposition ‘background’ regions a biodiversity-based empirical critical load of 1.4–3.15 kg N ha⁻¹ yr⁻¹ will protect lichen communities and other N-sensitive species in Jack pine forests across Northwestern Canada. Nitrogen deposition above the critical load may lead to adverse effects on plant species biodiversity within these forests. Full article

Cover crops and N fertilization strongly impact the forms of soil organic C and N and their availability, which change the responses of plants to N fertilization and soil organic C accumulation. Our study objectives were to evaluate the effects of cover crops and N doses on soil total and soluble C and N contents, N fractions, and potentially available N in a long-term no-till experiment. The experiment was conducted in a randomized block design with split plots and four replicates. The main treatments were cover crops species, jack bean, lablab bean, millet, velvet bean, and fallow cultivated prior to maize. Secondary treatments included two doses of mineral N (0 and 120 kg ha⁻¹). Soil samples were collected at depths of 0–5, 5–10, 10–20, and 20–40 cm, which were analyzed for total and water-soluble C and N contents, N fractions (acid hydrolysis method), and potentially available N (hot KCl solution and direct steam distillation methods). Cover crop velvet bean resulted in the highest soil organic carbon levels, and cover crop millet plus fertilization resulted in the highest levels of soil total N. The amino sugar was the largest N fraction, which decreased by 8% with N fertilization. The soluble C and N content strongly correlated with total and available N content. The changes in soil N were influenced by cover crop species and fertilization and the interactions of both, so the combination of fertilization regime and cover crops must be chosen with care. Additionally, legumes are a good source of plant and soil N in systems with low input of N via fertilizer; however, the combination of N fertilizer with legumes can reduce soil N reserves, leading to its long-term depletion. Full article

Nitrogen availability is among the major limiting factors for the production of organic crops. A central goal of organic farming, according to certification standards, is to rely on ecological and biological principles to build and maintain soil health. Nitrogen is among the most complex nutrient elements with respect to its different chemical forms and its flow within the environment at the soil, microbial, plant, aquatic, and atmospheric levels. Because, from an ecological perspective, all production variables on the farm are interrelated, a challenge for scientists and practitioners is to better understand nutrient cycles on the farm with respect to how particular production practices may improve N availability during particular stages of crop growth while minimizing potential environmental losses that may lead to contamination of the groundwater and aquatic habitats or to undesirable greenhouse gas emissions. Here, based on a selected review of the literature, we evaluate N cycles at the farm level and present key ecologically-based management strategies that may be adopted to improve internal N cycles. Given the location-specific nature of most ecosystem interactions, a participatory agroecology approach is proposed that incorporates the knowledge of indigenous and traditional cultures to better understand and design resilient and socially-equitable organic systems. Full article

Cover cropping is vital for soil health. Timing and method of termination are major factors influencing the agroecological benefits of cover crops. Delay in the termination of cover crops results in greater biomass production. Likewise, incorporation of cover crops during termination often speeds residue mineralization compared to no-till systems. We used four termination strategies for a late-terminated winter rye–legume mix (in tilled and no-till systems) and four N application rates in the succeeding sweet corn crop to examine how cover crop termination affected N response in sweet corn as well as the independent effects of N application rate and cover crop termination method. The experiment was conducted using a randomized complete block design with four replications. Increasing N fertilization up to 144 kg N ha⁻¹ promoted yield and quality in sweet corn as well as summer weed growth. The cover crop termination method did not affect sweet corn response to N fertilizer. This suggests that when rye is terminated late in the spring before planting cash crops, the incorporation of its residues may not greatly affect the soil N dynamics. This indicates that decisions to incorporate rye residues may be taken by farmers with an eye mainly towards management issues such as weed control, environmental impacts, and soil health. Full article

The scientific objective of this study was to answer the question of whether sink limitation is also true for high quality wheat varieties. We examined the incorporation of ¹⁵N and ¹³C during phenological phases into vegetative parts and grains of Elite wheat Triso (E) and Quality wheat Naxos (A) when the spike is halved. Three splits of fertilizer were applied at EC 11, EC 30, EC 59, whereby 10% at EC 30 and EC 59 was ¹⁵N, and plants were also labelled with ¹³CO2. The application of only the third split as ¹⁵N, combined with spike-halving, resulted in a significantly higher ¹⁵N-content (+11%) of 0.486 mg ¹⁵N/g DM, compared to the control (0.437 mg¹⁵N/g DM). Labelling whole plants with ¹³CO₂ at EC 59 resulted in a significantly higher ¹³C-content—40%—(0.223 mg ¹³C/g DM) of the grains of the control for Triso at the fully-ripe stage (EC 89), compared to Naxos (0.160 mg ¹³C/g DM). This superiority was reduced to 34%, and was also demonstrated by spike-halving (0.226 mg ¹³C/g DM, 0.169 mg ¹³C/g DM). Remobilization of ¹⁵N for control and spike-halving treatments were 68.2% and 61.1%, respectively. This clearly demonstrates that the reduction of the sink size by spike-halving leads to a 7% reduction in the remobilization of ¹⁵N from vegetative to reproductive tissues. Full article

The use of fertilizers is of the utmost importance for food security on a global scale. However, fertilizer production and overuse may yield environmental issues. In this research, Life Cycle Assessment (LCA) was used to estimate eighteen environmental impact categories for six different fertilizer products: three synthetic (ammonium nitrate; calcium ammonium nitrate; and urea ammonium nitrate) and three organic (cattle manure; compost; and a mixture of compost and synthetic fertilizer). The processes for fertilizer production were obtained from the Agribalyse database. The system boundaries were from cradle to factory gate (or farm gate in the case of animal waste), and the impact indicators were calculated per kg of nitrogen (N). The data showed that the organo-mineral fertilizer (a mix of compost and synthetic fertilizer) had the highest environmental impact according to the results for most of the impact categories. The median values for this product regarding water consumption, fossil resource use and global warming potential were 322.5 L, 3.82 kg oil equivalent and 13.70 kg CO₂ equivalent, respectively, per kg of N. The respective values for cattle manure, for which the lowest environmental impact was observed, were 0.23 L of water, 0.002 kg oil-eq and 3.29 kg of CO₂-eq, respectively, per kg of N. Further research should focus on the determination of the impact from other stages of the life cycle (e.g., transportation and application to the field) which were not included in this work. This research could support the selection of N fertilizer in sustainable food production. Full article

Reducing chemical fertilizers is critical for maintaining soil health and minimizing environmental damage. Biochar-based organic fertilizers reduce fertilizer inputs, improve soil fertility, increase crop productivity, and reduce environmental risks. In this study, a pot experiment was conducted in a greenhouse to assess the potential of biochar-based organic and inorganic fertilizers to improve soil fertility and Okra yield. Seven treatments with three replicates were arranged in a completely randomized design (CRD). Three treatments included biochar-blended formulations (i) biochar mixed with mineral NPK fertilizer (BF), (ii) biochar mixed with vermicompost (BV), and (iii) biochar mixed with goat manure (BM); two treatments included biochar enrichment formulations (iv) biochar enriched with cow urine (BCU) and (v) biochar enriched with mineral NPK fertilizer in aqueous solution (BFW), and the remaining two included control treatments; (vi) control (CK: no biochar and no fertilizers) and (vii) fertilized control (F: only recommended NPK fertilizer and no biochar). Mineral NPK fertilizers in BF, BFW, and F were applied at the recommended rate as urea, di-ammonium phosphate (DAP), and muriate of potash (MOP). Organic fertilizers in BV, BM, and BCU treatments were applied in equal quantities. All biochar-amended treatments showed improved soil chemical properties with higher pH, organic carbon, total N, and available P and K compared to the two non-biochar control plots (CK and F). Biochar blended with goat manure (BM) showed the highest effect on soil fertility and fruit yield. BM (51.8 t ha⁻¹) increased fruit yield by 89% over CK (27.4 t ha⁻¹) and by 88% over F (27 t ha⁻¹). Similarly, cow urine-enriched biochar (BCU) (35 t ha⁻¹) increased fruit yield by 29% and 28% compared to CK and F, respectively. Soil pH, OC, and nutrient availability (total N, available P, and available K) showed a significantly positive relationship with fruit yield. The study suggests that using biochar-based organic fertilizers, such as BCU and BM, could outperform recommended mineral fertilizers (F) and produce higher yields and healthy soils, thereby contributing to mitigating the current food security and environmental concerns of the country. Full article

Numerous biological and chemical methods have been proposed over the years for estimating the nitrogen (N) mineralization capacity of soils; however, none of them has found general use in soil fertility testing. The efficacy of a recently proposed alkaline hydrolysis method for assessing N availability in soils compared with the standard long-term incubation technique for determining potentially available N was evaluated. The nitrogen mineralization of 12 surface soils incubated under aerobic conditions at 25 °C for 26 weeks was determined. Field-moist soils were direct-steam distilled with 1 M KOH or 1 M NaOH; the NH₃ released was trapped in boric acid, and its concentration was determined successively every 5 min for 40 min. The cumulative N mineralized or hydrolyzed was fitted to the first-order exponential equation to determine the potentially mineralizable N (N_o) and an analogous “potentially hydrolyzable N (N_max)” for the soils. The flush of CO₂ (fCO₂) following the rewetting and incubation of air-dried soils under aerobic conditions for 3 days was also determined. The results showed that the N_max values differed considerably among the soils, indicating differences in the chemical nature and reactivity of the organic N content of the soils, and were significantly correlated with N_o and fCO₂ values. The estimated N_max and N_o values ranged from 105 to 371 mg N kg⁻¹ and 121 to 292 mg kg⁻¹, respectively. Based on the simple and inexpensive nature of the alkaline hydrolysis procedure, the reduction in the incubation time required to obtain N_o (months to minutes), and the strong association between N_max and N_o, we concluded that N_max is a good predictor of the biologically discrete and quantifiable labile pool of mineralizable soil organic N (ON), and the use of the alkaline hydrolyzable ON as a predictor of N_o merits consideration for routine use in soil testing laboratories for estimating the N-supplying capacity of soils. Full article