How Does Specialization in Agricultural Production Affect Soil Health?

: The aim of the study was to assess the impact of the specialization of agricultural production on selected parameters of soil health, i.e., soil organic carbon content (SOC), soil acidification, soil nutrient status, i.e., total nitrogen content (N T ), available forms of phosphorus, potassium, and magnesium, and microelements content, as well as the content of selected potentially toxic metals (PTMs). For the study, 18 farms located in the Masovian Voivodeship in Central Poland were selected. They were grouped into six types, and each type was represented by three farms. The study included organic farms; farms specializing in: crop, vegetable, poultry, dairy cattle, and pigs production. A total of 144 soil samples were analyzed. The results showed that the specialization of agricultural production and fertilizer management had a significant impact on most of the tested soil health parameters, except SOC and N T content. Despite the high organic fertilizer doses introduced into soils in poultry (170 kg N per hectare as poultry manure) and pig farms (150 kg N per hectare as pig manure), there was no significant influence of these amendments on SOC content. This may indicate low organic carbon sequestration potential in some Polish agricultural soils. Organic farms had the lowest levels of plant nutrients in the tested soil samples, which may limit soil productivity. All the tested soils were strongly acidified, which could restrict both production and regulatory soil functions. Based on the synthetic index of soil fertility (SSFI), vegetable and poultry farms were characterized by very high fertility, while crop, dairy cattle, and pig farms fell into the medium fertility class. Organic farms were in the lowest fertility class. However, the study suggests that the SSFI may not be the best indicator for assessing soil fertility and health; therefore, further research is needed.


Introduction
The soil of the arable land is a natural resource, which is crucial for food production and the functioning of terrestrial ecosystems.The growing population and dietary habits of societies create a demand for the increased production of food and feed.The intensification of agricultural production associated with an increase in chemical usage in plant and animal production often results in the unsustainable management of organic fertilizers, indicates soil acidification and water eutrophication, and increases the concentration of potentially toxic metals, pesticides, and other groups of pollutants, thus causing the degradation of soils in rural areas.Moreover, urbanization, erosion, and soil compaction reduce the area of arable land and thus limit the volume of agricultural production [1].It has been estimated that the costs associated with eliminating or mitigating the effects of soil degradation in the EU currently exceed EUR 50 billion per year [2].Considering that, it is extremely important and urgent to change the approach to soil management in the EU.To eliminate these negative effects in 2021, the European Commission delivered an EU soil strategy setting a vision to achieve healthy soils by 2050 and implement concrete actions by 2030.This strategy will contribute to the objectives of the European Green Deal and it is a key deliverable of the EU biodiversity strategy by 2030 [3][4][5].The implementation of the healthy soils concept in the EU seems to be very ambitious, but healthy soils are crucial to achieve, e.g., climate neutrality, halting land degradation, and safeguarding human health.
Soils perform various environmental functions.From an agro-environmental point of view, these soil functions are the production of food and other biomass, storage and cycling of nutrients, supporting gas exchange and biological activity [6], immobilizing and filtering organic and inorganic substances, and providing a habitat suitable for maintaining high levels of biodiversity [7].Properly functioning soils (of good quality) can provide ecosystem services to the benefit of humans and usually they are defined as healthy [8,9].Although awareness of the importance of soil health is constantly increasing, currently, there is no global consensus on how it should be defined and measured, as well as which indicators have to be applied [10].As a result, numerous studies on soil health are currently carried out in different regions of the world.They consider applications of various soil properties, modelling methods, and statistical calculations, but they are still under development.Moreover, there are no harmonized standards or soil property thresholds allowing for a uniform assessment and delineation of healthy soils worldwide [11].Recent studies on soil health often use soil quality indicators (SQI), which are defined and calculated based on different approaches [10].It seems that important criteria for selecting proper soil health indicators are the sensitivity to soil management and changes in environmental conditions.Soil health indicators should respond to the extent to which soil can perform functions necessary for crop and animal welfare, the natural environment, and human health.Moreover, they should combine various soil information, useful for making multiobjective soil management decisions in agriculture [12].
The reason for the methodological diversity of soil health assessment is the regional climate and soil variability, as well as diverse methodologies of soil properties analysis adopted by individual countries in their soil-monitoring systems.The development of a harmonized soil health index for the whole European Union has recently become a serious challenge for soil scientists and agronomists across different countries.In Poland, several indicators are used to assess soil quality and their use for agriculture, i.e., the index of valorization of the agricultural production area (VAPA), the soil physical quality index, the soil biological quality index, and the Riehm index [13].Since 2012, the synthetic soil fertility index (SSFI) developed by Ochal [14] has also been used.It is applied to assess the agrochemical fertility of soils.SSFI was developed using the so-called factor analysis, based on data describing the potential soil pH as determined in 1 mol KCl•dm −3 and the content available for plants of phosphorus, potassium, and magnesium.Critical values for the SSFI index have also been developed to enable soil fertility assessment [14].
One of the crucial factors influencing soil properties is the technique of soil management, which depends on the intensity and specialization of agricultural production.To achieve the EU strategic goals, it is necessary to find out the impact of various soil management practices on its health and to determine the state at which soil is correctly functioning for the implementation of various ecosystem services.Therefore, the authors hypothesize that in similar climatic and soil formation conditions, the soil state may differ due to the type of farms characterized by a different intensity and specialization of production and depends on the adopted soil-management system.Therefore, the aim of the study was to assess the impact of various types of agricultural production systems on soil characteristics and the soil fertility index as a potential component of the agricultural soil health evaluation.

Agricultural Farms Characteristics
For the study, 18 farms located in the Masovian Voivodeship (N: 53°28′55″ N; S: 51°00′47″ N; W: 19°15′33″ E; E: 23°07′42″ E) in Central Poland were selected (Figure 1).They were grouped into six types, and each type was represented by three farms.The study included organic farms (E1, E2, E3, marked with the symbol E1-E3, mix crop with dairy cattle production); farms specializing in crop (R1, R2, R3, i.e., R1-R3), vegetable (W1, W2, W3, i.e., W1-W3), poultry (D1, D2, D3, i.e., D1-D3), dairy cattle (B1, B2, B3, i.e., B1-B3), and pigs (T1, T2, T3, i.e., T1-T3) production.Farm types differed in the intensity and specialization of agricultural production.During the interview, farmers were asked to complete a questionnaire to provide information about their agricultural production (Table 1) and the average area of agricultural land and the type and dose of nutrients applied in fertilizers (Table 2).Farm types differed in the intensity and specialization of agricultural production.During the interview, farmers were asked to complete a questionnaire to provide information about their agricultural production (Table 1) and the average area of agricultural land and the type and dose of nutrients applied in fertilizers (Table 2).The crop structure of the farms R1-R3 was dominated by cereals (75% of the area), and rapeseed (approximately 25%).On the farms W1-W3, bulb vegetables, brassicas, root vegetables, and cucurbits dominated, accounting for 42%.The area of cereals on the W1-W3 farms was on average 22.5%, and potatoes occupied 26.5%.In both groups of farms R1-R3 and W1-W3, the basic source of nutrients for plants constituted mineral fertilizers.Organic farms (E1-E3) were mixed farms integrating annual arable crops, permanent grasslands, and dairy cattle.Arable land dominated on organic farms, accounting for 65.3% of agricultural land, while grasslands accounted for 28% and orchards for 6.7%.Cereals dominated among the cultivated plants (55.7%).In this type of farm, the only source of nutrients was organic fertilizers from own production.On organic farms, together with plant production, animal production was also carried out, i.e., dairy cattle.The average milk yield of cows was approximately 3850 L animal −1 per year.The average animal density was 1.11 livestock units (LSU) per 1 hectare.On these farms (E1-E3), animals were kept in shallow barns.Organic fertilizers were stored on manure plates, and the slurry flowing from the heaps in tight storage tanks.Due to the specificity of production on dairy cattle farms (B1-B3), there was a high share of grassland, i.e., almost 48%.These farmers used both organic and mineral fertilizers.The average milk yield of cows was approximately 8127 L animal −1 year −1 .The average animal density was 1.78 LSU ha −1 .On the B1-B3 farms, animals were kept in shallow barns.On farms with pig production (T1-T3), arable land dominated (93.4%), and grassland accounted for a small percentage of agricultural land, i.e., 6.6%.Cereals predominated among the cultivated plants, and most of them were intended for fodder purposes.The average animal density was 4.39 LSU ha −1 .On poultry farms (D1-D3) in which D1 and D3 were focused on breeding turkeys, D3 also reared laying hens and D2 bred geese and ducks, and 100% of the farm area constituted arable land.The average animal density was 18.01 LSU ha −1 .All of the tested farms employed a conventional tillage system.

Soil Sampling and Laboratory Analysis
The research was conducted in 2022.In autumn, after the plants' harvest, samples representing the topsoil (0-25 cm) were collected.The soil survey procedure included the collection of individual 30 samples per 1 ha.Soil samples were separately collected from 8 fields of the farm and then accurately mixed to obtain an averaged sample representing individual farmland.A total of 144 soil samples were tested (6 farm types × 3 farms in each type × 8 fields in each farm).
Soil samples were transported to the laboratory and air-dried.Then, samples were sieved through a sieve with a mesh diameter of 2 mm and the following parameters were determined: available for plants forms of phosphorus (P) and potassium (K) using the Egner-Riehm DL method.Available forms of magnesium (Mg) were determined using the Schachtschabel method in a solution of 0.0125 mol dm −3 CaCl 2 (soil: extractant ratio = 1:10, after 1 h extraction).The potentially toxic metals (PTMs) Ni, Cd, Cu, Mn, Fe, Pb, and Zn were measured in 1 mol dm −3 HCl (soil: extractant ratio = 1:10, after 1 h extraction).In the obtained soil extracts, P concentration was determined using the colorimetric method (spectrophotometer Genesys 10 UV-Vis (ultraviolet and visible light region)), Thermo Electron Corporation, Madison, WI, USA), and K, Mg, Ni, Cd, Cu, Mn, Fe, Pb, and Zn were determined by the atomic absorption spectroscopy method (AAS) (SOLAAR, Thermo Elemental, Cambridge, UK).In our study, the air-acetylene flame was employed.Soil organic carbon (SOC), and total nitrogen (Nt) content were measured by dry digestion using a Vario MacroCube, Elementar, Germany.Soil pH was determined by the potentiometric method in 1 mol.dm−3 KCl solution using a pH meter (Schott, Mainz, Germany).

Statistical Analysis
Basic statistics were calculated, i.e., mean, standard deviation, and coefficient of variation, using the Statistica PL 13.3 software (Tulsa, OK, USA).The results were also subjected to a one-way analysis of variance (ANOVA), and the confidence intervals for means were determined by Tukey's test at the significance level of p < 0.05.The correlation coefficients were calculated at p < 0.05.

Soil Organic Carbon
The content of soil organic carbon (SOC) and SOC stocks in soil are considered the most important indicators of soil health [10].SOC affects important soil functions and all the ecosystem services provided by soil.Currently, approximately 41-95 Mt C is accumulated annually in the soils of European Union countries [15].However, as a consequence of the observed climate changes and the specialization of agricultural production, the SOC content in soil is expected to decrease and the ability of soils to sequester carbon will decline as well [16].De Rosa et al. [17] reported that SOC stocks in agricultural soils have recently decreased by approximately of 0.75% in the period 2009-2018.The countries with the highest estimated losses of SOC were primarily located in the central and northern regions of Europe.
Generally, Polish soils are characterized by a very low SOC content.The results of soil chemistry monitoring [18] showed that, in Poland, the average carbon content in soils has remained at a constant level of approximately 1.12% for many years.
The SOC content in the collected soil samples varied depending on the agricultural specialization of the farm, although these differences were not statistically significant (Table 3).The lack of statistical differences resulted from significant differences in the carbon content in soils within farms of the same type.On average, the highest carbon content was found in soils from D1-D3 (1.53%) and B1-B3 (1.50%).In soils collected from the R1-R3, E1-E3, and T1-T3 farms, the average carbon content was 0.96%, 1.04%, and 1.04%, respectively.The source of carbon in the soils of the farms B1-B3, T1-T3, D1-D3, and E1-E3 was the organic fertilizer.The content of SOC in the soil is regulated by the balance between the inflow of carbon (e.g., in crop residues and organic fertilizers) and its outflow (mainly as a result of the emission of CO 2 and CH 4 -products of the decomposition of organic matter).Disruption of this balance in agricultural soils results in a loss of SOC and deterioration of soil quality.Organic fertilization increases the SOC content in the soil [19], but it may also increase CO 2 emissions from the soil [20].Sosulski et al. [20] showed that the CO 2 -C emission from soils fertilized with minerals was 2975.0 and 3933.1 kg CO 2 -C ha −1 , and from soils fertilized with organic fertilizers 5149.2 and 5365.0 kg CO 2 -C ha −1 .The effectiveness of organic fertilizers in maintaining or increasing the SOC content in soil varies and depends on many factors [21].According to Emde et al. [22], these factors are climatic conditions, properties of soil, and organic materials, their doses, and methods of application.Regarding the studied farms, the highest doses of organic fertilizers were used on farms D1-D3 (170 N kg ha −1 ) and T1-T3 (150 N kg ha −1 ).Despite high doses of organic fertilizers, no significantly higher SOC content was found in the soil on these farms, which may confirm literature reports that Polish soils have little potential to accumulate carbon stock [20].Audette et al. [23] showed that the effect of tillage practice on the SOC is more pronounced than fertilizer practices.It is well known that soil aggregate contains SOC with different degrees of physical protection against microbial decomposition [24].According to Six et al. [25], a no-till (NT) system leads to the formation of stable soil microaggregates rich in POM (particulate organic matter), contributing to greater carbon being stabilized and sequestered.Conventional tillage (CT) systems cause decreases in soil C content by increasing soil aeration, promoting the mineralization of organic matter, and the release of CO 2 -C into the atmosphere [26].However, the tillage system did not differentiate the SOC content in the tested soil samples because all farms used a conventional tillage system (CT).If soil organic carbon content decreases below a certain threshold (so-called critical limit), this will be reflected in all major soil functions, leading to soil degradation [27].The minimum thresholds for SOC content in soils occurring in the temperate climate zone were proposed by Drexler et al. [28] and Wessolek et al. [29].According to these authors, the minimum thresholds of SOC content depended on the fertilizer management carried out on a given soil [29]; in the system of exclusive mineral fertilization (used on the farms R1-R3 and W1-W3), the limit content was set at 0.73% C, in the mineral-organic fertilization system (used on the farms B1-B3, T1-T3, and D1-D3) it was 0.93% C, and on organic farms (E1-E3) with exclusive organic fertilization-0.8%C in the soil.Comparing the carbon content measured in the soils on individual farms with the abovementioned limit values, it can be concluded that on no farm was the soil found to be degraded, although, according to Niedźwiecki et al. [30], their carbon content was very low (<1%) or low (1-2%).

Soil Nutrient Status
The content of nutrients in soil impacts the growth of plants in both natural and agricultural settings.Appropriate nutrient levels positively affect biomass production and crop yield.The nutrient status of soil also affects the diversity of soil microorganisms, soil animals, and plant species, as well as water quality.In agricultural soils, crop growth is strongly stimulated by N and P input, but here the risk for N losses to the atmosphere and N and P to water and related eutrophication impacts is very high.Regrettably, there are no defined target levels or critical levels for the total nitrogen content in agricultural soils, although it is considered an indicator of soil health [10].
The content of total nitrogen in the tested soils ranged from 1.17 g N kg −1 on organic farms to 1.59 g N kg −1 on farms specializing in dairy cattle production (Table 4).The differences were not statistically significant, so there was no influence of the specific agricultural production on the nitrogen content in the soil.The conducted research showed that the average nitrogen content in samples taken from 18 farms (1.40 g N kg −1 ) was lower than the average for the country (1.59 g N kg −1 ) [31].Such a low nitrogen content in Polish soils compared to other European countries, where the average nitrogen content in the soil is approximately 2.26 g N kg −1 [31], may indicate a lower ability of Polish soils to store organic and mineral nitrogen compounds, which is confirmed by literature data [20,32].The highest nitrogen content in the soil has been recorded in Ireland (3.61 g N kg −1 ), Slovenia (3.48 g N kg −1 ), and Austria (3.06 g N kg −1 ).It should be noted that in both Ireland and Slovenia very high doses of nitrogen have been used since the 1990s, i.e., 170 and 141 kg ha −1 , respectively (average for the years 1992-2020) [33].In the same period, the average nitrogen doses used in Austria and Poland were similar and amounted to approximately 78 kg ha −1 [33].Nonetheless, Polish soils had a significantly lower nitrogen content than Austrian soils.The content of available forms of P in soil is one of the soil health indicators.Various methods are employed to evaluate the level of available soil P, such as P-Bray [34], P-Olsen [35], P-ammonium oxalate [36], P-calcium lactate [37], and P-Mehlich [38].Each extraction method indicates a different amount of P in a soil sample.Although it would be best to use a unified extraction method, many countries have linked P in a specific soil extraction method to crop yields and therefore prefer to stick with their own approach.Due to the lack of a harmonized method for evaluating soil abundance in P, it is currently considered that in order to prevent reduced crop yield and environmental damage, it is essential to maintain available soil P levels within a specific range.According to Mallarino and Blackmer [39], these levels should be above a critical point below which crop yield is limited, but also below which P runoff and leaching become a concern (such as noted in Li et al. [40]).
In Poland, the Egner-Riehm test is used to assess the content of available forms of P in soils.Despite soil acidification which favors the immobilization of phosphorus [41], the content of P available forms in soils was usually very high (except for the organic farms E1-E3) (Table 4).Particularly abundant in phosphorus were the soils of the D1-D3 farms, where the share of soils with very high phosphorus content was 95%.On the farms R1-R3 and T1-T3, the share of such soils amounted to approximately 83.5% on average.The soils of the E1-E3 farms had significantly the lowest phosphorus content (on average, approximately 51 mg P kg −1 ).As many as 46% of the soils on these farms were characterized by a low phosphorus content.On organic farms, the only source of phosphorus was organic fertilizer, with only 9 kg P ha −1 introduced into the soil (Table 2).The obtained results confirm the relationship between soil abundance in phosphorus and the dose of phosphorus applied in fertilizers (y = 3.1896x + 68.337R 2 = 0.5776, where y is the soil's phosphorus content (mg P kg −1 ) and x is the dose of P in fertilizers (kg P ha −1 )).Even though the total amount of phosphorus received with fertilizers on the farms T1-T3 was over 43% higher than on the farms D1-D3, the soil phosphorus content was lower there by over 68%.In the farms T1-T3, the amount of phosphorus entering the soil with organic fertilizers was nearly 2.8 times higher than its dose in mineral fertilizers.This indicates that the ability to accumulate phosphorus in the soil from organic fertilizers is lower than from mineral fertilizers.In both types of farms specializing in plant production, i.e., W1-W3 and R1-R3 (with exclusive mineral fertilization), although less phosphorus as superphosphate (37 kg P ha −1 and 17 kg P ha −1 , respectively) was applied with fertilizers than on the farms T1-T3 (50 kg P ha −1 in pig manure), the phosphorus content in soil was similar or even higher.According to Szara et al. [42], phosphate anions compete in the soil for sorption sites with organic compounds (with a negative charge) originating from the decomposition of organic fertilizers.Therefore, the phosphorus losses from organically fertilized soils may be greater than from those applying mineral fertilizers.It can, therefore, be concluded that in soils fertilized with high doses of phosphorus, especially in the form of organic fertilizers, the regulatory function related to water purification may be disturbed.Additionally, according to the literature data, soil acidification increases the solubility of organic compounds, and thus phosphorus associated with organic matter can be transported to deeper levels of the soil profile and to groundwater [43,44].
The distribution of potassium in soil is primarily influenced by the chemical composition of the soil parent material and the climate.In the sandy soils of north-eastern Europe, there is a tendency for lower concentrations of K due to higher levels of potassium loss caused by leaching.The average potassium content in the soils of European countries is approximately 130 mg K kg −1 , while in Polish soils it is approximately 107 mg K kg −1 [31].Among the analyzed farm types, the soils of the W1-W3 farms were the richest in potassium (178.32 mg K kg −1 ) (Table 4).Soils with very high and high potassium content constituted almost 60% of this type of farm.Soils rich in potassium were also found on the D1-D3 and T1-T3 farms (K content there amounted to 166.55 and 132.40 mg K kg −1 , respectively).These types of farms used high doses of K in fertilizers (potassium chloride, 50% K), which promoted the accumulation of this element in the soil (Table 2).In the case of farms specializing in dairy cattle production, the average soil potassium content was 101.60 mg K kg −1 , and the share of soils with a low and very low content of this nutrient was as high as 68%.The soils least rich in potassium were found on organic farms (59.61 mg K kg −1 ).More than 2/3 of the soils of organic farms were characterized by a low and very low potassium content.Considering that the most optimal, from the point of view of the production and regulatory functions of the soil, is the average soil content in potassium, it can be concluded that the best conditions prevailed on farms with field crop production (R1-R3).These farms had the highest share of soils with average potassium content (almost 50%) among all farm types analyzed.
According to the available literature data, magnesium deficiencies may occur on light and acidic soils [45].In such conditions, magnesium is washed deep into the soil profile.Polish (acidified) soils are characterized by a relatively low content of available magnesium (the average content of magnesium is approximately 83 mg Mg kg −1 ) [18].Since the share of acidic soils in the Masovian Voivodeship is relatively higher than in the rest of the country, their average magnesium content is only 68 mg Mg kg −1 .The research results indicate that in the studied farms (except for organic farms), the magnesium content in the soil was higher than the average for the country and voivodeship (approximately 92.29 mg Mg kg −1 ) (Table 4).The highest magnesium content in soil was found on the B1-B3 farms.As many as 76% of the soils on these farms belonged to the class of high or very high magnesium content.Soils from the W1-W3 and D1-D3 farms were equally rich in magnesium.The average soil magnesium content in these types of farms did not differ significantly and amounted to 93.84 and 97.92 mg Mg kg −1 , respectively.The least magnesium content was found in the soil of the R1-R3 farms (average approximately 71.85 mg Mg kg −1 ) and in organic farms (approximately 49.43 mg Mg kg −1 ).On organic farms, over 40% of the soils belonged to the class of low and very low magnesium content.

Soil Micro-Nutrients and Heavy Metals Status
In the soil, in addition to the micronutrients nickel (Ni), zinc (Zn), copper (Cu), manganese (Mn), and iron (Fe), heavy metals occur that have toxic effects on living organisms: lead (Pb) and cadmium (Cd) [46].Nevertheless, high concentrations of micronutrients could also be harmful to crop and humans due to their toxicity.Microelements are involved in many metabolic processes in the plant, where they increase the use of macroelements, thus positively influencing the yield of plants and their general condition and health [47].The availability of microelements for plants depends mainly on soil acidification, soil granulometric composition, and organic matter content [47].The type and rate of fertilization are also important for the bioavailability of microelements for plants.It is commonly believed that on farms using organic fertilizers, the soil content of microelements is higher than on those using exclusively mineral fertilizers [48].However, the research results presented in this study suggest a different relationship (Table 5).The content of Ni, Zn, and Fe was the highest in the soil of farms where no organic fertilizers were used, i.e., R1-R3 and W1-W3 (Table 5).Means marked with different letters in the columns differ significantly at p < 0.05.
The limit numbers used to assess soil abundance in microelements in the national agricultural advisory system depend on the agronomic category of the soil and/or soil pH [47].When the content of microelements is low, fertilization with microelements is recommended.However, their accumulation in the soil exceeding the high concentration level may be toxic to plants, as well as they may be dispersed from the soil, leading to environmental pollution.
The content of zinc in the soil on all the farms studied was greater than 5 mg kg −1 , which, considering their acidification, indicated a high abundance [47].From the soil abundance of this micronutrient found in farms specializing in vegetable production (W1-W3), crop production (R1-R3), and poultry production (D1-D3) (Table 5), exceeding 5 mg kg −1 by two-or threefold, it can be concluded that the zinc content in soils deteriorated its quality and health.The content of copper in the tested soils shown in Table 5 indicates that only on organic farms (E1-E3) was the abundance of this microelement low.However, in the farms T1-T3, R1-R3, and in W1-W3, the copper content in soils was much higher than the critical values, determining its high abundance.Considering the soil pH, it was estimated that the manganese content in the soil in all the researched farms was very high and considerably exceeded the assumed critical values.Disturbing soil health with excessive concentrations of microelements may pose a threat to its productive function and to the quality of the natural environment.
According to the Environmental Quality Standards Directive [48], cadmium is classified as a priority hazardous substance and is considered one of the most toxic environmental chemicals.The obtained research results indicate that significantly more cadmium was present in the soils of farms specializing in vegetable production (W1-W3) and crop production (R1-R3) (on average, approximately 0.17 mg Cd kg −1 ) (Table 5) than in other types of farms (0.08 mg Cd kg −1 ).The highest lead content was found in the soils of farms specializing in vegetable production (22.31 mg Pb kg −1 ).In the soils of other farms, the lead content was similar (6.62-8.37 mg Pb kg −1 ).The vegetable farms are often located in peri-urban regions.Therefore, soils in these areas may contain higher levels of heavy metals compared to rural regions [49].The farms W2 and W3 are located 15 km from Warsaw (capital of the country), which could have resulted in an increase in the content of heavy metals in soil samples collected from these farms.The proximity of a large city may have favored greater atmospheric deposition of Pb into the tested soil.According to Liu et al. [50], atmospheric deposition is one of the major sources of Pb in soils.Moreover, intensive vegetable production creates pressure to use larger quantities of mineral fertilizers, pesticides, and organic wastes (i.e., wastewater irrigation and sewage sludge) compared to other crop production [51].The application of pesticides and phosphorus fertilizer could lead to the accumulation of Cd in soil [52].

Soil Acidification
There are multiple indicators that can be used to assess soil acidification, such as pH, base saturation, aluminum (Al) concentration, the ratio of Al to base cations, and cation exchange capacity (CEC) [53].In agricultural soils, soil pH is the primary indicator used to evaluate the acidity level of the soil and determine if liming is necessary.Excessive soil acidification negatively affects all soil functions [10].Hence, it is one of the most important indicators of soil health [54].The obtained test results indicate that in all types of farms the soil was excessively acidified (Table 6, Figure 2).As many as 67% of the collected soil samples were acidic or strongly acidic, of which 36% were soils with a pH below 4.5.Several studies have shown that crop production is limited when the soil pH levels fall below 5.5-6.0 because of the restricted availability of Ca, Mg, K, and P [55].These findings strongly suggest that pH 4.5 is considered critically low.About 1/3 of the tested soils were slightly acidic (20%) and neutral (13%).The obtained results proved that the structure of soil pH varied depending on the specialization of agricultural production on the farms studied (Figure 2).In the scientific literature, there are studies that highlight the impact of mineral nitrogen fertilizers on soil acidity [56].On the other hand, the application of organic fertilizers, mainly manure, reduces soil acidification [57].It can, therefore, be expected that the state of soil acidification on organic farms and farms specializing in animal production should be lower than on farms specializing in plant production.The results obtained indicate yet a different relationship.The results of the conducted research show that most of the strongly acidic soils were found on organic and pig farms (Table 6) where organic fertilizers have been used for years.In each of these types of farms, over 50% of the soils were highly acidic (Figure 2).However, the highest percentage of soils with a neutral pH was found on vegetable farms (33%), where the only source of nutrients was mineral fertilizer.Since the main method of regulating soil pH is soil liming, the obtained research results prove that on farms specializing in plant production, maintaining the soil pH close to the optimal one for plant growth and yield is more important for the economic results of the farm than on farms specializing in animal production.In this type of farm, income is generated based on the sale of animal products.On farms with animal production, pigs and poultry in particular, animal nutrition is based mainly on the purchased feed.Therefore, the expenditure on maintaining optimal or even satisfactory soil fertility is less important there.The loss of plant yield caused by soil acidification is compensated by a greater supply of feed from external sources.On organic farms, awareness of the importance of optimizing soil conditions for agricultural production should be the highest or even prioritized, although it seems that the economic effect of organic production in Polish conditions limits the amount of expenditure on soil liming, which increases its acidification level.A positive significant correlation between soil pH and the content of SOC and NT in the soil was found only on the farms R1-R3 and W1-W3 (Table 7), where manure was not applied.According to the literature data [59,60], a greater production of crop residues and the formation of organic carbon and nitrogen compounds that are more resistant to decomposition and leaching in soils with a higher pH promote the accumulation of SOC and NT.In the soils of the farms W1-W3, D1-D3, and B1-B3, the content available for plants of phosphorus and magnesium in the soil was positively and significantly correlated with soil pH (in the farms E1-E3 only with the content of available magnesium).The beneficial effect of a higher soil pH on the content of available forms of P and Mg is widely known in the literature [41,61].Therefore, in the soils of the farms W1-W3 (with a relatively high soil pH), the obtained relationships confirm the commonly accepted views on the chemistry of these nutrients in the soil.A positive significant correlation between soil pH and the content of SOC and NT in the soil was found only on the farms R1-R3 and W1-W3 (Table 7), where manure was not applied.According to the literature data [59,60], a greater production of crop residues and the formation of organic carbon and nitrogen compounds that are more resistant to decomposition and leaching in soils with a higher pH promote the accumulation of SOC and NT.In the soils of the farms W1-W3, D1-D3, and B1-B3, the content available for plants of phosphorus and magnesium in the soil was positively and significantly correlated with soil pH (in the farms E1-E3 only with the content of available magnesium).The beneficial effect of a higher soil pH on the content of available forms of P and Mg is widely known in the literature [41,61].Therefore, in the soils of the farms W1-W3 (with a relatively high soil pH), the obtained relationships confirm the commonly accepted views on the chemistry of these nutrients in the soil.

The Synthetic Index of Soil Fertility (SSFI)
The analysis of the chemical properties of soils collected from six types of farms of different specialization in agricultural production allowed for the determination of the synthetic soil fertility index (SSFI) developed by Ochal [14] and used to assess the quality of agricultural soils by IUNG-PIB in Puławy [13].The results of the SSFI calculation are presented in Table 8.Soil SSFI values ranged widely, i.e., from −1.16 on organic farms to 2.27 on farms specializing in poultry production.Based on the SSFI values of the soil in the farms W1-W3 and D1-D3, they were classified as very high fertility soils.However, the soils from the farms R1-R3, B1-B3, and T1-T3 were classified as soils of medium fertility, and the from organic farms (E1-E3) were of low fertility.While the obtained results regarding soils from organic farms do not raise any doubts, because these soils were excessively acidified and were characterized mainly by a low and very low content of phosphorus, potassium, and magnesium, the quality assessment of the soils in other farms raises certain concerns.On farms whose soils are classified as being of medium or even very high fertility, the indicator values seem overestimated.Since these soils were acidified, it would be difficult to classify them as high or even medium quality soils.Perhaps from the point of view of the production function, these soils (despite their strong acidification) are able to supply plants with nutrients (P, K, and Mg), but their other functions, including the regulatory function, may be disturbed.

Conclusions
Increased concern about global warming has enhanced the interest in understanding the role of mineral and organic fertilizers in SOC accumulation in soils.The obtained results indicate that the tested soils had little potential for carbon sequestration.The SOC content was assessed as low or very low, even in the soils of farms where large doses of organic fertilizers have been used for years.The soils of organic farms were characterized by the lowest content of plant nutrients among all of the farm types examined.Moreover, it was stated that the ability to accumulate phosphorus in the soil from organic fertilizers is lower than from mineral fertilizers.In practice, promoting organic agriculture may gradually reduce the soil's abundance of plant nutrients.This may limit the realization of the production function of these soils.The conducted research suggests that shortening supply chains by locating vegetable farms close to recipients in peri-urban regions may lead to soil contamination with heavy metals.The high content of microelements and toxic heavy metals in the soil of vegetable farms may pose a threat not only to the health of the soil but also to people.The study suggests that individual soil parameters reflect soil fertility and health more accurately than the synthetic soil fertility index, SSFI.It appears that the assessment of soil health requires further research aimed at specifying the method of determining a universal index for assessing the quality of agricultural soils taking into account their various uses.

Figure 1 .
Figure 1.Localization of 18 farms in the Masovian Voivodeship.All farms are represented by green points.

Figure 1 .
Figure 1.Localization of 18 farms in the Masovian Voivodeship.All farms are represented by green points.

Table 1 .
Characteristics of farms where soil samples were collected.

Table 2 .
The average area [ha] and nutrients doses applied in the form of organic and mineral fertilizers in selected types of farms.

Table 3 .
Soil organic content (SOC) in soil samples collected from different types of farms (in %).

Table 3 .
Cont. -an average value for 8 farmlands of each farm; means for individual farms marked with different lowercase letters in the columns differ significantly at p < 0.05; means for farms marked with different uppercase letters in the columns differ significantly at p < 0.05. *

Table 4 .
Average content of total nitrogen (N) and available forms of phosphorus (P), potassium (K), and magnesium (Mg) in the soil from various types of farms.

Table 5 .
The average content of microelements (plant nutrients) and potentially toxic metals (PTM) in soils in individual types of farms.

Table 6 .
Soil pH in different types of farms.

Table 6 .
Cont. -an average value for 8 farmlands of each farm; means for individual farms marked with different lowercase letters in the columns differ significantly at p < 0.05; means for farms marked with different uppercase letters in the columns differ significantly at p < 0.05.-an average value for 8 farmlands of each farm; means for individual farms marked with different lowercase letters in the columns differ significantly at p < 0.05; means for farms marked with different uppercase letters in the columns differ significantly at p < 0.05. **

Table 7 .
Correlation coefficients (r) between soil pH and soil chemical parameters.

Table 7 .
Correlation coefficients (r) between soil pH and soil chemical parameters.