*3.3. Changes in K2O and P2O5 Content*

Potassium and phosphorus are, besides nitrogen, macronutrients of essential importance in plant nutrition. Phosphorus is one of the compounds building plant cells. Its presence in the soil and absorption by plants determines the absorption of other nutrients, mainly nitrogen. Phosphorus plays an important role in various plant life processes (regulates cell division, root development, flowering processes, seed setting, and maturation processes). The factor that strongly limits phosphorus absorption is the low pH of the soil, and the content of organic matter also plays an important role in this process [35,42,43]. On the other hand, potassium, unlike nitrogen and phosphorus, does not form basic organic substances of the plant. The natural content of potassium in soils depends on their mineralogical structure and granulometry, especially the content of clay minerals, the presence of which is reflected in the share of floated parts in the soil composition. In light soils, the natural potassium content is usually lower than in more compact soils, but with the increase in the share of colloidal parts, the absorption of potassium decreases, because it is strongly bound in the inter-packet spaces of clay minerals. The available forms of potassium are subject to losses due to plant uptake and leaching, especially in light soils. [35,42,43]. Average content of available forms of potassium and phosphorus in relation to the land use type are presented in Figure 8.

In general, it can be stated that in all researched sites, nine cases were observed for phosphorus and six cases for potassium, in which the content of these elements determined in the fallow fields, in relation to arable land, fell below the adopted critical value (10 mg per 100 g of soil). We can conclude that the research hypothesis has been rejected for 15% of cases (α ~ 0.15) and 10% (α ~ 0.1) respectively for each of these elements.

Contrary to carbon content and pH, potassium and phosphorus are the more labile elements in the soil. In this case, the greater variance in the sample set is expected and natural. Despite this feature, only in one case (Table 1: SD of K2O for wooded) the values of the standard deviation exceeded the critical value (10), and in others they are close to half of its value. It should be noted, however, that in the case of potassium, the mean values of its content in fallow soils are most often higher than in arable soils (Figure 8).

#### *3.4. Principal Component Analysis (PCA)*

The analysis showed different relations between individual parameters depending on the land use type. In the case of arable agricultural land (AL), the positively correlated parameters are: Corg content and pH, which are not related to the other group of parameters: content of floatable parts < 0.002 mm and available form of potassium. In case of this group, a negative correlation with the content of available phosphorus can be observed (Figure 9A).

**Figure 9.** Variables charts showing relationships between different parameters depending on landuse types. Subfigures: (**A**)—arable land, (**B**)—grassland, (**C**)—grassland fallow, (**D**)—goldenrod fallow, (**E**)—bushy fallow, (**F**)—wooded/afforested.

In the case of grassland (GL), the variables form two groups. The first group of correlated parameters is pH, the content of potassium and, to a lesser extent, of phosphorus. The second group, on the other hand, is the Corg content and the share of fraction 0.05– 0.002 and < 0.002 (Figure 9B). A similar correlation between the Corg content and the granulometric composition can be observed in bushy fallow land—FB (Figure 9E), where we can also see a relationship between whose parameters and the content of potassium.

Grassland fallow (FGL) shows correlation between the content of Corg and pH, which demonstrates in high values of those parameters compared to the other groups (Figure 9C), as it is in the case of grassland, where the content of potassium and phosphorus is negatively correlated with the content of floatable parts < 0.002 and fraction 0.05–0.002.

Moreover, in this group it can be observed that the first two principal components (PC1 and PC2) explain the largest percentage of the total variation, respectively (58.1% and 41.9%). Goldenrod fallow (FG) shows some similarities in relation of some factors with regard to arable land (AL), and afforested fallow areas (FW), applying mainly to potassium, which depends on the content of fraction < 0.002 and fraction 0.05–0.002. The content of Corg, on the other hand, is not correlated with any other parameter (Figure 9D).

The last group, which represents advanced succession with trees (FW), shows correlation between the content of Corg and phosphorus, which at the same time are negatively correlated with pH (Figure 9F). In this case, the higher content of Corg the lower pH value (Figure S1).

#### **4. Discussion**

#### *4.1. Impact of Fallowing on Soil*

The obtained results generally show that fallowing on weak soils does not lead to such deterioration of chemical properties, which would make it difficult to restore plant production. In the case of fallowing agricultural land as grassland—without allowing secondary succession, even an improvement in the condition of the soil can be observed, i.e., no tendency to soil acidification, high humus and potassium content, which confirms the results published by Kazlauskaite-Jadzevice et al. [21]. Similar consistency with the results described by Foote and Grogan [22] was obtained for the assessment of the impact of mature succession (afforestation), but in this case, the effect of carbon sequestration was not so noticeable, which could have been influenced by the selection of sites for testing (poor and very poor soils).

Another important research result is the negative impact of goldenrod (*Solidago* L.) succession. This invasive species is now common in the agricultural landscape of the country and, as shown in the research of Orczewska [12], Sekutowski and his team [13], as well as this work—it has a negative effect on soil and environmental conditions. For this reason, one of the recommendations for maintaining the soil in good condition should be the requirement to mow fallow land with this type of succession.

The works carried out by the teams of Stolarski [15] and Matyka [10,18] prove that plantations of perennial energy crops can be established even on the weakest soils, classified as marginal soils. This work proves that setting aside agricultural land with poor soils is not an obstacle in restoring it to the production of this type of biomass. This applies in particular to the goldenrod succession sites—in such case, conversion may also contribute to a more sustainable use of agricultural production space.

#### *4.2. Possibility of Returning Fallow Land to Agriculture*

Michna et al. [1] wrote about the purposefulness of restoring fallow land to agricultural production. To the conclusion that: "Land of individual farmers may be transformed or used for other purposes only with the consent of the owners. Thus, there is the ownership barrier to transformation and changes in the forms of land use of all, including fallow, soils", we can now add the fact that over the three decades of intensified changes in agricultural production in Poland, which unfortunately show the constantly growing trend of abandoning agricultural land, not much intervention by the administration has taken place. Also, the expected changes after Poland's accession to the EU structures in 2004 did not result in any visible process of returning fallow land to production, but only slowed down the trend of land abandonment [5].

Moreover, the subsequent solutions in the field of bioeconomy, promoting an increase in the share of biomass in the energy mix, did not change the observed situation. Although the RED Directive [44] obligated EU countries to increase the sustainable use of biomass, it was not reflected in the relevant national regulations, which would provide tangible support to farmers and target recipients of the biomass produced [45]. This resulted in

a lack of response from industry and energy, which could lead to the creation of a stable biomass market and allow the recovery of fallow land for the production of this raw material, or, alternatively, the resumption of food production.

The new concept of the Green Deal is another attempt to draw attention to the sustainable use of biomass in agriculture and bioeconomy [46]. These activities once again stimulate the interest of the energy and industry sectors in biomass of agricultural origin. This was manifested by the need to estimate the raw material potentials reported to the Ministry of Agriculture and Rural Development and the need to regulate the possibility of non-agricultural use of biomass along with the possibility of reusing waste from biomass processing as fertilizer for soil conservation. The solutions can directly contribute to the greater use of straw, hay, and manure in the bioeconomy, but they can also have a significant impact on the consolidation of fallow land and their recovery to biomass production.

In the BioMagic project, which financed this work, the presented results were used to estimate the theoretical, technical, and economic potentials of producing perennial industrial plants on fallow soils. In subsequent works carried out on this subject, a remote sensing tool will be developed for the remote sensing recognition of the degree of natural succession, and the density and type of biomass on the analyzed land plot.

#### **5. Conclusions**

Fallow land in Poland constitutes a significant percentage of agricultural land. Most of it has great potential to be restored for food or biomass production for bioeconomy purposes—such as providing raw materials for the chemical and pharmaceutical industries and energy. The observed effects of fallowing are not an agro-technical problem in this case. Based on the obtained results, it can be concluded that:


**Supplementary Materials:** The following are available online at https://www.mdpi.com/2077-047 2/11/2/148/s1, Figure S1: A ternary diagram of the soil texture triangle showing the USDA-based soil texture classifications [S1]., Figure S2: Primary results characterizing each tested site.

**Author Contributions:** Data preparation, M.K.; calculations and data analysis, M.K.; discussion of the results, M.K. and R.P.; writing of the paper, M.K. and R.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This paper is the result of a study carried out at the Institute of Soil Science and Plant Cultivation—State Research Institute, Department of Bioeconomy and Systems Analysis, and it was financed by the National (Polish) Centre for Research and Development (NCBiR), entitled "Environment, agriculture and forestry", project: BIOproducts from lignocellulosic biomass derived from MArginal land to fill the Gap In Current national bioeconomy, No. BIOSTRATEG3/344253/2/NCBR/2017.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We would like to thank Małgorzata Wydra for her help with editing the English manuscript of this paper.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**

