The increase of phosphorus in the minesoil was in line with organic matter content increase.Soil organic matter plays an important role in P sorption as the main constituent of the soil sorption complex which is responsible for binding of anions in the soil material.Phosphorus leaching can be significantly affected by soil adsorption-desorption properties,and it has been shown that application of organic fertilizers could significantly improve the soil phosphorus adsorption capacity.Mengmeng et al.found,that organic fertilization together with mineral NPK increased available phosphorus by 150 % comparing to NPK alone.Application of organic matter is also supposed to improve the soil phosphate availability by promoting microbial activity.The increase in potassium content was slowier than that of phosphorus.During the first three years,this increase was not statistically significant.It was only in 2017 that a statistically significant increase in the potassium content was noted,to the level of 95 mg kg of soil.In the last year of the experiment ,the potassium content reached the value within the norm for this element,ranging from 120 to 200 mg,reaching the value of 175 mg kg of soil.Disturbance to soil by mixing subsoil with topsoi may decrease potassium content.In the studied soil the initial amount of K was very low.On top of that it is known that potassium readily leaches from organic matter as it decomposes.found,that when staw was left on the field,most of its K content was leached during the first month of storage.That can explain the delayed increase of this element level in the course of soil remediation – even though annual potassium fertilization was used.On the other hand hemp is known to take up potassium when the element is not needed and hemp has a lower requirement for potassium than other crops.Most of the absorbed potassium absorbed by Cannabis is concentrated in the stem.
That could explain the finally observed increase of the potassium in the studied soil as this part of the plant is particularly resistant to decay due to the high content of celulose and lignins and could store the absorbed amount of potassium.It is worth noting that the content of phosphorus and potassium reached a value above the lower limit of the norm for these elements only after three and four years of soil rehabilitation respectively,cannabis grow tent despite the annual intensive fertilization with these elements.This confirms the important role of organic matter in the absorption of potassium and phosphorus in the soil.A significant increase was also observed in the case of magnesium content.In the base year,the magnesium content was below normal and amounted to 40.6 mg kg of soil.The level of macro-nutrient gradually increased and in 2017 it amounted to 92 mg / kg of soil,reaching the value of 119.2 in the last year of the study.The standard for the content of magnesium in the soil is from 61 to 100 mg kg.The rate of movement of magnesium in soil depends on soil texture,rainfall and soil pH.It is generally believed that it is easy to migrate in neutral to acidic soil and difficult to migrate in alkaline soil.That suggests that soil conditions in the course of the experiment were in favour to limit leaching of this element and as a result progressive absorption within the organic matter delivered into soil.The manganese content in the tested soil increased in the years 2015–2017 as reclamation was carried out and fell within the norm,which is from 85 to 830 mg.This increase was statistically significant.Also,comparing the base year with 2017,a significant increase in the level of manganese in the studied soil was observed.It has been reported that soil Mn was primarily found in the exchangeable and organic fractions,regardless of soil pH but solubilization of organic acids under alkaline conditions may result in increased complexation of Mn inhibiting its precipitation in the soil environment.It has been also observed that stronger bonding of manganese ions by organic matter was induced by increased soil pH.These previous observations well explain our results showing that Mn content in the soil directly followed changes in the soil pH.The boron content in the soil varied in the studied period.Its level increased significantly from 0.41 mg/kg of soil in 2014 to 0.99 mg/kg of soil in 2017.There was no significant change in the boron level between the starting year 2014 and 2015.Between 2015 and 2016,a significant increase of this micro-nutrient content was observed.Despite the increase,the content of this micro-element was below the standard for Polish soils,which ranges from 2.2 to 7.2 mg kg of the soil.
Although many investigations have been performed with the influence of certain constituents on boron behavior in soil,there is no general agreement on the role they play in B adsorption and desorption.There is general belief that organic matter plays an important role in B adsorption and its availability in the soil.Besides organic matter,also soil pH plays an important role in B behaviour in the soil environment.Gu and Lowe found that B adsorption by humic acids was strongly pH-dependent,being low and relatively constant in the pH range 3.0–6.5,increasing markedly up to a peak at near pH 9.5,and then decreasing at higher pH values.It has been concluded that complexation of B with soil organic matter should only be important at alkaline pH.Our results are in agreement with those previous observations.We have noted the increase of B content in the minesoil together with organic matter accumulation and with soil pH increase.In recent years there has been a development in many areas of plant fibre-based materials ,which has led to an ever-increasing demand for flax scutched fibres,particularly in Europe,which produces 80 % of the world production of flax and hemp.A study conducted by ADEME,the French environment agency,in 2015 predicted that it would be necessary to sow 145,951 ha of fibre crops in France by 2030 in order to meet the demand.This would represent about 1,000,000 tons of straw.In 2018,a total area of only 107,000 ha of textile flax was cultivated in Europe,including 89,000 ha in France.Due to the need for a mild and humid climate and long crop rotations with flax cultivation being repeated on the same land only once every six to seven years to avoid soil depletion and the proliferation of diseases ,the traditional flax production areas are at their maximum production capacity and cannot satisfy an ever-increasing demand for flax fibres.It is therefore necessary to find an additional crop to increase the production of high added value fibres for textile and technical applications to reach the targeted surface of 145 000 ha suggested.In the past,hemp was cultivated for such applications ,in particular for the rigging of sailing ships.A decline in its use during the 20th century led to a sharp decrease in its cultivation worldwide.Hemp cultivation in Europe rose from 15,000 ha in 2013 to 47,000 ha in 2016,of which 16,400 ha in France.A study conducted by FRD and whose results were published in an ADEME report showed that 200,000 ha of hemp is likely to be cultivated in France in the future for different end uses such as fibre,cannabidiol ,shives for building,but this work only investigates the production of long line fibres.
Indeed,unlike flax,hemp is adapted to the climatic and soil conditions of most areas of France and Europe,which allows its establishment over a large geographical area in Europe or in many places in the world such as China.However,the possibility to perform dew retting advantageously is more favourably conducted in mild and humid areas even though it was shown it can be conducted in many different European climates with increased durations for example.The textile flax production zones in Europe where the extraction capacity by scutching/hackling is already present corresponds to the most favourable zone for dew retting of flax and also for hemp.Hemp could also be favourably inserted within flax crop rotations due to its limited fertilizer and pesticide requirements and for its competition against weeds.In the 19th century,harvesting was performed mainly by hand in Europe and in China.In Europe,the first machines to perform fibre extraction using breaking rollers and beaters appeared in 1820.In Eastern Europe,hemp was mainly cultivated for textiles for its long line fibres and countries such as Hungary and Romania developed specific scutching and hackling devices to extract the fibres.These machines,however,required as input,well-retted stems.Water retting was traditionally performed prior to fibre extraction.These processing lines could process whole hemp stalks and very long line scutched fibres could be obtained.However,the resulting fibres were subsequently cut into sections of about 70 cm to be hackled on flax machines.These devices are now very old and have been decommissioned for their dependency on water retting that has been banned in most countries due to its high environmental impact and the risk for humans and animals health.Moreover,the hemp industry in Eastern Europe was labour-intensive,particularly for the harvesting stages and this negatively affects the economic sustainability of traditional value chains.Indeed,the hemp sector has not been able to draw inspiration from the mechanisation of the flax sector and there is currently no complete mechanised hemp harvesting chain for long line hemp fibre production.China,for its part,has invested considerable resources to modernise and recreate an economic sector entirely based on the flax value chain and field retting.This means using flax machinery for the management of the harvesting and fibre extraction.However,this requires that the stem length is shorter than 1 m.In the field a hemp mower,grow lights for cannabis a swath turning machine to homogenise the field retting and an adapted baling system are necessary.
A similar production system proved to be technically feasible in the early years 2000 with the “baby hemp” cultivation in Italy ,where stems were kept short by applying an herbicide when the plant was approximately 120 cm high.In China,manual labour is still used to perform dew retting management and cutting hemp stems in 1 m pieces.If in the past,the numerous attempts to develop hemp harvesters were not completely satisfactory ,suitable hemp mowers are now on the market.However,a machine to cut on the field the mown stems is still not available but this is necessary if flax turning and bailing machinery is to be used.This type of machine is under study and advanced prototypes were tested in summer 2021 with a global success even though improvements need to be completed.With the success of such a prototype,a complete value chain could be created using flax processing lines.Nowadays,hemp fibres for paper pulp,or insulation materials are extracted using hammer mills.This process is very efficient but it damages the fibre and reduces its length.Hemp fibre price,used for technical non-structural automotive applications,is generally much lower than the price of scutched textile flax.However,the mechanical properties of hemp fibres extracted using a hammer mill remain generally low.These fibres cannot be used for load-bearing applications.The possibility to obtain load-bearing grade fibres from hemp would open a complementary market to the one of flax fibres,which is globally saturated and guarantee a higher price than that for the fibres extracted using hammer mills.Ideally,this price should be lower than that of flax long line textile fibres,too expensive for numerous applications in the automotive or other industries.As mentioned above,the old East European hemp scutching and hackling lines are no longer operating,and only flax dedicated equipment are available industrially to extract long line fibres.Preliminary scutching and hackling trials of hemp stems on industrial flax lines were performed by with low scutching yields of long line fibres and high amounts of scutching tows.Vandepitte et al.also used industrial scutching facilities with some of the process parameters changed for hemp extraction purposes with a wide range of European hemp varieties.Higher levels of long fibre scutching yields were globally obtained but this one was dependant on the batches/varieties/levels of dew retting.Following scutching,hackling is generally performed to start the division of technical fibres.During this process tows may also be generated.In hemp stems,the mass of fibres represents,depending on the varieties,about 30–35 % of the mass of the stem.To value the hemp straw and particularly its fibres in the most advantageous way,it is essential to maximise the amount of long line fibres obtained at the end of the extraction process.Main objective of this work is to investigate if hemp could become a source of long line fibre for load bearing composites in complement to the flax ones.To reach this objective,this study proposes to study the long fibre yields obtained at the end of the scutching/hackling process and the quality of fibres that can be obtained.