control mechanisms of clays and their specific surface

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Control mechanisms of clays and their specificsurface area in growing media – assessmentof clay properties and their parametrization forthe optimization of plant quality

SummaryThe addition of clay to growing media aims atconstant supply of potassium, phosphorus andmicro nutrients, pH-buffering, improvement ofrewettability, and cohesion of growing media.The identification of suitable clays and theirclassification is a prerequisite for product andcultivation safety. A range of different clays hig -hly variable in their mineral parameters wereselected for experiments on nutrient buffering(P) and Mn toxicity, their ability to improve therewettability and binding capacity of growingmedia. From batch experiments and growthtrials it was derived that a threshold value forthe sum of exchangeable and easy reducibleMn in clays for growing media is not justified,as even very high Mn contents in clay were notphytotoxic. The P binding capacity of clays wasstrongly correlated with the oxalate extractableFe and Al content. A newly developed capilla-ry rise method (WOK) was used to characteri-ze the rewettability of growing media. Thespeed of rewetting mainly depends on thefineness of the amended clay. Surface freeenergy (SFE) data of the growing media indica-

te that those with a good rewettability showalso high values for SFE. Compared to the ka o -linitic and illitic clay amendments, bentonitesshow no significant increase in the SFE. Sur -faces of clay minerals exhibiting originally polarand hydrophilic surfaces, can render hydro-phobic when coated with weakly or non-polarorganic matter moieties. Dissolved organic car-bon (DOC) sorption was found to be positivelycorrelated with the specific surface area (SSA),cation exchange capacity (CEC) and amount ofdithionite extractable Al and Fe. Clays contai-ning expandable clay minerals with high CECand SSA (e.g. smectites) and those rich in Al-and Fe-oxides seem to be less effective forimproving rewettability, whereas an additionof non-expandable clays with lower SSA, CEC(e. g. kaolinitic and illitic clays) and amorphousoxide content appears more promising. Newinsights on the adsorption of DOM on claymineral`s surfaces will be obtained in the thirdyear by chemical analysis of the surface with X-ray photoelectron spectroscopy (XPS), surfacetopography analysis and contact angle measu-rements. A new method for the determination

Dultz S. (1)*, Schellhorn M. (2), Schmilewski G. (3), Schenk M.K. (4), Dombrowski, I., (4), Schmidt, E. (2),

Walsch, J. (1), Below, M. (1)

(1) Institute of Soil Science, Leibniz University Hanover, e-mail: [email protected],

[email protected]

(2) Stephan Schmidt KG, Langendernbach, e-mail: [email protected],

[email protected]

(3) Klasmann-Deilmann GmbH, Geeste-Groß Hesepe, e-mail: [email protected]

(4) Institute of Plant Nutrition, Leibniz University Hanover, e-mail: [email protected],

[email protected]

*Coordinator of the project

SR16_Mineraloberfl_V8.qxp:Abstract_InfosysII.qxd 08.10.10 13:25 Seite 140

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of the binding capacity of clays in blockingmedia is currently being tested and will bestudied together with determinations on K-dynamics.

Introduction

Concept on function und effect of clays ingrowing media related to practiceA suitable rooting medium is fundamental forplant growth. Horticultural crops have certainrequirements which the grower needs to fulfilwith the help of individually tailored growingtechniques and cultivation measures. For theselection of growing media constituents eachshould possess optimum characteristics for thespecific culture. Modern horticulture withcomputer-controlled irrigation and fertilisationprogrammes, potting machines, pricking ro -bots, climate-controlled greenhouses and just- in-time production requires dependable, quali-ty-assured growing media. Specialist compa-nies rely on ready-made growing media whichare either part of the manufacturer's standardrange or special mixtures produced at the gro-wer's request. For the development of formu-lations and the production of growing mediasuitable for this market a large number of che-mical, physical, biological and economic cha-racteristics of the constituents must be takeninto account (Table 1).

Due to its characteristics Sphagnum peat hasbeen the most important constituent of orga-nic growing media for several decades. A highwater and air capacity in combination withother favourable features make peat ideal forthis purpose. After fertilising and liming, peatis the sole constituent of many growing media.Nonetheless, peat has also some drawbacks

arising from the relatively poor wettabilitywhen dry, limited nutrient buffering capacity,and structural stability. Mineral amendments,especially those rich in clays, are commonlyincorporated into the organic media in orderto improve physical and chemical conditions. In horticultural practice, clays are amended topeat in amounts of 10-80 kg/m³. The amend-ment aims mainly to improve the followingthree different properties of the growingmedia:1. Buffering of nutrients. Clay amendment

shall ensure that optimum conditions adju-sted at start of the crop remain constantduring cultivation. Relative large amounts ofclay are needed.

2. Rewettability during irrigation. Due to thelarge specific surface area and charged surfa-ce sites, hydrophilic mineral surfaces of clayminerals coating the surfaces of peat shouldhave a positive effect on the wettability ofinherently water repellent growing mediaand the amount of plant available water.Relative low amounts of clay are needed.

3. Binding capacity. For the use of prickingrobots when transplanting young plants andalso for the production of peat blocks (e.g.4 x 4 x 4 cm) for mechanical sowing the co -hesion of the growing media shall be impro-ved by the amendment of a suitable clay.Relative low amounts of clay are needed.

As a certain clay can hardly support all of therequired properties, also blends of differentclays are used in growing media industry. Claysavailable show great differences in their mine-ral parameters, e. g. texture, mineralogical com -position, layer charge, and oxide content.Chemical as well as physical characteristics ofthe growing media strongly depend on thekind of clay amendment. Currently no proven

Table 1: Properties of growing media and their constituents that pertain to »quality«(adapted from Schmilewski, 2008).


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standards are available for evaluation of clayfor use in growing media and to design gro-wing media perfectly for specific crops andproduction methods. Here improved knowled-ge of the mechanisms and processes of theseclays in growing media helps to identify suita-ble clays and to make the prognosis of clayproperties more reliable.

Surface chemistry of growing media con-stituentsThe amendment of clays or clay blends to peattogether with the addition of fertilizers andlime in a growing medium has a strong effecton the surface chemistry of its constituents.Clay and peat stem from very different envi-ronments and a distinct number of exchangereactions can be assumed. E. g. the clays usedin this project from Mesozoic-Tertiary weathe-ring mantle of the Rhenish Massif are undersa-turated for the binding of dissolved organicmatter (DOM), whereas peat is known to be asource for DOM. For this reason the externalsurfaces of clay minerals might act as a sink forDOM, whereby in turn the adsorption of DOMcan modify the surface properties of the clayminerals. Here the interaction of water withpeat and clay minerals defines the physical andchemical properties of the growing medium,as virtually all chemical and biological reactionstake place at the solid-water interface. A sche-

me of the interactions of water with the gro-wing media constituents is shown in Fig. 1.

The solid-water interface is important becauseit controls the retention and transport ofnutrients and pollutants and provides physicalsupport for plants. Also contained in the poresolution besides inorganic solutes is DOM, itsfate and transport highly depending on interac-tions of the solid-water interface. On addition,many chemical species in the growing mediasolution can interact with each other. Solutebehaviour is controlled by multiple solid phases,different clay minerals, oxides and organic mat-ter, which have a complex and heterogeneousnature. The type of surface reaction has a deci-sive role for the potential of the solid phase forthe desired properties listed above.

Water repellent surfaces of growingmediaWater repellent (hydrophobic) growing mediacan cause problems as, after drying out, theyrequire a long time to rewet (Michel et al.,2001). For that reason the rewettability couldbecome a property with major repercussionsfor plant growth (Doerr et al., 2000). Organicmaterial in peats used for growing media com-prises many substances which are only weaklyor non-polar. As a consequence they are notable to form hydrogen bonds with water mole-

Figure 1: Scheme for the interactions growingmedia constituents withsolution.


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cules and hence exhibit often distinct waterrepellence (hydrophobicity). On the otherhand, charged mineral surfaces are usuallyhydrophilic as they are capable of forminghydrogen-bonds. Thus, when incorporatedinto a water repellent sandy soil or a growingmedium, it is expected that charged clay mine-rals show a positive influence on the wettabili-ty, by increasing the surface area, maskinghydrophobic organic surfaces and exposinghydrophilic clay surfaces (McKissock et al., 2002;Lichner et al., 2006). The effect of mineralamend ments on the wettability of soils wassubject to several studies. Nevertheless, theresults were somewhat inconsistent. Indicationwas obtained that the variations observeddepend to a great extent on the formation oforganic coatings on mineral surfaces. Theadsorption of weakly or non-polar organicmatter moieties can render a hydrophilic mine-ral surface hydrophobic (Chenu et al., 2000;Doerr et al., 2000). The hydrophilic reactivity ofmineral surfaces and accordingly their positiveinfluence on wettability can be reduced oreven lost, when the minerals tend to accumu-late hydrophobic organic compounds derivedfrom soil organic matter or peat on their exter-nal surfaces. For soils sorption of dissolved organic matteronto mineral surfaces was found to be affec-ted by the parameters CEC, SSA and contentof amorphous oxides (e. g. Kaiser and Gug gen -berger, 2000; Kahle et al., 2004). The stabilityof organic coatings depends on the type ofbond, which is not only dependent on the che-mical composition of DOM, but also on mineralsurface properties. Basically it has to be assu-med that wettability is largely influenced by thesorption of DOM and, as the wettability is adynamic property, also desorption processes. In the study on rewettability of growing media,the effect of certain clay amendments, highlydifferent in their mineral parameters (mineralo-gical composition, specific surface area, oxala-te and dithionite soluble Fe- and Al-oxides, tex-ture) was determined. Processes and reactionsresponsible for the variations in growing mediawettability are assessed and used to rate theeffectiveness of the amendment. The results

will be used to find reliable criteria for the selec-tion of suitable clays for improving wettability.

Clays for improving P buffering capacityThe amendment of clay to peat-based growingmedia is supposed to improve both physicaland chemical properties of growing media.Pure white peat is nearly non-buffered and thebuffering capacity for the nutrients potassiumand phosphorus can be increased by the addi-tion of clay. A buffered growing media canabsorb nutrients in large amounts by bindingthem and is able to release them again in thesolution of the growing media when plantsdeplete the nutrient concentration. By this,fluctuations of plant demand or fertigation canbe balanced and optimal growing conditionscan be maintained. The buffering capacity of a clay in growingmedia is mainly caused by its contents ofamorphous Fe- and Al-oxides. Therefore P fixa-tion varies widely between clay minerals andthe choice of the »right« clay is very important.In horticultural practice the basic fertilizationof a growing media is generally not sufficientto supply the plants over the whole vegetationperiod with nutrients and subsequent fertiga-tion is necessary. The nutrient concentration inthe fertigation solution should reflect thenutrient demand of plants. Growth rate andnutrient uptake of plants differs widely bet-ween species and cultivars and is dependenton environmental conditions. Therefore opti-mum fertigation concentration can be deter-mined only with uncertainties. Thus, i) clay characteristics affecting P buffe-ring capacity were determined and ii) the in -fluence of the buffering capacity of peat/clay-growing media on the safety of plant cultiva-tion at varied concentrations of P fertigationwas investigated.

Progress within the projectThe work in the first two years focused on thetwo sections »Buffering of nutrients« (1) and»Rewettability during irrigation« (2). In prelimi-nary studies in the section »Binding capacity«(3) different methods were tried out and ex -tensive experiments on blocked growing me -


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dia by mechanical agitation will be carried outin the third year.Determinations on Manganese toxicity (section1) on clays with different contents of active Mn(sum of exchangeable and easy reducable Mn)reveal that a threshold value for Mn in clays forgrowing media is not justified (Dombrowskiand Schenk, 2010). Even very high Mn con-tents in clays only resulted in small increase ofMn concentration in the solution of the gro-wing medium. Additionally plants toleratedhigh Mn-concentrations in the growing mediasolution because, i) Mn was complexed by DOMand thus is less phytotoxic ii) silicic acid dissolvedin growing media solution alleviated the harm-ful Mn effects. Results on the P binding capaci-ty of clays, strongly correlated with the oxalateextractable Fe and Al content of clays, are des-cribed in detail in the result section.For the determination of the rewettability ofgrowing media the capillary rise method wasused. For this purpose a WOK-apparatus allo-wing 8 simultaneous measurements was in -stal led at Klasmann-Deilmann GmbH. The ef -fectiveness in improving the wettability of gro-wing media was found to depend on the sur-face properties of the clay minerals amendedwhich govern the binding of DOM. Amend -ments with non-expandable clay minerals withlow SSA, CEC (e.g. kaolinite and illite) and clayswith a low amorphous oxide content appearmost promising (Walsch and Dultz, 2010). Thefineness of the clay was observed to be a deci-sive factor for improving the rewettability.

Material and Methods

RewettabilitySeven clay samples with distinct differences inmineral parameters (Tab. 1) and a blend of dif-ferent Sphagnum peats with a moderatedegree of decomposition were used.

The growing media was limed (6.0 kg/m3) andfertilized (1.1 kg NPK 14-16-18 standard ferti-lizer per m3 growing media. The rewettabilityof the growing media mixtures was assessed interms of capillary water uptake (WOK, DutchRHP foundation) and contact angle (CA) mea-surements (OCA 20, Data Physics). In the WOKapparatus dry samples are placed in cylindricalrings (500 cm3) on a layer of water and theincrease in water content is logged over timefor 24 h (Fig. 2).

Hydration of particles in the growing mediawas determined in an environmental scanningelectron microscope (ESEM; Quanta 200, Fei).As peat surfaces consisting mainly of C arecovered with clay minerals rich in Si, thedegree of coverage of the peat surface can bedescribed by the C/Si ratio. By energy dispersi-ve X-ray spectroscopy five different sectionseach of 6 mm² size were determined.The adsorption of peat derived DOM on mine-ral surfaces was determined in batch adsorp-tion experiments. DOM solutions for adsorptionexperiments were extracted from limed and fer-tilized peat. Batch adsorption experiments werecarried out for 24 h at 20 °C in the dark. DOCconcentrations were measured with a totalorganic carbon analyser (liquiTOC, elementar).Aromaticity of the adsorbed DOC and DOM-fractionation phenomena due to adsorption

Table 1: Physical, chemical and mineralogical properties of seven clay samples from the Westerwald area, representingsaprolithic clay (S), bentonite (B), translocated clays (U) and blends (M).


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were determined with a Cary 50 UV-Vis spec-trometer (Varian). According to Chin et al. (1994)absorbance values were recorded at a wave-length of 280 nm, where electron transitionsoccur for a number of aromatic substances.

P buffering capacityThe influence of P buffering of peat/clay gro-wing media on the safety of plant cultivationwas characterized in a plant experiment. Atfirst nine clays with different P fixation capaci-ty were selected for the determination of plantavailability of P in peat/clay growing media. Toassess the P fixation clays and growing mediawere shaken in a P solution (1000 mg P L-1) ata ratio of 1:10 for 24h. Oxalate soluble Fe andAl were extracted using 0.2 M oxalate solution.Based on these data two clays with a low P fixa-tion (04S and 01S) and two with a high P fixa-tion (06B and 39W) were selected for the plantexperiment and mixed with peat (80 vol-%peat, 20 vol-% clay). For each growing media a calibration curvewas established to determine the amount of Pneeded to obtain a CAT extractable P concen-tration of 25 mg P/L growing media, which isusually adjusted in horticultural practice. CAT(0.01 M CaCl2 + 0.002 M DTPA) is commonlyused for the extraction of potentially plantavailable P in horticultural growing media (Alt& Peters, 1992). The growing media wereequilibrated in an oven at 50°C for 48 h, thenat room temperature for another 48 h prior todetermining the CAT extractable P concentra-tion. Previous work showed that this incuba-

tion procedure is highly correlated with CATsoluble P after 9 weeks of storage. Seedlings of Impatiens walleriana F1 »Candy ®

Coral Bee« were planted in 600 ml plastic pots( 12 cm) filled with the different growing me -dia and pure white peat as control. Volumeweight of growing media was determinedaccording to standard method of VDLUFA(1991). All growing media were fertilized with1.5 g L-1 of a P free compound fertilizer (Ferti8® – N : P2O5 : K2O = 20 : 0 : 16 + micronu-trients) and pH was adjusted with CaCO3 topH 6 (0.01 M CaCl2). The CAT soluble P con-tent of 25 mg P/L growing media was obtainedby addition of Ca(H2PO4)2 according to thecalibration curves. The fertigation solution con-tained in mg/L solution: 120 N as KNO3 andNH4NO3, 130 K as KNO3, K2SO4 and KH2PO4,10 Mg as MgSO4*7 H2O and 200 Flori® 10.The P concentration of the fertigation solutionwas varied (in mg P/L solution): 0 (= no P), 17(= sufficient P) and 35 (= excess P). Plants wereharvested after 10 weeks cultivation after mea -suring plant height and diameter and length ofinternodes. The P concentration in plant d.m.was measured after wet digestion and the Pconcentration in the solutions was determinedwith the ammonium-vanadate-molybdate me -thod. Treatments were replicated five times(each replicate consisted of 5 plants) in a com-pletely randomized design and statistical ana-lysis was performed with the program R 2.8.1.Means were compared between treatments at = 0.05 using Tukey-Test.

Figure 2: WOK apparatusused for the determinationof the time dependentwater uptake capacity.


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Results

Rewettability

ESEM observationsGrowing media submitted to hydration showclearly that the condensation of water startson the hydrophilic clay particles coating theexternal surfaces of the peat compounds (Fig.3a, b). The formation of water drops on thesurface of peat indicates high contact angelsand strongly hydrophobic properties.

Saprolithic and translocated clays have thestrong est effect on surface coverage of peatcompounds with clay minerals (Fig. 4). Herealready 20 kg/m³ are sufficient to reach lowestC/Si ratios indicating maximum coveragedegree. Bentonites and clay blends still increa-se the coverage degree of the peat`s surface bythe addition of 30 kg/m³. Bentonites showsmall effects even at the largest amendedamount. Here C/Si-ratios are comparable withgrowing media amended with saprolithic andtranslocated clays in amounts of 10 kg/m³.

Water uptake is improved for all samples bythe amendment of clay (Fig. 5), strongly de -pen ding on the clay parameters. Clays consi-sting mainly of illite and kaolinite (saprolithicand translocated clays) show fastest wateruptake (50 vol.% within 10 min).

At C/Si ratios <20, where the surfaces of peatare most completely coated with clay minerals,all clay-peat systems show the highest wateruptake rate. Highest variability is observed for asaprolitic clay rich in illite and kaolinite, wherethe water uptake rate ranges from 2.6 to 15.5(%v/v)/min. Bentonites show only minor effects,water uptake ranges from 1.47 to 3.63 (%v/v)/min. The translocated clay is most effective toimprove water uptake rate with small amen-ded amounts. Larger added amounts showonly slight improvements. For the clay blendand bentonite, the absolute amount of amen-ded clay showed minor effects, but wateruptake rate is quite higher than that of originalpeat (1.3 (%v/v)/min). The results show a good

Figure 3a, b: Environmental scanning electron microscope(ESEM) observation of a peat-based growing media amen -ded with a translocated clay (27 U) submitted to hydra-tion conditions. The image shows a leave of Sphagnummoss peat covered with clay platelets in the dry state (a)and directly after beginning of condensation (b). Notethat condensation starts at sites where clay minerals coatthe peat´s surface. Water droplets are marked with arrows.

Figure 4: Coverage degree of peat surfaces with clayminerals expressed by the C/Si ratio (n=5). Low valuesindicate highly covered surfaces.


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correlation of the surface coverage degree ex -pressed by the C/Si ratio and rewettability deter-mined by the capillary rise »WOK« method. Thedetermination of the C/Si ratio in a relativelyshort procedure is a suitable method to identifysuitable clays and the amount of clay needed.

Effect of clay particle size on water uptakecharacteristics Time dependent water uptake measurementson growing media with clays of different parti-cle size fractions reveal that the initial particlesize of the amended clay has a strong influen-ce on the water uptake rate. For fine aggrega-te sizes <0.5 mm the fastest water uptake wasobserved (Fig. 6). Note that in horticulturalpractice irrigation is often performed in theebb and flow mode. If the uptake rate is highthe time needed for irrigation can be shorte-ned. After the total duration of the experimentof 1440 min (24 h) in the WOK apparatus thedifferences in water uptake rate are low. This isnot relelevant for horticultural practice, wherethe differences at the beginning of irrigationare most important. Here the growing mediawhere the smallest aggregate size (< 0,5 mm)and the highest amount of amended clay (20kg/m³) shows the fastest water uptake rate inthe time period up to 15 min.

Effect of different clays on water uptake cha-racteristics From the water uptake curves, it becomes evi-dent that the growing media differ significant-ly in their rewetting behaviour (Fig. 7). Purepeat showed the slowest water uptake, whe-reas the increase in the volumetric water con-tent of mineral amended growing media wasgenerally faster. Up to 50% water saturation,also marked variations in the rewetting beha-viour among the different mineral amend-ments were observed. Amendments of smecti-te rich clays (06B and 07B) caused only a minorincrease in water uptake velocity, while kaolini-tic and illitic mineral amendments notablyenhanced the rewetting. It can be concludedthat the wettability of a growing media is adynamic property, which is to a great extentinfluenced by the clay mineralogy and alsoother mineral parameters.

Formation of hydrophobic coatings onclay mineral surfacesMineral surfaces provide a variety of reactivefunctional groups on their surface, whichallows strong interaction with dissolved orga-nic and inorganic substances. Consequently,batch adsorption experiments with peat deri-ved DOM revealed that both, the bulk samples

Figure 5: Effect of four different clays on the water upta-ke rate. Growing media are amended in amounts of 10,20 and 30 kg/m³ clay. Water uptake rate is determined bythe time (min) needed to 50% saturation. The dashedline is the 95% confidence interval of the linear regres-sion (solid line).

Figure 6: Time dependent water uptake of a growing me -dia (fineness of peat: 0-10 mm) to which translocated clay36M was added in two different amounts (10 and 20 kg/m³)und two particle size fractions (<0.5 and 0.5-2.0 mm).


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as well as the clay fractions adsorb relativelyhigh amounts of dissolved organic substances(Fig. 8). For the samples under investigation,the adsorbed C content varies between 20 and180 mmol C kg-1, with the highest amount be -ing adsorbed by the samples rich in smectite(06B and 07B).

The composition of the organic coatings onthe mineral phase was determined by UV-Visanalyses of the equilibrium solutions after thebatch adsorption experiments (Fig. 9). Here, adecrease in light absorbance values (Abs280)indicates a high removal of aromatic organicsubstances from the solution due to the sorp-tion onto mineral surfaces.The extent of sorption was found to be depen-dent on the mineral parameters of the samples(Fig. 9, a, b, c). Linear regression analysis sho-wed that sorption of aromatic compounds cor-relates well with the SSA, CEC and amount ofoxalate soluble Al and Fe, which is consistentwith findings reported elsewhere (e.g. Kaiserand Guggenberger, 2000; Kahle et al., 2004).Absorption at 280 nm shows strongest corre-lation to the SSA (r2 = 0.86), which is most like-ly due to the higher surface area available for

DOC sorption. Considering liming and fertiliza-tion, it can be suggested that DOM sorption ismost likely also mediated by the solution che-mistry. For instance Feng et al. (2005) foundthat sorption of peat derived humic acid ontokaolinite and montmorillonite increased withincreasing ionic strength and decreasing pH,and that the presence of Ca2+ largely enhan-ced the sorption. Subsequently, DOC linkageto negatively charged mineral surfaces viacation-bridges appears to be an important bin-ding mechanism, which is also indicated by thepositive correlation (r2 = 0.66) between adsor-bed DOC and the CEC of the clay amendment.The correlation with the sum of oxalate extrac-table Al and Fe (r2 = 0.71) is indicative for DOCbinding onto poorly crystalline Fe and Al mine-rals (Mikutta et al., 2005). It can be assumedthat the amount and composition of adsorbedDOM as well as the mineral parameters are thekey to explain the observed variations in wet-tability and accordingly the differences in theeffectiveness of the mineral amendments.

Effect of hydrophobic mineral coatings onwater uptake rate Adsorption of peat derived DOM onto mineral

Figure 7: Time dependent water uptake of peat-based growing media amended with differentclays in an amount of 30 kg/m3. For comparison, the water uptake of the pure peat is shown.


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surfaces influences the wettability of the gro-wing media in a negative way (Fig. 10). Fromthe non-linear regression it becomes evidentthat variations in wettability of the growingmedia can be attributed to a high extent todifferences in the affinities of the mineral sur-faces for aromatic DOM compounds. Mostprobably the hydrophilic behaviour of themineral surfaces is affected by the adsorbedorganic compounds forming hydrophobicsurface coatings, diminishing their positiveeffect on the wettability.

Conclusions on rewettabilityIt was shown that the absorption of aromaticorganic compounds changes the hydrophilicityof clay mineral surfaces. The intermolecular in -ter actions involved in the formation of hydro-phobic coatings can be very complex. However,sig nificant effects can be expected if specificpro perties of the dissolved organic mat ter isvaried (molecular size, configuration and pola-rity or hydrophobicity) and also specific surfaceproperties of the adsorbents (texture, minera-logical composition and the distribution ofreactive functional groups). Additio na lly, theproperties of the organic and inorganic reac-tants can be affected by aqueous phase proces-ses (e. g. hydrolysis, hydration, dissolution, andprecipitation). Here the composition and pro -perties of the aqueous solution (e. g. pH, ionicstrength, presence of bridging-ca t ions or com-petitive anions) have to be considered as well.

For instance, Tarchitzky et al. (2000) explainedthe role of polyvalent cations for water repel-lency with the enhanced formation of coatingson mineral surfaces, resulting in a reduction ofthe hydrophilic reactivity. Additionally it has

Figure 9: UV-Vis light absorbance values of the equili-brium solutions after batch adsorption experiment. Note,a decrease in the Abs280-values indicates a high removalof peat derived aromatic organic substances due to sorp-tion onto mineral surfaces. Abs280-values are shown inrelation to mineral properties a) CEC, b) SSA and c) oxala-te soluble Fe and Al. The dashed line is the 95% confi-dence interval of the linear regression (solid line).

Figure 8: Sorption of peat derived DOM on bulk clay sam-ples and the clay fractions.


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been suggested by Hurraß and Schaumann(2006) that bridges by polyvalent cations maycause an aggregation of the humic substancestht would also result in a lower wettability.

The results of our study show that the obser-ved differences in rewettability can be mainlyattributed to the extent and kind of organiccoatings on the mineral surfaces. Obviouslythe formation of these coatings relies on diffe-rences in the sorption properties of the mine-rals and, in turn on distinct mineral properties.Mineral amendments containing expandableclay minerals with high CEC and SSA (e. g.smectites) and amendments rich in amorphousFe and Al compounds seem to be less effecti-ve, due to their high affinity for peat derivedDOM, especially for hydrophobic DOM consti-tuents (Fig. 11). Accordingly, an addition ofnon-expandable clays with lower SSA, CECand amorphous oxide content (e.g. kaoliniteand illite) seems more promising.

P buffering capacityThe nine selected clays differed clearly in theirP binding capacity and content of ∑Feox+Alox

(Fig. 12). The P binding of the clays correlatedpositively with the ∑Feox+Alox, but could notfully explain the whole variation in P fixation. Itis assumed that the surface of the oxides dif-fers widely, which will be investigated in furt-her experiments.

After ten weeks cultivation the varied P ferti-gation resulted in clear differences in plantgrowth and quality among the treatments andgrowing media. Without P fertigation the elongation of shootswas reduced in pure white peat and the twoclays with low P binding capacity 04S and 01S,compared to peat/clay blends containing clay06B and 39W. Also P fertigation increasedinternode length in these blends (Fig. 13). Thesame trend was observed in plant diameterand height (data not shown).

Figure 10: Water uptake of growing media amended with different clays in relation to the adsorption of aro-matic compounds from peat derived DOM. The measure for the absorption of aromatic organic compoundsfrom the equilibrium solution is the absorbance at 280 nm.


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In all growing media the dry matter yield of theplants not fertigated with P was reduced com-pared to those receiving P. Without P fertiga-tion yield of treatments in pure white peat andin the peat /clay blend 04S was lower than inthe other three peat/clay mixtures (Fig. 14).

The increasing P supply via fertigation resultedin increased P conc. in plant d.m. in all growingmedia. Without P fertigation, P concentration inshoot d.m. was the lowest in treatment purewhite peat and the peat/clay blend 04S and thehighest P concentration was obtained in the tre-atment 06B. The same differences between gro-wing media were observed with fertigation con-centration 17 mg P L-1 solution, but no longer atthe highest P fertigation level (Fig. 15).

Figure 11: Water uptake of peat-based growing media mix -tures amended with different clays with an amount of 30kg m-3 and its relation to mineral properties a) CEC, b) SSAand c) oxalate soluble Fe and Al. Dashed line is the 95%confidence interval of the linear regression (solid line).

Figure 12: Relationship between the content of oxalatesoluble Fe and Al and the P binding of nine different claysin a batch experiment with a P supply of 10000 mg kg-1.Marked clays were used in the plant experiment.

Figure 13: Length of internodes of Impatiens wallerianadepending on growing media type and P conc. in fertiga-tion solution. Different capital letters indicate significantdifferences between growing media at the same P ferti-gation level and different small letters indicate significantdifferences between fertigation treatments (p<0.05).


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To characterize the capacity of different claysto supply P to plants the P amount containedin shoot d.m. was calculated for the treatmentwithout P fertigation (Fig. 16). The clays 01S,06B and 39W supplied about twice as much Pto plants compared to pure peat and peat/clayblend 04S. In the latter treatments plants didnot exhaust the P amount determined by CATextraction whereas in the other treatmentseven more P was taken up than characterizedas plant available by the extraction. In all gro-wing media more P was fertilized than deter-mined as plant available or taken up in shoot

d.m., which was more pronounced in blendswith clays having a high binding capacity (01S,06B and 39W). At the same CAT level, in thiscase 25 mg/L growing medium, peat/clayblends containing clay with a high P bindingcapacity supplied more P to plants indicating thecapacity to buffer fluctuating P supply. In horti-cultural practice P availability in growing mediais characterized by CAT extraction procedure.

Conclusions on P buffering capacityThe P binding capacity of clays was stronglycorrelated with the oxalate extractable Fe andAl content. However, this factor did not fullyexplain the observed variability. Low availabilityof P reduced internode length and supportedcompaction of plants. But at the same timed.m. production was decreased indicating thatshortage of P is not a successful strategy to pro-duce more compact and better shaped plants.Peat-clay blends containing clay with a high Pbinding capacity provided up to twice as muchP to plants than those with a lower bindingcapacity, thus buffering fluctuations in P uptakeand P fertigation. The common extraction pro-cedure for horticultural growing media tendedto underestimate the plant available P in peat-clay blends with high binding capacity. Toimprove the understanding of P binding of claysspecific surface of oxides will be investigated.

Figure 14: Dry matter of Impatiens walleriana dependingon growing media type and P conc. in fertigation solu-tion. Different capital letters indicate significant differen-ces between growing media at the same P fertigationlevel and different small letters indicate significant diffe-rences between fertigation treatments (p<0.05).

Figure 16: Amounts of fertilized and CAT-extractable Pper plant container (0.6 L growing media) and P contai-ned in shoot d.m. of Impatiens walleriana depending ongrowing media type.

Figure 15: Phosphorus conc. in d.m. of Impatiens walleri-ana depending on growing media type and P conc. in fer-tigation solution. Different capital letters indicate signifi-cant differences between growing media at the same Pfertigation level and different small letters indicate signifi-cant differences between fertigation treatments (p<0.05).


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control mechanisms of clays and their specific surface

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