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Koelen, ontvochtigen, bevochtigen, lucht in beweging brengen, zijn teeltmaatregelen die met de opkomst van (semi-) gesloten kassen volop in de belangstelling staan. Onderzoeker Peter van Weel van Wageningen UR Glastuinbouw kijkt in dit artikel vanuit de plantfysiologie naar effecten van klimaatsturing op het gewas.

Voor wat betreft de drijvende teelt van gewassen die niet direct op het systeem worden gezaaid is één van de cruciale vraagstukken: Wat is de juiste combinatie van: het type drijver; de vorm en afmetingen van en het type opkweekmateriaal (pot/plug); de positie van de pot of plug t.o.v. de voedingsoplossing; de watergeefstrategie. Mits goed op elkaar afgestemd kunnen diverse systemen tot goede resultaten leiden: 1. Een systeem waarbij de pot/plug altijd boven de voedingsoplossing hangt en in het begin regelmatig bevochtigd moet worden. 2. Een systeem waarbij de pot/plug van begin af aan met de voet continu in de voedingsoplossing hangt. 3. Een eb-/vloedsysteem (dus een combinatie van 1. en 2.).

Voor wat betreft de drijvende teelt van gewassen die niet direct op het systeem worden gezaaid is één van de cruciale vraagstukken: Wat is de juiste combinatie van: het type drijver; de vorm en afmetingen van en het type opkweekmateriaal (pot/plug); de positie van de pot of plug t.o.v. de voedingsoplossing; de watergeefstrategie. Mits goed op elkaar afgestemd kunnen diverse systemen tot goede resultaten leiden: 1. Een systeem waarbij de pot/plug altijd boven de voedingsoplossing hangt en in het begin regelmatig bevochtigd moet worden. 2. Een systeem waarbij de pot/plug van begin af aan met de voet continu in de voedingsoplossing hangt. 3. Een eb-/vloedsysteem (dus een combinatie van 1. en 2.).

The present study suggests that many soils in the Netherlands may be water repellent to some degree, challenging the common perception that soil water repellency is only an interesting aberration. When dry, water repellent soils resist or retard water infiltration into the soil matrix. Soil water repellency can lead to the development of unstable wetting and preferential flow paths. The persistence of water repellency was examined in topsoils using the water drop penetration time (WDPT) test. The severity of water repellency measured on dried soil samples, the so-called “potential” water repellency, can be used as a parameter for comparing soils with respect to their sensitivity to water repellency. Measurement of the “actual” water repellency on field-moist samples determines the soil fraction excluded from direct solute and water flow. However, preferential flow is a dynamic process, which is why the ratio between water repellent and wettable soil is time dependent. The “critical soil water content”, below which the soil in the field is water repellent and above which the soil is wettable, was found to be a useful parameter in water repellency studies. Spatial and temporal variability in volumetric soil water content was studied in vertical transects by intensive sampling with 100 cm3 steel cylinders. Spatial variability in soil water content under grass cover was high, due to fingered flow. On arable land, vegetation and microtopography appeared to play a dominant role. This study provides examples of uneven moisture patterns in water repellent sand, loam, clay and peat soils with grass cover, and in cropped, water repellent sandy soils.

The variability of soil water content over short distances was studied at seven experimental fields, located in southern Sweden, eastern Germany, southern France, and in the Netherlands. The soils all in use as arable land, are a sandy soil, a stony sandy soil, a loamy sand, a sandy loam, and three clay soils. Samples with a volume of 100 cm 3 were taken at close intervals in trenches at several depths to determine potential water repellency and soil water content. When dry, the topsoils of the Mellby site in Sweden (loamy sand) and the Vredepeel site in the Netherlands (sandy soil) are water repellent, whereas the soils at the other five sites are wettable, according to the water drop penetration time (WDPT) test. The variation in water content within short distances was high at most depths at all sites, with differences of 7 to 18 vol% within a horizontal distance of only centimeters to decimeters. Drier as well as wetter soil areas were visualized by contour plots of the soil water content distributions in the transects. Large differences in wetting capacity between samples taken in the topsoil and in the subsoil at several sites were assessed by measurements of the wetting rate. In many cases, subsoil samples wetted faster than topsoil samples. The severity of water repellency of sandy samples from the Vredepeel site increased remarkably at oven temperatures above 65°C. The wetting rates of dried samples from this soil were also evidently influenced by the oven temperature; samples dried at 25°C wetted immediately and reached soil water contents of 23 to 32 vol% within one hour, whereas samples dried above 85°C hardly wetted during 72 hours, as a result of the increased water repellency at higher drying temperatures.

Wageningen UR Glastuinbouw heeft in opdracht van het PT en het ministerie van EL&I in het kader van Kas als Energiebron voorbereidend onderzoek gedaan naar een energiezuinig teeltconcept voor Freesia (“Het Nieuwe Telen”). De warmtevraag van de gangbare freesiateelt is berekend op 13 m 3 a.e./m 2 /jaar; dit gaat vooral naar het op temperatuur houden van de kas en naar het afvoeren van vocht uit verdamping en de bodem. De mogelijke energiebesparing bij Freesia wordt ingeschat op 7 m 3 /m 2 .jaar (excl. stomen en belichting). Dit is te bereiken met actieve ontvochtiging en het afdekken van de bodem. Circa 3 m 3 /m 2 besparing is te realiseren is door via aangepaste isolatie en watergift de verdamping uit de bodem tegen te gaan. Het perspectief hiervan is in een experiment aangetoond zonder negatieve effecten op de groei.

Wageningen UR Glastuinbouw heeft in opdracht van het PT en het ministerie van EL&I in het kader van Kas als Energiebron voorbereidend onderzoek gedaan naar een energiezuinig teeltconcept voor Freesia (“Het Nieuwe Telen”). De warmtevraag van de gangbare freesiateelt is berekend op 13 m 3 a.e./m 2 /jaar; dit gaat vooral naar het op temperatuur houden van de kas en naar het afvoeren van vocht uit verdamping en de bodem. De mogelijke energiebesparing bij Freesia wordt ingeschat op 7 m 3 /m 2 .jaar (excl. stomen en belichting). Dit is te bereiken met actieve ontvochtiging en het afdekken van de bodem. Circa 3 m 3 /m 2 besparing is te realiseren is door via aangepaste isolatie en watergift de verdamping uit de bodem tegen te gaan. Het perspectief hiervan is in een experiment aangetoond zonder negatieve effecten op de groei.

Het toepassen van bevochtiging in perenbewaring, voor een optimale beheersing van de vochtigheid, is in opkomst. De belangrijkste reden is dat telers en bewaarders het afdekken van kisten als te arbeidsintensief beschouwen. Een te hoge arbeidsbelasting in een toch al drukke periode. Vanwege deze ontwikkeling geeft Frank van de Geijn (Food & Biobased Research) enige richting in de keuze voor een bevochtigingtechniek.

Door de productieverhoging van Conference neemt de behoefte aan betere en veilige bewaring toe. Voor de afzet is het noodzakelijk dat een deel van de peren jaarrond bewaard kan worden. Om het vochtverlies zoveel mogelijk te beperken, biedt bevochtiging in de cel een interessante mogelijkheid.

Het praktijkonderzek 'Energie onder de knie' heeft aangetoond dat het goed mogelijk is om het energieverbruik terug te brengen met 40% met behoud van productie en kwaliteit. Schermen, lucht bevochtigen en drogen passen in dat plan. Verreweg de grootste winst is dat telers hebben meegemaakt dat het kan

Het praktijkonderzek 'Energie onder de knie' heeft aangetoond dat het goed mogelijk is om het energieverbruik terug te brengen met 40% met behoud van productie en kwaliteit. Schermen, lucht bevochtigen en drogen passen in dat plan. Verreweg de grootste winst is dat telers hebben meegemaakt dat het kan

Koelen, ontvochtigen, bevochtigen, lucht in beweging brengen, zijn teeltmaatregelen die met de opkomst van (semi-) gesloten kassen volop in de belangstelling staan. Onderzoeker Peter van Weel van Wageningen UR Glastuinbouw kijkt in dit artikel vanuit de plantfysiologie naar effecten van klimaatsturing op het gewas.

In semi-gesloten kassen bieden koeling en bevochtiging meer mogelijkheden om het klimaat rond de plant te beïnvloeden. Het is echter weinig bekend hoe die mogelijkheden goed kunnen worden benut. Binnen het programma Kas als energiebron is daarom aandacht besteed aan vocht, met de plant als uitgangspunt

Fytagoras, een uit TNO afkomstig onderzoeksbedrijf, ontwikkelde een meetmethode waarmee 'online' zuurstof in het wortelmilieu is te meten. Hoewel de resultaten indrukwekkend zijn, bestaat er nog enige discussie over de interpretatie van de gevonden waarden

Thin aqueous films formed on a solid surface play an important role in adhesion, spreading, and colloidal stability. These phenomena are all relevant for paint systems. Measuring surface forces in these films is an experimental challenge, and over the years several techniques have been developed to measure the interaction forces as a function of the thickness of the film, the so-called disjoining pressure isotherms. A thin film balance technique (TFB) in combination with an ellipsometer offers the possibility to study the surface forces in aqueous films formed at a silica surface.The aim of this project was twofold. Firstly, we wanted to set up the technique and test it by investigating interaction forces in thin aqueous films formed between an air interface and a solid interface (air/aqueous film/silica). Secondly, we intended to apply this technique for investigating interaction forces in aqueous films formed between an oilinterfaceand a solid interface (oil/ aqueous film/ silica).As happens so often in research, we were 'immersed' in the first system, and as a result of it, we did not reach the second aim. Hence, this thesis deals with the experimental investigations of the interaction forces in thin aqueous films formed between an air phase and a solid phase. A non-ionic polymeric surfactant is used as a stabilizer of these films. In addition, we have also used colloidal probe atomic force microscopy (CP-AFM) in order to see if phenomena similar to those in wetting films might occur in the interactions between two silica surfaces, immersed in solutions of the surfactant.In chapter 1 an introduction is given to wetting films, to the surfactant used and to the technique used in the present study. A short overview of the interaction forces in wetting films and the drainage of these films is presented. As foam films, which are aqueous films between two air phases (air/ aqueous film/ air), have much more extensively been studied than wetting films, we also discussed drainage of foam films, as well as and an interesting finding in foam films stabilized by similar surfactants.In chapter 2 the solution behaviour of the surfactant and its adsorption on silica-water and air-water interfaces is studied using light scattering, surface tensiometry and ellipsometry. Interaction forces in wetting films stabilized by the surfactant are measured and are interpreted in terms of a disjoining pressure as a function of a thickness of the film.Parameters of the systems are the concentration of the surfactant and the concentration of NaCl. The main findings are that addition of NaCl lowers the cmc, diminishes adsorption of the surfactant on silica, and increases the thickness of the wetting film stabilized by the surfactant.In chapter 3 we investigate the thinning of wetting films formed from aqueous solution of the surfactant on silica using a TFB andan ellipsometer. In addition, imaging ellipsometry is used to visualize the film structures at subsequent stages of their development. The main findings are that the thick films observed in chapter 2 are metastable. The time required for the formation of steady films strongly increases with the concentration of electrolyte. For sufficiently large amounts of salt, two characteristic relaxation regimes can be clearly identified: after initial quick thinning, further thinning slows down enormously. The interpretation we give of the observed features is that these typical kinetic regimes are the result of coupled dependencies of the bulk and interfacial properties of the surfactant on salt concentration.In chapter 4 the time-dependent thickness of wetting films formed from aqueous solutions of NaCl on a silica surface modified with the surfactant is revisited, because of a rather surprising finding: the thickness as a function of time could go through a maximum. It turned out that that the thickness of the films depends strongly on whether or not the surfactant was applied over the entire substrate or only at a spot coinciding with the investigated wetting film. In the former case, the thickness shows a transient thickness maximum, in the latter case, it equilibrates quickly. A possible interpretation of the transient thicknesses is given, and the equilibrium values of the thickness are discussed.In chapter 5 the effects of pH and different additives (NaCl, Na 2 SO 4 , NaSCN and urea) on the adsorption of the surfactant at the interfaces of wetting films, on film drainage and on the interaction forces in these films are examined using an ellipsometer and a TFB. These additives reduce the adsorption of the surfactant on silica and all retard the drainage of the films stabilized by the surfactant. Moreover, the film destabilizes at high pH values if 0.1 M NaCl is present. The reduction of the adsorbed amount is correlated to a dramatic slow down of film drainage. We show that these effects must be attributed to bridging attraction between a densely covered air-water interface and a very sparsely covered silica-water interface.In chapter 6 the interactions between two silica surfaces, immersed in solutions of the surfactant is studied by CP-AFM.The main findings are that (i) there are long ranged attractive forces between bare hydrophobic and hydrophilic surfaces in aqueous solution; and that (ii) a weak adhesion occurs between a hydrophobic and a hydrophilic surface in the presence of the surfactant in 0.1 M NaCl.Results (i) can be attributed to capillary bridging and (ii) to observation bridging of the surfactant between a densely covered hydrophobic surface and a sparsely covered hydrophilic surface, thus confirming the effect found in chapter 5.

Thin aqueous films formed on a solid surface play an important role in adhesion, spreading, and colloidal stability. These phenomena are all relevant for paint systems. Measuring surface forces in these films is an experimental challenge, and over the years several techniques have been developed to measure the interaction forces as a function of the thickness of the film, the so-called disjoining pressure isotherms. A thin film balance technique (TFB) in combination with an ellipsometer offers the possibility to study the surface forces in aqueous films formed at a silica surface.The aim of this project was twofold. Firstly, we wanted to set up the technique and test it by investigating interaction forces in thin aqueous films formed between an air interface and a solid interface (air/aqueous film/silica). Secondly, we intended to apply this technique for investigating interaction forces in aqueous films formed between an oilinterfaceand a solid interface (oil/ aqueous film/ silica).As happens so often in research, we were 'immersed' in the first system, and as a result of it, we did not reach the second aim. Hence, this thesis deals with the experimental investigations of the interaction forces in thin aqueous films formed between an air phase and a solid phase. A non-ionic polymeric surfactant is used as a stabilizer of these films. In addition, we have also used colloidal probe atomic force microscopy (CP-AFM) in order to see if phenomena similar to those in wetting films might occur in the interactions between two silica surfaces, immersed in solutions of the surfactant.In chapter 1 an introduction is given to wetting films, to the surfactant used and to the technique used in the present study. A short overview of the interaction forces in wetting films and the drainage of these films is presented. As foam films, which are aqueous films between two air phases (air/ aqueous film/ air), have much more extensively been studied than wetting films, we also discussed drainage of foam films, as well as and an interesting finding in foam films stabilized by similar surfactants.In chapter 2 the solution behaviour of the surfactant and its adsorption on silica-water and air-water interfaces is studied using light scattering, surface tensiometry and ellipsometry. Interaction forces in wetting films stabilized by the surfactant are measured and are interpreted in terms of a disjoining pressure as a function of a thickness of the film.Parameters of the systems are the concentration of the surfactant and the concentration of NaCl. The main findings are that addition of NaCl lowers the cmc, diminishes adsorption of the surfactant on silica, and increases the thickness of the wetting film stabilized by the surfactant.In chapter 3 we investigate the thinning of wetting films formed from aqueous solution of the surfactant on silica using a TFB andan ellipsometer. In addition, imaging ellipsometry is used to visualize the film structures at subsequent stages of their development. The main findings are that the thick films observed in chapter 2 are metastable. The time required for the formation of steady films strongly increases with the concentration of electrolyte. For sufficiently large amounts of salt, two characteristic relaxation regimes can be clearly identified: after initial quick thinning, further thinning slows down enormously. The interpretation we give of the observed features is that these typical kinetic regimes are the result of coupled dependencies of the bulk and interfacial properties of the surfactant on salt concentration.In chapter 4 the time-dependent thickness of wetting films formed from aqueous solutions of NaCl on a silica surface modified with the surfactant is revisited, because of a rather surprising finding: the thickness as a function of time could go through a maximum. It turned out that that the thickness of the films depends strongly on whether or not the surfactant was applied over the entire substrate or only at a spot coinciding with the investigated wetting film. In the former case, the thickness shows a transient thickness maximum, in the latter case, it equilibrates quickly. A possible interpretation of the transient thicknesses is given, and the equilibrium values of the thickness are discussed.In chapter 5 the effects of pH and different additives (NaCl, Na 2 SO 4 , NaSCN and urea) on the adsorption of the surfactant at the interfaces of wetting films, on film drainage and on the interaction forces in these films are examined using an ellipsometer and a TFB. These additives reduce the adsorption of the surfactant on silica and all retard the drainage of the films stabilized by the surfactant. Moreover, the film destabilizes at high pH values if 0.1 M NaCl is present. The reduction of the adsorbed amount is correlated to a dramatic slow down of film drainage. We show that these effects must be attributed to bridging attraction between a densely covered air-water interface and a very sparsely covered silica-water interface.In chapter 6 the interactions between two silica surfaces, immersed in solutions of the surfactant is studied by CP-AFM.The main findings are that (i) there are long ranged attractive forces between bare hydrophobic and hydrophilic surfaces in aqueous solution; and that (ii) a weak adhesion occurs between a hydrophobic and a hydrophilic surface in the presence of the surfactant in 0.1 M NaCl.Results (i) can be attributed to capillary bridging and (ii) to observation bridging of the surfactant between a densely covered hydrophobic surface and a sparsely covered hydrophilic surface, thus confirming the effect found in chapter 5.

Binnen het KwaliCon-project wordt de kwaliteitsverbetering van Conference na bewaring onderzocht. In dit artikel worden de praktijkproeven met bevochtigingssystemen beschreven

In this last chapter I will review and integrate the findings of the previous chapters and give suggestions for further research on this topic.In chapter 2 measurements of depletion interactions by means of colloidal probe atomic force microscopy (CP-AFM) are described. We found that the behaviour of the range of the depletion interaction is roughly in agreement with predictions of Fleer et aI. The strength of the depletion interaction is not in agreement with of predictions of Tuinier et al.2 and Louis3 and Bolhuis.4 It is much smaller. We explain this by the fact that the segment-surface interaction for polydimethylsiloxane (PDMS) and a stearylated silica surface is non zero. Comparison of our experimental values with values obtained by Scheutjens Fleer Self Consisted Field theory for different values of Xs leads to a reduced segment surface interaction parameter of 0.41. This corresponds to a decrease to about one order of magnitude of the maximal depletion interaction (see figure 2.13). This strong weakening is in agreement with experimental findings that significantly more PDMS is needed to reach the colloidal liquid-gas binodal than predicted, even if non ideality of the polymers is taken into account.5· 6In chapter 3 we present observations of wetting phenomena in depletion interaction driven, phase-separated colloidal dispersions. These dispersions consist of stearyl coated silica spheres in cyclohexane with PDMS as non-adsorbing polymer. The dynamic contact angle was determined by moving a thin fibre upward or downward through the colloidal liquid gas interface and drawing tangents to the meniscus at the fibre in the three phase contact line. The dynamic contact angle was measured as a function of the velocity of the fibre. From these dynamic contact angles the static contact angle was determined by interpolation to zero velocity. We observed no noticeable hysteresis. The contact angle of the colloidal liquid-gas interface at a solid substrate (coated glass) was determined for a series of compositions. We were able to locate a Cahn wetting transition7 from partial to complete wetting upon approaching the critical point. By this we have tested successfully the well known Cahn prediction regarding wetting transitions for coloid-polyrner mixtures.In the same way we determined the contact angle of the colloidal liquid-gas interface at a soft solid substrate for a series of compositions. A soft solid substrate was obtained by heating a glass fibre in a melt of PDMS. In this way we manipulated the colloidal particle-substrate interaction. Upon approach to the critical point, a transition occurs from partial to complete drying. The region where the wetting transition occurs was compared with that of density functional calculations of Brader et al. and Monte Carlo simulations of Dijkstra et al.8' 9

In this last chapter I will review and integrate the findings of the previous chapters and give suggestions for further research on this topic.In chapter 2 measurements of depletion interactions by means of colloidal probe atomic force microscopy (CP-AFM) are described. We found that the behaviour of the range of the depletion interaction is roughly in agreement with predictions of Fleer et aI. The strength of the depletion interaction is not in agreement with of predictions of Tuinier et al.2 and Louis3 and Bolhuis.4 It is much smaller. We explain this by the fact that the segment-surface interaction for polydimethylsiloxane (PDMS) and a stearylated silica surface is non zero. Comparison of our experimental values with values obtained by Scheutjens Fleer Self Consisted Field theory for different values of Xs leads to a reduced segment surface interaction parameter of 0.41. This corresponds to a decrease to about one order of magnitude of the maximal depletion interaction (see figure 2.13). This strong weakening is in agreement with experimental findings that significantly more PDMS is needed to reach the colloidal liquid-gas binodal than predicted, even if non ideality of the polymers is taken into account.5· 6In chapter 3 we present observations of wetting phenomena in depletion interaction driven, phase-separated colloidal dispersions. These dispersions consist of stearyl coated silica spheres in cyclohexane with PDMS as non-adsorbing polymer. The dynamic contact angle was determined by moving a thin fibre upward or downward through the colloidal liquid gas interface and drawing tangents to the meniscus at the fibre in the three phase contact line. The dynamic contact angle was measured as a function of the velocity of the fibre. From these dynamic contact angles the static contact angle was determined by interpolation to zero velocity. We observed no noticeable hysteresis. The contact angle of the colloidal liquid-gas interface at a solid substrate (coated glass) was determined for a series of compositions. We were able to locate a Cahn wetting transition7 from partial to complete wetting upon approaching the critical point. By this we have tested successfully the well known Cahn prediction regarding wetting transitions for coloid-polyrner mixtures.In the same way we determined the contact angle of the colloidal liquid-gas interface at a soft solid substrate for a series of compositions. A soft solid substrate was obtained by heating a glass fibre in a melt of PDMS. In this way we manipulated the colloidal particle-substrate interaction. Upon approach to the critical point, a transition occurs from partial to complete drying. The region where the wetting transition occurs was compared with that of density functional calculations of Brader et al. and Monte Carlo simulations of Dijkstra et al.8' 9

Door de productieverhoging van Conference neemt de behoefte aan betere en veilige bewaring toe. Voor de afzet is het noodzakelijk dat een deel van de peren jaarrond bewaard kan worden. Om het vochtverlies zoveel mogelijk te beperken, biedt bevochtiging in de cel een interessante mogelijkheid.

Het toepassen van bevochtiging in perenbewaring, voor een optimale beheersing van de vochtigheid, is in opkomst. De belangrijkste reden is dat telers en bewaarders het afdekken van kisten als te arbeidsintensief beschouwen. Een te hoge arbeidsbelasting in een toch al drukke periode. Vanwege deze ontwikkeling geeft Frank van de Geijn (Food & Biobased Research) enige richting in de keuze voor een bevochtigingtechniek.

Fingered flow in the unsaturated zone caused by wetting front instability enhances solute leaching to the groundwater. This paper reviews recent progress in fingered flow research, focusing on theoretical results and model development. A variety of stability criteria have been derived to predict wetting front instability, mainly through linear, and sometimes non-linear, stability analysis, but also by theoretically analyzing infiltration into dry soils, and by stochastic methods that take into account random variations of the soil properties, fluid pressure, and front location. These stability criteria are discussed and compared. Eight expressions for finger size are presented. They fall into three categories based on the dependence on the ratio of the infiltration rate to the soil hydraulic conductivity in the finger. Next, the modeling of finger growth is discussed. Because of its relevance to solute transport, recent advances in the measurement and modeling of finger behavior in moist soils are thoroughly reviewed. Various models (ranging from easily applied closed-form equations for risk assessment to sophisticated transient numerical codes) have been developed over the past 15 years. These are discussed and selected results are shown. Finally, some remaining gaps in our understanding are given. Among these are the role of the initial water content, the nature of the flow in the distribution zone above the fingers, the effect of soil heterogeneity on wetting front instability, and the combined effect of soil heterogeneity and wetting front instability on field-scale solute transport.

In semi-gesloten kassen bieden koeling en bevochtiging meer mogelijkheden om het klimaat rond de plant te beïnvloeden. Het is echter weinig bekend hoe die mogelijkheden goed kunnen worden benut. Binnen het programma Kas als energiebron is daarom aandacht besteed aan vocht, met de plant als uitgangspunt

Fytagoras, een uit TNO afkomstig onderzoeksbedrijf, ontwikkelde een meetmethode waarmee 'online' zuurstof in het wortelmilieu is te meten. Hoewel de resultaten indrukwekkend zijn, bestaat er nog enige discussie over de interpretatie van de gevonden waarden

Binnen het KwaliCon-project wordt de kwaliteitsverbetering van Conference na bewaring onderzocht. In dit artikel worden de praktijkproeven met bevochtigingssystemen beschreven