This process, which is called hygrostatic irrigation, is especially useful for optimized and un¬ attended control and delivery of water to the root system of a plant growing in soil, particularly a potted plant, as well as for water conservation in the agri¬ cultural irrigation of arid regions. It also permits the optimized delivery to a plant, on a continuous basis in proportion to the water consumed, of nutrients, e.g. fertilizers dissolved at appropriate low concentrations in the supply water.

- _B UKEALT

OMPI - ir-m _, WIPO - _j V

Background of the Invention The present invention relates generally to means for controlling and regulating the delivery of wa to a soil environment containing growing plants, as a function of the humidity or moisture in the soil. More, particularly, the invention relates to a method and sel contained apparatus for controlling the relative humidi of a soil environment at a pre-set value.

It is common horticultural practice to grow ornamental plants as well as seedlings and cuttings of forest and agricultural species in containers of natura or artificial soil, either outdoors, in greenhouses or hot-beds or indoors. Such potted plants require varyin amounts of watering depending on numerous factors such type of plant, rate of growth, relative humidity and ra of air flow around the leaves, and drainage through the container soil.

Where water supplies are plentiful, sprinklin or hosing from above the plants on some regular basis w sufficient excess to permit runoff usually provides a co venient means for meeting the needs of a group of plants outdoors or in a greenhouse or hot-bed. However, incre ing concern for conserving water resources makes this a less than ideal practice. Indoors, however, where runoff of excess wate poses housekeeping problems, it is necessary to careful water potted plants individually and manually. The cho is onerous and can easily lead to over- or under-wateri to the detriment of the particular plant. in field planting, especially in arid regions where the soil is often highly and variably porous, the terrain uneven, the temperature high and air humidity l water losses due to evaporation and gravitational seepa from soil volumes well removed from the root systems ca far exceed water lost by transpiration from crop foliag

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and be highly variable from plant to plant. Hence, valuable water supplies which might otherwise cost-effec¬ tively support agriculture, often fail with classical irrigation techniques or even with the most advanced trickle irrigation technology.

Summary of the Invention It is an object of this invention to provide a means for automatically delivering to each of one or a multiplicity of plants, on an essentially continuous bas just that amount of water that it requires.

Another object is to provide a low cost, compa self-contained, watering apparatus with long-term reli¬ ability, well ^' suited to both the watering" of house plant and to agricultural irrigation. A typical high-quality soil is porous, somewha absorbent, and has good drainage. Except for the period immediately following a heavy rain or watering, the root of a typical plant are not immersed in liquid water, but are exposed to humid air which surrounds them and the so particles. The water partitions between the gas phase a the moist soil. The humidity in the gas phase of a soil is controlled by the so-called matric potential of its water. This is usually expressed as equivalent osmotic potential or pressure in bars. The potential of pure wa is taken as zero, and the less available the water, the more negative the matric potential.

The following computations are provided in ord to define osmotic potential in relationship to soil mois ture: Let ^~~ equal the osmotic potential of an ideal

(but not necessarily dilute) solution of a non-volatile solute, s, in water; T, the absolute temperature in °K; V the partial molal volume of pure water (i.e., the vo ume of one gram molecular weight of water under standard conditions, which is 18.02 ml.: and 1/V = 55.5 moles of water per liter), R, the gas constant ( ^w 8.314 x 10 ^_/ 9 lite bars/mole-°K) ; and Xw is the mole fraction of water in a ideal solution.

At 27 °C = 300 °K, TΓ = - 1,384 In X _w bars (1 bar = 0.987 atmospheres) . Let the molal concentration of water be c , w' and of solute be c .

"== r- w _ w , therefore

T = 1,384 In [ c _s + c _w ) bars.

w

- In (1 - X s)' = + 1/2 1/3 X

+ For X _o << 1 (i.e. c _w » o _s ) , - m _w = x _s =

C. c _g /c = c / 55.5, therefore π ~ 25 c bars,

which is the classic van't Hoff's Law for dilute solu¬ tions.1

The relative humidity, RH, of air in equilibrium with such an ideal solution is also X. (Haoult's Law).

TΓ Therefore RH = e ^{" 1} ' ³⁸⁴

The osmotic potential of an aqueous solution of water soluble long-chain polymers departs from ideality at high concentrations. The pressure rises above that com¬ puted for ideal behavior for two reasons: Increased hydro- gen bonding of an increasing fraction of water by the polymer reduces c . In addition, as the polymer chain ends get entangled with one another, the momentum transfers on collision are mainly those of short segments of un¬ hindered polymer chain, producing the effect of a higher concentration of lower molecular weight molecules.

Therefore the required concentration of a polymer to produce a given osmotic potential will be so what lower than that computed, assuming ideality.

Since a cross-linked aqueous gel is essentiall a single molecule, its molal concentration is infinitesm and conversely, the mole fraction of water, X , in the g is unity, independent of its weight concentration. If the behavior _v were ideal, the osmotic pressure of a gel would therefore be zero. - , In'fact, the behavior of cross-Linked gels is such that they only exhibit negligible osmotic pressure at high levels of cross-linking. The fewer and further between are the cross-links, the more closely their osmo behavior approaches that of an equivalent solution of lo chain polymers. That is, they behave as if the solution consisted of an appreciable molal concentration of short chain segments, yielding finite and quite appreciable osmotic potentials. (As such gels swell in contact with water, however, tension develops along the chains, and t number of unhindered and unconstrained segments diminish reducing the osmotic potential much faster than can be accounted for by the increasing volume of the gel.)

Capillary forces, surface adsorption forces, t chemical binding forces of hygroscopic substances (e.g., hydrogen bonding) , and the osmotic potential associated with dissolved solutes, all reduce the tendency for soil water to evaporate into the gas phase of the soil. Beca of these forces, even at equilibrium, in wet but well- drained soil at its so-called "field capacity", the rela tive humidity is slightly less than 100%, typically abou -0.3 bars matric potential.

As the soil dries out, the more weakly bound water in the largest capillary spaces is the first to be used up, followed by that in small capillary spaces, etc and the humidity drops and the matric potential becomes

more negative. When the relative humidity of the soil gas-phase drops to the so-called "permanent wilting po¬ tential", plants may die for lack of soil water. The permanent wilting potential typically falls between - 10 and - 15 bars, equivalent to about 99% relative humidity, varying only slightly with soil type, and hardly at all with plant type.

For minimal stress and maximal plant growth, the soil water matric potential should be kept below - 0.3 bars, that is, the soil should not be water-logged, and above about - 6 bars.

Different soils, e.g., clays, loams, sandy loams, and fine sand, will hold different amounts of water per unit volume at the same matric potential. But it is the matric potential which determines the availability of the water to the root system, and with a steady optimal matric potential, soils with very low water capacity can function as effectively as higher quality soils. Therefore, a device which is constructed to sense the relative humidity (matric potential) of the soil in the root zone, and through negative feedback, to control the flow of water to maintain a relatively constant matric potential will work equally well for virtually all kinds of ^" plants in all kinds of soils. Accordingly, this invention relates to- a method and apparatus for controlling the relative humidity in a soil environment around the roots of a growing plant to regulate the moisture content in said environment which method comprises subjecting said environment -to a self- contained, relative humidity sensor-regulator which is pre¬ set to control the delivery of water to the environment to maintain a desired level of humidity. The apparatus em¬ ployed to effect the above-stated process, termed an os¬ motic relative humidity sensor-regulator valve, is also within the purview of this invention.

The relative humidity sensor-regulator contai as the sensing element, an enclosed volume of water con taining a water-soluble or water-swellable material whe in the concentration of said material senses and sets t relative humidity of-the environment to a pre-set value, said enclosed volume of water separated from the enviro ment by a semipermeable membrane which is impermeable t said water-soluble or water-swellable material but perm able to water, wherein the amount and direction of flow of _. water which passes through said membrane as a result osmotic transfer is dependent on the relative humidity the ambient environment and controls flow of water from otherwise isolated water supply to the environment by . varying the pressure on a flexible section or flow-cont diaphragm in a water supply line.

Fundamentally, the herein disclosed method co trols the relative humidity of an environment to a pre- value using an enclosed prescribed volume of a water- soluble or swellable material at a predetermined water centration, said enclosed volume separated from the ext nal environment by a semirigid, semipermeable membrane which is impermeable to the contained material but high permeable to water. The enclosed volume of liquid or h drogel communicates, hydraulically or by a hydraulicall driven mechanical member, to a water impermeable flexib section through which, or flow-control diaphragm by whi water flows from a pressurized water supply to the envi ment to be controlled. When for example, as a result o irrigation, the humidity of that environment rises abov the pre-set level, water is transported osmotically acr the membrane into the enclosed volume, increasing the internal pressure thereby constricting the flexible sec or diaphragm.and reducing the flow of water from the ir gation supply. Conversely, when the environmental humi drops- below the preset value, water is transported

osmotically in the opposite direction, the internal pressure drops, the constriction opens and water flow increases to the environment. i

If the device responds reasonably rapidly, and the delivery rate is not excessive (i.e., there is sufficient hydraulic resistance in the system) , the flow will be continuous and smooth, rather than oscilla¬ tory. The speed of response of the device is proportional to the ratio of the outer surface area of the membrane to the change in volume required to pinch off the water flow. It is also proportional to the water permeability of the semipermeable membrane and of the hydrogel.

Semipermeable membranes prepared for ultra- filtration and desalination of sea water by reverse osmo- sis, have properties very close to those required for this process. They pass fluxes of water of the order of 1

2 microliter per cm per second for pressure differences of 1 atmosphere, and can withstand a few atmospheres with¬ out rupturing. A disadvantage of a design which uses a semi¬ permeable membrane with a filling of a solution of a rela¬ tively high molecular weight solute is that the membrane and chamber seals must be absolutely leak-proof if such a device is to operate reliably over a period of a year or more.

Preferably, the "filling solution" comprises an appropriately compounded, slightly cross-linked gel, and the leak problem disappears. In fact, a rather large pore (e.g., 0.2 mm) semirigid mesh can serve as the "semiperme- able membrane". The mesh will of course pass water freely, but the swelling of the gel will not result in significant bulging through the mesh pores, and as osmotic flow occurs into the device, the gel swells inwardly leading to the constriction of the flexible section of the waterline.

In a preferred embodiment, said osmotically- sensitive means comprises a chamber surrounding a water- impermeable flexible tube; water-swellable material occupying the chamber surrounding the flexible tube; and a semirigid water-permeable membrane, impermeable to the water-swellable material, forming part of the outer wall of the chamber.

In another preferred embodiment, said os ot- ically-sensitive means comprises a chamber a portion of one wall of which is a water-impermeable flexible tube.; a semirigid water-permeable membrane which is impermeabl to a water-swellable material which membrane forms a second side of the chamber, a piston in the chamber in contact with the flexible tube; water-swellable material occupying a part of the chamber between said membrane and said piston and rigid means located adjacent the fle ible tube on the side opposite the piston.

In another preferred embodiment, said osmot- ically-sensitive means comprises a chamber one side of which is a semirigid water-impermeable- diaphragm, a wate permeable membrane which is impermeable to a water-swell able material which membrane forms a second side of the chamber, a piston in the chamber in contact with the diaphragm,- water-swellable material occupying a part of the chamber between said membrane and said piston, and rigid means located adjacent the diaphragm on the side opposite the piston.

The osmotic sensor-regulator valve of this in¬ vention embodies a self-contained, closed container fill with a volume of an aqueous solution of a solute or swel able gel which serves as the humidity sensor. At least part of the wall of the container is composed of a semi¬ rigid, semipermeable membrane. It can be readily buried in the soil among the plant roots. The membrane is high permeable to water but impermeable to the solute (or

water-swellable material). The solute concentration typically is set so that the chemical potential of water contained in the membrane-enclosed-volume is near that of air at 99% relative humidity. If the solute has a molecular weight of about 5,000 and is present at about

30% concentration by volume, the water concentration will be about 70%, and assuming ideal behavior, the osmotic potential will be about 2.1 bars. From Raoult's ^' Law, the relative humidity of air in equilibrium with the device will be about 99.8%. If the humidity in the soil incre-ases above 99.8%, water will pass osmotically inwardly through the membrane, and if the membrane is relatively rigid and therefore the increase in enclosed volume due to the osmotic swelling is small, the hygrostatic pressure with- in the valve will rise to almost-2.1 bars, at which point, the net flow across the membrane will .cease.

The rise in pressure in the enclosure constricts the flow of water through the compressible means deliver¬ ing water to the soil and provides the required negative feedback for the device. As the plant consumes water, the relative humidity around the roots will gradually drop below 99.8%. Water will then pass back from the enclosed volume through the membrane into the environmental soil, the pressure within the valve will drop, relieve the con- striction, and water flow to the soil increases.

Thin-walled silicone rubber tubing or diaphragm is well suited as the material for the compressible wall of a waterline within the sensor.

jUREAl _/ ^*

¹ V

Description of the Drawings The invention will be described with reference to the accompanying drawings which will give a clearer understanding of the invention and preferred method of practicing the invention.

FIG. 1 is a longitudinal sectional view of the valve in a non-constricted embodiment;

FIG. 2 is a longitudinal view of the valve see in FIG. 1 in the constricted configuration; .. _ FIG. 3 is a longitudinal sectional view of sti another preferred embodiment of the valve disclosed and claimed herein.

FIGS. 4 and 5 are longitudinal sectional views of still other preferred embodiments of the valve disclo and claimed herein.

Description of the Preferred Embodiments Suitable construction for the valve is described in FIGS. 1, 2, 3, 4 and 5. Referring now to FIG. 1, which is illustrative of the apparatus of this invention, the • valve 2 contains a body section with osmotically-sensitive means comprising a water-permeable membrane 4, a chamber 6, a flexible tube 8 arranged to pass through the chamber 6, end pieces 10 and 12 and water-swellable material 14 which occupies the chamber 6. The end piece 10 is formed with an inner wall 16 and an outer wall 18 and is provided with a passage 20. The passage 20 extends beyond both walls 16 and 18 in the form of nipples 22 and 24 respectively. The end piece 12 is formed with inner wall 26 and outer wall 28 and is provided with a passage 30. The passage 30 ex- tends beyond the inner wall 26 and the outer wall 28 in the form of nipples 32 and 34 respectively. In the preferred embodiment the membrane 4 and end pieces 10 and 12 form the body of the valve 2. The inner walls 16 and 26 of the end pieces 10 and 22 define the chamber 6 which water from the ambient environment enters into or leaves via membrane 4.

The flexible tube 8 or diaphragm is preferably comprised of silicone rubber, although any compressible material which is inert in an aqueous environment such as polyurethane , PVC or rubber is applicable. The body section is typically constructed of polypropylene, however, other materials such as polymeric acetals, nylon and polyester, materials which are stable to moisture and possess reasonable strength are employ¬ able. The water-soluble or water-swellable material 14 is typically a hydrogel such as a solid gel derived from polyacrylamide, polyvinyl alcohol formulations, etc. Pre¬ ferred is a hydrogel capable of swelling compliantly to about 25 times its dry volume when in equilibrium with water at 100% humidity.

The semipermeable membrane 4., usually in mesh form, when used with such a hydrogel, is composed gener¬ ally of the same material as the body section. In some instances a stainless steel mesh is applicable.

The nipples 22 and 32 connect to the ends of t flexible tube 8 and the nipples 34 and 24 attach respec¬ tively to the supply line 36 and the line 38 to the soil environment.

FIG. 2 illustrates the valve of FIG. 1 in the constricted or closed configuration. As seen, the swell- able material 14 in chamber 6 is expanded in volume and exerts sufficient pressure on the flexible tube 8 to con¬ strict or completely collapse the water passage defined by the flexible tube in the region between nipples 22 an 32 thereby preventing the flow of water between inlet 36 and'outlet 38.

Referring now to FIG. 3 which is illustrative o another preferred embodiment of this invention, valve 102 comprises a body section 103, generally cylindrical with its axis vertical in this section, having osmotically- sensitive means comprising a water-permeable membrane 10 a chamber 106, water-swellable material 114, partially occupying chamber 106, piston 109 within chamber 106 havi a flat upper surface 111 and a conical lower surface 113 tapering to a blunt point 115 and free to move axially so as to restrain passage of water through flexible tube 108 by downward movement actuated by expansion of swellable material 114 in chamber 106. The flexible tube 108 is supported by the body section 103 and passes through the lower portion of chamber 106. The bottom surface of the flexible tube 108 is supported by relatively rigid means such as plate 117, the lower wall of body section 103, located adjacent the flexible tube 108 on the side oppos piston 109. in operation, the swellable material 114 in

chamber 106 is expanded in volume and exerts sufficient pressure on the upper surface of the piston 109 causing the bottom conical portion of• piston 109 at point 115 to compress or collapse flexible tube 108 at the point of contact thereby preventing the flow of water between inlet 36 and outlet 38.

Conversely, upward movement of the -piston 109 follows the shrinkage in the water-swellable material 114 in the chamber 106 removing the constriction in the flex- ible tube 108 at point 115 thereby allowing the passage of water through the tube 108 between inlet 36 and outlet 38. ^*

Referring now to FIG. 4 which is illustrative of another preferred embodiment of this invention, valve 202 comprises a generally cylindrical body section 203 having osmotically-sensitive means comprising a water- permeable membrane 204, a chamber 206, water-swellable material 214 partially occupying chamber 206, piston 209 within the chamber 206 having flat upper and conical lower surfaces 219 and 221 respectively free to move axially so as to restrain passage of water through the flexible tube 208 by downward movement actuated by expansion of material 214 in chamber 206. The flexible tube 208, supported by the body section and a seat in piston 209, passes through the lower portion of chamber 206. The bottom surface of ^* flexible tube 208 is supported by relatively rigid means 223 located adjacent the flexible tube 208 on the side opposite the piston 209. The relatively rigid means 223 is a threaded member which is screwed into the lower por- . tion of body section 203 having a conical upper surface 225 tapering to a blunt point 215 at the bottom surface of the flexible tube 208.

The operation of valve 202 is similar to that described for the valve 102 of the embodiment of FIG. 4. The swellable material 214 in chamber 206 is expanded in volume and exerts sufficient pressure on the upper surface

219 of the piston 209 causing the bottom surface 221 of the piston 209 to collapse or compress the flexible tube

208 at the point of contact with the tapered end 215 of member 223 thereby preventing the flow of water between inlet 36 and outlet 38.

Conversely, the upward movement of the piston

209 caused by shrinkage in volume of the swellable mater 214 in the chamber 206 removes the constriction in the flexible tube 208 at point 215 thereby allowing the pass age of water through the tube 208 between inlet 36 and outlet 38.

The threads on members 203 and 223 permit the adjustment of the pre-set humidity over a wide range.

Because of the large ratio of the area of the tops of the pistons to the area of the conical tips in FIGS.3 and 4, these versions have substantial gain in their feedback loops so that they become much less sensi tive to water-supply pressure variations than the versio in FIGS. 1 and 2. In the embodiments in FIGS. 3 and 4, there are no internal connections to the water supply which should lead to fewer assembly failures.

Referring now to FIG. 5, which is illustrative of another preferred embodiment of this invention, valve 301 comprises a generally cylindrical upper body section 302 and a lower body section 303 having osmotically- sensitive means comprising a water-permeable membrane 30 water-swellable material 314 partially occupying a chamb 306, ^' a piston 309 within chamber 306 having a substan- tially flat upper surface 317 and a conical lower surfac 319 tapering to a blunt point 315, a circular flexible diaphragm 308 conforming to the lower surface area of th piston 309 and sealed between the edges 325 and 327, respectively, of body sections 302 and 303. A bottom portion of the body section 303 has an inlet opening 336

for the passage of water, the upper surface of said inlet configured in a concavity 312 conforming to the shape of the conical lower surface of piston 309. An outlet 338 for the passage of water is adjacent inlet 336 and com- , ^* prises an opening in rigid member 303.

In operation, the water-swellable material 314 in the chamber 306 is expanded in volume and exerts suffi¬ cient pressure on the upper surface 317 of the piston 309 to be displaced axially to cause the diaphragm 308 sur¬ rounding the bottom surface of'the piston 309 to close the opening between tapered point 315 and the ^• concavity 312 of the inlet opening 336 thereby preventing the flow of water between inlet 336 and outlet 338.

Conversely, the upward movement of the diaphragm 308 caused by shrinkage in volume of the water-swellable material 314 located in the chamber 306 allows for the passage of water through the inlet 336 into a cavity 330 and out through outlet 338.

This procedure, termed hygrostatic irrigation, i _s especially useful for unattended control and delivery of water to the root systems of individual potted or con¬ tainerized plants as well as for water conservation in the agricultural irrigation of arid regions ^'" . It also permits the optimized delivery to a plant, on a continuous basis in proportion to the water consumed, of nutrients, e.g. fertilizers dissolved at appropriate concentrations in the supply water. This procedure is also applicable to the control of the humidity of environments other than soil.

This invention is also concerned with a device termed an osmotic relative humidity sensor-regulator valve which is devised to sense the relative humidity of the soil in the root zone, and through negative feedback, to control the flow of water to maintain a relatively constant matric potential for virtually all kinds of plants in all kinds of soils. This device can similarly control humidity

in environments other than soil.

Accordingly, the apparatus disclosed herein fo the purpose described above is an osmotic relative humid sensor-regulator valve comprising: a body section; means, for attaching the valve body section in a pressurized waterline; compressible means in the waterline for openi and closing the waterline; and osmotically-sensitive mea for opening and. closing the flexible means in the water- • line. It should be understood by those skilled in th art ^' that various modifications may be made in the presen invention without departing from the spirit and scope thereof as described in the specification and defined in the appended claims.