WO1998012154A1

WO1998012154A1 - Soil additive

Info

Publication number: WO1998012154A1
Application number: PCT/CA1997/000667
Authority: WO
Inventors: Kenneth Roger Williams; Marlene Elizabeth Conway
Original assignee: Envirolutions Inc.
Priority date: 1996-09-17
Filing date: 1997-09-15
Publication date: 1998-03-26
Also published as: GB9619415D0; CA2304239A1; AU4196797A

Abstract

A soil additive comprising a super absorbent polymer and a growth-promoting additive. The super absorbent polymer is a polyacrylate and is in a particulate form, and the growth-promoting additive is absorbed therein.

Description

1

TITLE

SOIL ADDITIVE

Field of the Invention The present invention relates to a soil additive comprising a super absorbent polymer particulate and a growth-promoting additive absorbed therein. The soil additive is intended for addition to soil in low concentrations to promote growth of plants, especially to assist m facilitating growth of plants in a sustained manner during dry periods. In particular embodiments, the present invention relates to a controlled release fertilizer that is based on slow release of plant nutrients encapsulated in sodium polyacrylate polymer, formed by incorporation of the nutrients into the polymer when it is m a water swollen gel state, after which the product is dried.

Background to the Invention Controlled release fertilizers have been available for some time. Typically they are composed of chemical fertilizer granules with a porous coating which in moist soil allows diffusion of plant nutrients (ions) into the adjacent soil environment. An example is the controlled release fertilizer available from Scotts Co. under the trademark "Osmocote" . By varying the nature and/or thickness of the coating, nutrient availability over a range of time periods can be achieved. However, as the moisture environment around the granules m the soil is controlled by the moisture content of the soil, under dry conditions the transfer rate of nutrients to the roots from the fertilizer granules tends to be greatly reduced. Technology for increasing the retention of moisture m soils is also known. Both natural products, e.g. peat moss and the like, and synthetic water absorbing polymers are used in horticultural/agricultural applications. Examples of such polymers are those available under the trademarks "Liqua-Gel" and "SuperSorb". Such synthetic water absorbing polymers utilized m agricultural end- uses absorb high volumes of water as soon as they are placed in soils. Adequate moisture supply is critical to roots, especially to plants whose growing medium is subject to long periods of moisture deficiency. Various superabsorbent polymer (SAP) gels have been offered commercially to address the problems of inadequate moisture supply to roots. So called agricultural SAP chemicals, which are acrylamides or acrylamide copolymers, are non-ionic or have a very low anionic character. As a consequence of the non- ionic state, there tends to be a relative msensitivity to the presence of cations in the soil and hence the degree of swelling of the agricultural SAP tends to remain constant over repeated wet/dry cycles. In contrast, anionic SAP's which are normally sodium polyacrylate, tend to lose their ability to absorb large quantities of water in a cyclic wet/dry environment because of exchange of cations from the surrounding soil, particularly from clay soils.

During dry periods, sodium polyacrylate tends to condense and form crosslinks that inhibit re-swelling when it is re-wetted. Even when used in situations where a limited number of wet/dry cycles are experienced, sodium polyacrylate inhibits plant growth or in some cases is toxic to plants. This inhibition of plant growth or toxicity is believed to arise because the sodium ions in the sodium polyacrylate network are exchangeable and these ions are adsorbed by clay particles or tend to undergo exchange with cations on the surface of plant roots. The consequence is a condition that is analogous to an alkali soil, which generally tends to adversely affect or inhibit plant growth.

Super absorbent polymers have been studied for many years for use as soil additives to increase available water for plants. R.H. Fikhof et al . Proc . 12^th Nat Agr Plastics Conf . 1974 studied the influence of a hydrophilic polymer on the water requirements of container grown plants. Gehring and Lewis reported on the effect of hydrogel on wilting and moisture stress of bedding plants, (J. Amer. Soc . Hort . Sci. 105, 511-14 (1980)). Later, W.G. Pill HortScience 23, 998-1000

(1988) studied the use of acrylamide based polymer gels as growth media for tomato seedlings.

Sensitivity of hydrogels to the presence of salts has caused the focus to be on acrylamide-based polymers rather than ionic polyacrylates , although even polyacrylamide superabsorbents show a decrease in water absorption in the presence of soluble salts. Bowman, Evans and Paul, J. Amer. Soc. Hort. Sci. 115: 382-86 (1990) reported that divalent cations at a concentration of 20 meg/liter reduced water pickup by a polyacrylamide gel to about 10% of the level observed in distilled water. Lamont and O'Connell, reported in Scientia Horticulture 31: 141-49 (1987) that there was no improvement in bedding plant dry shoot weight when polyacrylamide and polyacrylamide copolymer was used, compared with controls.

U.S. 5,405,425 of Pieh et al relates to the addition of a sulphonyl group to acrylamide polymers and copolymers to reduce the deswelling effects of salts present in soil. U.S. 4,906,276 and 4,985,062 of Hughes disclose polymerization of acrylic acid using special polymerization procedures with potassium and ammonium ions to provide ion species for plant growth when the product is swollen in moist soil. U.S. 4,997,192 of Martinau et al . discloses incorporation of a fine grain inorganic powder, clay, during polymerization of a cross- linked water-absorbing polymer or copolymer composed of acrylic acid and acrylamide.

Canadian 1,309,070 of Cooke describes a polyacrylate useful in dry sandy soils to retain moisture. Nutrients or bacterial strains that increase plant yield can be absorbed by the swollen gel which is then dried and added to the soil. The patent is particularly directed to polymerizing an acrylamide monomer, the polymer product obtained being subsequently swollen in an aqueous medium containing additive substances e.g. plant nutrients.

U.S. 4,559,074 of Clarke relates to use of cross- linked non- ionic polyacrylamide as an additive for a plant growth medium.

Use of polyacrylamides in horticultural or agricultural end uses tends to be modest, primarily because of cost. Although crop yield improvements have been reported, applications are generally restricted to some horticultural uses. Inclusion of plant nutrients in polyacrylamide applications would be expected to further increase costs.

In preferred embodiments, the present invention makes use of SAP widely used as absorbents in the hygienic disposables industry. A process exists for recovering such SAP, developed by Knowaste Technologies Inc. of ississauga, Ontario and illustrated m PCT application WO 92/07 995 of M.E. Conway et al , published May 14, 1992. Such SAP may be used in the preparation of the soil additives described herein.

Improvements in existing soil additives would be beneficial, especially an increase in water absorption of super absorbent polymers over repeated wet/dry cycles in the soil to effect a gradual release of captured nutrient

Summary of the Invention

A soil additive formed from a super absorbent polymer and a growth promoting additive has now been found, which is more effective in producing plant growth than the super absorbent polymer and the growth-promoting additive when added separately to the soil.

Accordingly, an aspect of the present invention provides a soil additive comprising a super absorbent polymer and a growth-promoting additive, said super absorbent polymer being a polyacrylate and being in the form of a particulate and said growth-promoting additive being absorbed into the super absorbent polymer particulate, said super absorbent polymer containing growth-promoting additive having an absorption capacity index in the range of about 4 to 50, where absorption capacity index is defined as: (wt of water saturated gel polymer - polymer dry wt) /polymer dry wt .

In a further aspect, the present invention provides a soil additive comprising a super absorbent polymer and a growth-promoting additive, said super absorbent polymer being a polyacrylate and being in the form of a particulate and said growth-promoting additive being absorbed into the super absorbent polymer particulate, said super absorbent polymer having been treated, when it is in a swollen aqueous gel state, with a composition, a major portion of said composition being inorganic compounds and at least part of said composition being growth-promoting additive, said treatment with said composition effecting shrinkage of the super absorbent polymer such that the absorption capacity index of said super absorbent polymer containing growth-promoting additive is in the range of about 4 to 50, where absorption capacity index is defined as: (wt of water saturated gel polymer - polymer dry wt) /polymer dry wt .

In preferred embodiments of the invention, the growth-promoting additive is urea or a nitrate, especially ammonium nitrate or calcium nitrate.

In another embodiment of the invention, the super absorbent polymer is sodium polyacrylate.

In a further embodiment of the invention, the super absorbent polymer is recycled super absorbent polymer, especially super absorbent polymer separated from a process for recovery of components from personal care products.

In yet another embodiment, the soil additive is added to soil in an amount of 0.01-0.5 percent by weight.

In another embodiment, the inorganic compound is calcium nitrate, the growth promoting additive s urea, the ratio of urea to super absorbent polymer is 0.5-3:1 by weight and the ACI of the superabsorbent polymer, after urea addition, is in the range of 2-30.

In another aspect of the invention, there is provided a method of forming a soil additive comprising a super absorbent polymer and a growth-promoting additive, said super absorbent polymer being a polyacrylate and being in the form of a particulate and said growth- promoting additive being absorbed into the super absorbent polymer particulate, said method comprising the steps of treating super absorbent polymer in a swollen aqueous gel state with a composition, a major portion of said composition being inorganic compounds and at least part of said composition being growth-promoting additive, said treatment with said composition effecting shrinkage of the super absorbent polymer such that the absorption capacity index of said super absorbent polymer containing growth-promoting additive is in the range of about 4 to 50, where absorption capacity index is defined as: (wt of water saturated gel polymer - polymer dry wt) /polymer dry wt, and separating said soil additive.

Brief Description of the Drawings The present invention is illustrated by the drawings, as follows:

Figure 1 is a graphical representation of results obtained in Example III; and

Figure 2 is a graphical representation of results obtained in Example VI.

Detailed Description of the Invention

The present invention is a granular soil additive, and the related soil treatment, that incorporates slow fertilizer release in a specially formulated hydrogel that undergoes gradual expansion in a wet environment . The soil additive is formed from a sodium or potassium polyacrylate super absorbent polymer (SAP) that has been treated in its gel state with growth-promoting additives. The treatment is adjusted to produce a controlled degree of gel deswelling. After it is dried, and during use in soil, the soil additive undergoes slow reswelling in a moist soil and the trapped growth-promoting additive is released slowly over time. Typically during a growing season, soils undergo repeated wet/dry cycles. During each cyclic wetting, there is an additional release of growth-promoting additives into the soil. As noted above and illustrated herein, the present invention relates to a soil additive comprising a super absorbent polymer and a growthpromotmg additive, and use thereof. The growth-promoting additive is absorbed into the super absorbent polymer, and is not merely an admixture of super absorbent polymer and growth-promoting additive.

Anionic super absorbent polymers are preferred as the SAP used in the preparation of the soil additive of the present invention, especially because their level of swelling m aqueous solutions tends to be dependent upon the cation concentration of the solutions. Potassium and sodium polyacrylates are especially preferred and furthermore have the advantage of being available commercially because they are widely used for absorption of body fluids m hygienic disposable products, e.g. baby diapers, sanitary napkins, adult incontinence proαucts and the like.

Super absorbent polymers that are acrylate polymers are normally cross-linked during the manufacturing process. Any cross-linking referred to herein is m addition to cross- linking that may have occurred in the processes for the manufacture of the polymer.

The super absorbent polymer may be virgin polymer, but it is particularly intended that the super absorbent polymer would be such polymer that has been recovered from another process, one example of which is recovery from used disposable diapers or other absorbent sanitary paper products, also referred to herein as personal care products, during processes for recycling and recovery of components of such processes for future use.

In embodiments of the invention, dry solid sodium polyacrylate is swollen in water, using ratios of about 1:25 to 1:300 of sodium polyacrylate: water. At ratios below about 1:75, the gel absorbs essentially all of the water, and dry ionic solids or even slightly soluble molecules such as calcium hydroxide or water-soluble organic molecules such as urea may be added directly to the gel. The solids essentially dissolve in the bound water in the gel, which undergoes extensive deswelling.

As a result of inherent water absorbent properties, superabsorbent polymers tend to swell on contact with water. The super absorbent polymer, after treatment for use in the soil additive of the present invention, preferably has an absorption capacity index (ACI) that is in the range of about 4-50, especially in the range of about 10-45. As noted above, ACI is defined as: (wt of water saturated gel polymer - polymer dry wt) /polymer dry wt . The measurement of ACI is described herein.

The ACI of an anionic super absorbent polymer, such as the polyacrylate polymers, may be decreased by cross- linking of the polymer with cations. Examples of chemical compounds that may be added to the aqueous solution to effect cross-linking include soluble salts of at least one of an alkaline metal, an alkaline earth metal, aluminum, copper (II), iron (III) and zinc. Examples of such salts include calcium chloride, calcium nitrate, dicalcium phosphate, tricalcium phosphate, magnesium chloride, magnesium nitrate, magnesium sulphate potassium nitrate, dipotassium phosphate, superphosphate, disodium phosphate, barium chloride, barium nitrate, disodium phosphate, trisodium phosphate, sodium nitrate, aluminum sulphate, aluminum nitrate, zinc sulphate and zinc nitrate. In addition, salts of ammonium ions, e.g. diammonium phosphate, triammonium phosphate and especially ammonium nitrate may be used. Calcium hydroxide may also be included to aid in deswelling the SAP. In preferred embodiments of the present invention the cation used is potassium, calcium or ammonium or a combination of these cations, and the anion is nitrate. The amounts of cross-linking agent and growth- promoting additive are adjusted so that the absorption capacity index (ACI) of the super absorbent gel polymer is preferably in the range of about 4-50, as indicated above. This is substantially less than ACI typically characteristic of super absorbent polymers, which is substantially above 100.

The particulate gel super absorbent polymer that has been treated as described herein is separated from the aqueous solution and subjected to drying procedures, preferably in a heated air stream at about 60°C or lower. In embodiments, drying is allowed to proceed until a hard solid of about 1-10% moisture content is obtained, which is then ground to size for adding to soil.

The growth-promoting additives that may be used herein include the nitrate and phosphate compounds mentioned herein as cross-linking agents, as such compounds may function as both cross- linking agents to deswell the SAP and as growth-promoting agents. Urea is another growth-promoting additive that can be incorporated into the super absorbent gel matrix to produce a delayed release fertilizer. Urea is a water soluble organic compound that is slightly basic. However, it is not cationic, and thus it does not deswell anionic super absorbent polymer gels. As will be shown in an example, addition of urea to a SAP gel, that was previously treated with an inorganic compound such that the SAP gel had an ACI in the 5 to 50 range, results in absorption of the urea into the SAP gel, dissolving in the bound water within the gel. There is no further deswelling with this treatment, and it is believed that all of the urea is retained within the gel. The preferred ionic deswelling agents are compounds that contain ions beneficial to plant growth, e.g. ammonium, potassium, nitrate, phosphate etc. It is preferred that the resulting dry solid have a controlled reswelling characteristic i.e. the first expansion in the presence of water is moderate and subsequent wetting with pure water brings on an increase in swelling over several cycles. This behavior may be achieved with divalent ions such as calcium or magnesium. Thus, for example, calcium salts can be used with ammonium salts. Relatively insoluble calcium compounds can be used e.g. calcium hydroxide. Water soluble organic compounds may be introduced into the gel network either before or after the deswelling agent is added. In embodiments, sufficient urea is added, for example, to yield a final product with more than 32% nitrogen. In other embodiments, when the sodium polyacrylate :water ratio is in the range of 1:30-50, urea dissolves in the bound water in the gel with little or no deswelling of the gel. A high concentration of urea in the sodium polyacrylate increases its rewet ACI. By incorporation of high urea concentration in the gel structure it is possible to achieve nitrogen levels not achievable using super absorbent polymer polymerization processes with use of nitrogen containing salts. For processes for recovery of SAP after use, m which it is swelled and mixed with other ingredients, for example, in a process for recovery of components of soiled hygienic disposable products, such as that described in the aforementioned PCT application WO

92/07995 of M.E. Conway et al , a multi-step process as described above has certain advantages. The first treatment, using the aforementioned cross-linking agents, reduces the ACI of the swollen gel to a desirable level for its separation from the other recycle products.

Introducing fertilizer ingredients at this point in such a process causes a significant amount of the fertilizer ingredients to be lost since it remains in the slurry from which the dewatered gel SAP is recovered. Also, the process waste water would likely be unacceptably high in nitrogen for most municipal sewage treatment operations, if nitrogen is part of the fertilizer composition. Multivalent, low pH salts are disclosed in the aforementioned PCT application of M.E. Conway et al . that do not create sewage disposal problems. Alternatively, basic compounds such as calcium hydroxide or calcium carbonate may be used to dewater the gel SAP.

While a growth-promoting additive may be added in a separate step, it is preferred that the treatment and formation of the particulate form of the super absorbent polymer and addition of the growth-promoting additive be carried in one step by utilizing a cross-linking agent that is in itself also a growth-promoting agent. Nonetheless, it is to be understood that for practical reasons it may be necessary to utilize two or more steps to effect deswelling and incorporation of a growth- promoting additive. As an example, use of ammonium nitrate for both deswelling and as growth-promoting additive may require the use of environmentally- unacceptable amounts of ammonium nitrate. If the growth promoting additive has nitrogen, phosphorus and potassium components, it may be preferable to utilize a three-step process to formulate the soil additive.

The soil additive of the invention is added to soil, for instance by using techniques typically used for the addition of fertilizers to soil. The amount of soil additive added to soil may be varied over a wide range of concentrations. Nonetheless, a concentration of soil additive that is sufficient to effect promotion of growth of plants within the soil but not substantially in excess of such a concentration should be used, for practical reasons. For example, typical concentrations may be in the range of 0.05-0.5% based on the dry weight of soil, with a preferred range of 0.1-0.4%, although it should be understood that the concentration to be used will depend on the concentration of the growth-promoting agent used, the soil composition and the type of plants grown. The invention discloses a novel way to provide a growth-promoting additive that is released slowly over time into soil as the soil undergoes alternate wet and dry periods. As these growth-promoting additives are released, they are in an environment of relatively high moisture content which surrounds each gel super absorbent polymer particulate. As the soil dries out, the zone surrounding the super absorbent particulate better retains its moisture and this zone also has a greater concentration of the growth-promoting additive. Thus, it is believed that a more desirable environment is created for the plant roots in these zones. The particulate of this invention undergoes an increase in absorption capacity index when subjected to alternate wet and dry cycles; therefore, additional amounts of growth-promoting additive are released over time to the soil for absorption by plant roots. For this mechanism of diffusion of growt -promoting additives out of the SAP particulate into the soil to take place over repeated wet and dry cycles, it is necessary that the SAP not swell to its maximum extent when it is first placed in water. On the other hand, it is believed that some increase in swelling must occur over time in order for the growth- promoting additives to diffuse out of the particulate.

The absorption capacity index (ACI) test used herein was as follows: l.Og of the dried particulate product was placed in 200 ml of water for a period of time. The resultant gel was collected on a fine mesh screen and the weight of the gel was measured, from which the ACI value was calculated. The procedure was repeated, after discarding the water not absorbed in the gel, using a further 200 ml of water and the ACI value was recalculated. This procedure was repeated for 5 or more cycles. This testing cycle was used as a simulation of the moisture behaviour found in soil. For instance, under wet soil conditions, where there is runoff and/or loss to the water table in the soil, the SAP should experience swelling similar to immersion in water. As the soil dries out, water diffuses out of the SAP along with trapped salts and it will reach the moisture content measured in the "gel" state. Nonetheless, it is understood that in actual conditions in a soil, further soil drying will also reduce the water content of the SAP, but this loss will be influenced by the osmotic forces developed in the soil . This was deemed to be outside the scope of measurement in laboratory tests used to assess the present invention. The present invention is illustrated by the following examples.

Example I Table 1 contains a summary of the composition and properties of a series of formulations of super absorbent polymer compositions. The same super absorbent polymer source, Stockhausen Favor "Fam" sodium polyacrylate, was used in all of the formulations in the Table. The columns showing water ratios list the solid/water ratio for each of the ingredients. A value of zero indicates that the solid was added as a dry salt. When two or more salts were added sequentially, with filtration and collection of the gel between addition of the salts, the second salt is shown in the column "SLT 2". When two salts were added without an intermediate filtration of the gel, both salts are listed in "SLT 1" along with their respective weights.

The footnote to the Table shows the names of the compounds that were used. ACI values were measured as described herein. "Rewet ACI" shows the ACI values of the formulations after having been rewet 1, 2, 3 or "n" times .

Table 1

Formulations and Properties

N

13. 30. 29.

Table 1 (Cont.)

Formulations and Properties

Water Ratios

Run WTR/ WTR/ WTR/ SAP GEL SAP SLT 1 , SLT2, REWET ACI %

No. SAP SLT 1 SLT 2 Wt, g ACI Wt ACI t ACI

1 2 3 N

20 150 0 0 10 COH, 6* 5 U,20 5 30 22 253 80 92 ,103

29 50 59 8.2 35 55

6.1 55 42 54 33 2 9/4.3 14

27 50 78 4/200 9.3 32 51 56 30. 4.3 6/30 2 6.3 24 29

3/50 3 40 65 77

Codes :

AN - Ammonium Nitrate CN - Calcium Nitrate COH Calcium Hydroxide CS - Calcium Sulfate

KN - Potassium nitrate

KP - Potassium Phosphate U - Urea Sug - Sugar Woods - Woods ^,M Rooting Compou

The first step in the procedure used in the 'Runs of Table 1 above was to dissolve the SAP in water. In Runs 1-12 and 14-23, large quantities of water (150:1 to 300:1) were used so that there was excess of water i.e. free water, present with the swollen gel. In Runs 24-34, lower water quantities were used (20:1 to 35:1) . In these Runs, the gels obtained were semi -solid i.e. there was not free water present with the gel . Run 13 was a run in which granular SAP was added to a higly concentrated calcium nitrate solution, without addition of water.

The quantity of water present with the gel during the subsequent deswelling by ionic salts affected salt concentration, which in turn influenced the ACI values that were obtained. Thus, the ACI values recorded in Runs 1-12 and 14-23 were greater than those obtained in Runs 24-34.

In a number of Runs, the composition had water : calcium nitrate in a ratio of 1:0.6. The runs with the higher water content viz. Runs 12, 17, 19, 23, showed higher ACI values than the Runs with the lower water content viz. Runs 24, 25, 27 and 34.

The data show that reversing the order of addition of two salts can affect polymer reswelling characteristics and the amount of nitrogen in the final product (see for example Runs 15 and 17) .

When urea used in Run 17 was substituted with sugar (Run 19), the rewet values were similar. These organic and essentially nonionic compounds appear to sterically hinder the collapse of the gel network and subsequent crosslinking of the SAP by the calcium ion during drying. As a result, on rewetting a larger amount of water is absorbed.

The anion present in the calcium salt also affects rewet characteristics. A large hydrated ion reduces crosslinking which results in higher rewet values after the treated SAP has been dried. For instance, Run 1 used calcium nitrate whereas Run 5 used calcium hydroxide. The initial rewet of Run 1 is low, but it increases with repeat cycles. In contrast, in Run 5, use of calcium hydroxide resulted in a treated SAP that essentially does not reswell. Similarly, in Run 16 a high concentration of calcium nitrate (15 g of calcium nitrate with 10 g SAP) after addition of 40 g of urea, showed an initial rewet with a low ACI (3) , but on subsequent rewet cycles the ACI value increased significantly.

Example II

Two compositions from Table 1, Runs 16 and 31, were investigated for their retention of nitrogen compounds, ammonium ions and nitrate ions, when the compositions were immersed in water. The procedure used was to place 2.0 g of each composition in 400 ml of water and then analyze the water from each composition for ammonium and nitrate ions after various periods of time.

The samples of composition in water were stirred at a moderate level using a Sybron N/4 stirrer. Ion concentrations in Table 2 are reported in mg/L. Diffusion of Ions into Solution Over Time

Table 2

Time, min Run 16 Run 31

NH₄ ⁺ N0₃ ^" NH N0₃ ^"

1 1.0 90 8.6 20.6

10 0.3 12.8 9.7 44.7

1000 8.6 24.9 14.9 49.7

Ion diffusion out of the gel of Run 16 is lower than the diffusion from the gel of Run 31, which could be expected from its lower ACI value. Also, diffusion of the nitrate ion from the gels is greater than ammonium ion diffusion. Even after 1000 min of stirring, the nitrogen remaining in the gel of Run 31 is estimated at over 90% based on a total nitrogen measurement of the original compound.

Example III

The product of Run 34 was used in a plant trial to determine the effect of the superabsorbent polymer product on plants, and especially on the roots of plants.

"Red Robin" tomato seedlings were grown in a 50/50 spagnum moss/vermiculite medium in 6" pots containing the product of Run 34, using 250g of medium and lOg of the treated SAP of Run 34. After a period of 45 days, the test was terminated.

It was found that the plants had an average fresh weight of 282g. The sphagnum moss/vermiculite medium was examined at the end of the trial and a number of swollen SAP gels of >2mm diameter were observed. Most of the gels were penetrated by roots and in some cases the roots also exited i.e. passed right through, the gels.

It was concluded that the roots were not adverse to the presence of the treated SAP product, and actually sought out the nutrients and water absorbed within the product. It is believed that the roots have the capability of extracting nutrients such as nitrate, phosphate and potassium ions directly from the treated gels of the SAP product.

Example IV

A series of compositions of super absorbent polymers and plant growth promoting additives is shown in Table 3. The method of preparation of each composition is given, together with the ACI of the product prior to 5 drying. "Favor ' Sodium polyacrylate polymer, FAM type, from Stockhausen was used as the starting material for all compositions .

Table 3

Sample No. Preparation

1 45 g NH₄N0₃ in 1 liter water added to 20 g. SAP in 3 liters water followed by 5 g Ca(N0₃)₂ in 500 ml water. Gel was filtered (ACI = 35.6) and dried.

2 12.5 g Ca(N0₃)- granules added to 20 g SAP in 3 liters water, Gel collected (ACI = 34.6) Add 20 g NH₄ N0₃ to gel. Collected dewatered gel (ACI = 5.0) and dried. 3 20 g SAP granules added to 80 g Ca((N0₃)₂ in 100 ml water. Collected gel (ACI = 1.1) and dried. 4 50 g Ca((N0₃)₂ in 1 liter water added to 20 g SAP in 3 liters water. Gel collected (ACI = 2.4) and dried.

5 20 g Ca(0H)₂ in 200 ml water added to 10 g SAP in 3 liters water. Granular material collected and dried. Note: Samples 1, 2, 4 and 5 are Runs 3, 12, 1 and 6 respectively of Table 1.

The rate and degree to which these products reswell in water are shown in Figure 1. The ACI vs Water Treatment is also shown for the SAP polymer used in preparation of the above compositions, identified as untreated SAP in Figure 1, which is believed to be typical of polyacrylate polymers used in hygienic disposable products. Measurable swelling in water is very rapid, taking place in less than one minute. An unmodified commercial sodium polyacrylate polymer would not be a satisfactory medium for controlled, delayed release of growth promoting substances in soil.

Samples 1-4 when placed in water, for the ACI test, show a markedly slower rate of swelling than the untreated SAP. The rate and degree of reswelling correlates with the ACI values shown in Table 1. Sample 3 had a very low (1.1) ACI which is attributed to its method of manufacture. In the preparation of the sample, granular SAP rather than water swollen SAP, was introduced into a highly concentrated Ca(N0₃)₂ solution and it is believed that the SAP never became fully hydrolyzed.

Sample 5 did not swell or take on gel characteristics after repeated ACI tests. This formulation would not be a suitable candidate for slow release, into soil, of a growth-promoting additive because it is believed that diffusion of trapped ions

(nutrient) out of the composition would be insufficient to benefit plants. However, lower concentrations of Ca(0H)₂ combined with SAP did yield products that reswelled in water.

Sample 5 demonstrates that sodium polyacrylate polymer has a high affinity for Ca" ion since it absorbs this ion from dilute solution causing most of the calcium hydroxide, which has a low solubility product, to dissolve. The calcium ion concentration in Sample 5 is actually lower in the SAP than it is in Sample 4, yet Sample 4 shows greater reswelling properties in water. Its resistance to reswelling is attributed to steric effects. In the case of Sample 4, the presence of the larger nitrate anion in the gel vs the hydroxyl anion in Sample 5, is believed to prevent the SAP network from condensing as much during drying and it is more amenable to subsequent swelling in water. This test result demonstrates that difficultly-soluble salts can be incorporated into water swollen SAP in appreciable quantities. Thus, it is believed that growth-promoting substances with low solubility can be incorporated into the gel structure if they have an ionic character.

Example V The SAP used in preparation of the samples of Example IV was treated as in Table 3. 70 g Ammonium nitrate in 1.5 liters of water was added to 30 g SAP in 6 liters of water to give a gel ACI of 45 (Run 3 of Table I) . This sample was coded SAP/AN. The composition was tested as a soil additive, and compared with the use of each component separately as a soil additive viz. the use of each of sodium polyacrylate and ammonium nitrate as a soil additive, as well as addition of a mixture of SAP and ammonium nitrate in admixture i.e. in which the ammonium nitrate was merely admixed with the SAP but not absorbed therein.

Two week old "Orangeade" Marigold (Burpee) transplants were grown in 500g of growing medium in 16 oz

(473ml) plastic containers under experimental conditions shown in Table 11. Two soil types were used: 100% garden soil (S) and a 50/50 soil/sand mix (SS) . Two levels of water application were employed in each watering cycle: High (H) and Low (L) . All soil treatments were given equivalent water volumes in each cycle but the amount of water did not remain constant throughout the cycles because of different degrees of soil dryness and the amount of water transpired by the plants as their root mass changed. Thus soil treatment was studied under four conditions: high and low water addition in soil and high and low water addition in the soil/sand mix.

Table 4

Run Treatment Soil Water Soil Reci;

No. Code Type Addition SAP AN SAP/AN

35 SAP S H 1.5

36 SAP SS H 1.5

37 SAP s L 1.5

38 SAP SS L 1.5

3399 SSAAPP//AANN sS HH 2.25

40 SAP/AN SS H 2.25

41 SAP/AN s L 2.25

42 SAP/AN SS L 2.25

43 CONTROL S H

44 CONTROL SS H

45 CONTROL s L

46 CONTROL SS L

47 SAP+AN s H 1.5 0.75

48 SAP+AN SS H 1.5 0.75

49 SAP+AN s L 1.5 0.75

50 SAP+AN SS L 1.5 0.75

51 AN s H 0.75

52 AN SS H 0.75

53 AN S L 0.75

54 AN

s L 0.75

In the Table, Control refers to samples that were watered but which were not treated with SAP and/or AN in any form. SAP/AN refers to the modified SAP of Example III i.e. samples of the invention, whereas SAP+AN refers to addition of the SAP and AN in admixture only.

Plant height was measured over time. Table 5 shows the average results from the four testing conditions applied to the five soil treatments over the indicated period of time.

Table 5

Run No. Test Condition Growth (cm) Growth (cm

Three Weeks Four Weeks

55 SAP 13 19

56 SAP/AN 23 30

57 CONTROL 14 18

58 SAP+AN 18 26

59 AN 18 28

The marigold plants grown in containers with the treatment of the present invention, SAP/AN, showed the greatest growth. This growth was superior to use of the known fertilizer, ammonium nitrate (AN) or to the use of an admixture of SAP and AN. The treatment of SAP alone at this concentration, 0.03 weight %, resulted in plant growth that was indistinguishable from the untreated control.

Example VI

"Aztec" Hybrid Sweet Corn (Ferry Morse) kernels were germinated directly in a 50/50 soil sand mix in 16 oz plastic containers. Three modified SAP samples of Example 1, in which ammonium nitrate and calcium nitrate were added, were compared with: (1) a treatment consisting of separate additions of granules of super absorbent polymer and ammonium nitrate, [coded SAP + AN] , and (2) untreated soil controls. The treatments are shown in Table 4. Table 6

Run Relative

No. Treatment Plant Size

60 SAP+AN (0.26g SAP, 0. 1 5g AN); No Leaching 20

61 SAP/AN (0.4lg SAP/AN) ; No Leaching 1

62 Control; No Leaching 9

63 SAP/AN+CN (0.4lg SAP/AN+CN) ; No Leaching 3 64 SAP/CN+AN (0.4lg SAP/CN+AN) ; No Leaching 8

65 SAP+AN (0.26g SAP, 0.15g AN); Leaching 13

66 SAP/AN (0.4lg SAP/AN) ; Leaching 5

67 Control; Leaching 10

68 SAP/AN+CN (0.4lg SAP/AN+CN); Leaching 2 69 SAP/CN+AN (0.4lg SAP/CN+AN); Leaching 4

70 SAP+AN (0.26g SAP, 0. 1 5g AN); No Leaching 14

71 SAP/AN (0.4lg SAP/AN) ; No Leaching 12

72 Control; No Leaching 16

73 SAP/AN+CN (0.4lg SAP/AN+CN); No Leaching 6 74 SAP/CN+AN (0.4lg SAP/CN+AN) ; No Leaching 18

75 SAP+AN (0.26g SAP, 0. 1 5g AN); Leaching 15

76 SAP/AN (0.4lg SAP/AN) ; Leaching 11

77 Control; Leaching 17

78 SAP/AN+CN (0.4lg SAP/AN+CN); Leaching 7 79 SAP/CN+AN (0.4lg SAP/CN+AN); Leaching 19

In the Table, "Leaching" indicates that the amount of water added in each cycle was sufficient to cause drainage from the container and salts were leached from the soil, whereas "No Leaching" indicates that the amount of water added was insufficient to cause drainage from the container. The plants were compared and rated for overall size, (1 for largest and 20 for smallest) after 6 watering cycles, at which time they were becoming rootbound .

The average of the four measurements for each treatment indicates that modified SAP, when its ACI is above 3.0, results in increased corn growth. The results, in which the lower number indicates better results, were as follows: Run No . Treatment Avg . Size Rating

80 SAP/AN+CN 4 . 5

81 SAN/AN 7 . 3

82 SAP/CN+AN 12 . 3

83 No Treatment 13 . 0

84 SAP+AN 14 . 3

The growth results from the five treatments fall into two groups. Plants grown with modified SAP treatments, SAP/AN+CN and SAP/AN, were substantially more vigorous than those grown with the other treatments. At the low concentration of the treatments used in this test, the treatments SAP+AN and SAP/CN+AN were little different from the Control. In the case of the SAP/CN+AN treatment this is attributed to its low, 3.0, ACI value which indicated it did not, under the test conditions used, provide sufficient moisture and nutrients to the plants .

Example VII

In this Example, a series of experiments were conducted to illustrate the effect on gel SAP properties of the order of addition to gel SAP of an ionic nitrogen containing compound (calcium nitrate) and a covalent organic nitrogen compound (urea) . Two concentration levels of the nitrogen- containing compounds were used. In particular, the effect of the order of addition and compound concentration was determined for: (i) the ACI of the gel SAP, (ii) the nitrogen concentration in the dried SAP products, and (iii) the reswelling characteristics of the dried SAP products.

In Step 1 of each experiment, 10.0 g of Stockhausen "Favor' Fam type sodium polyacrylate were mixed with 1.5 liters of water. The resulting gel had no free water present. After about 30 minutes, the first nitrogen containing compound was added. After a further one hour, the dewatered SAP gel was filtered from any free water.

The gel obtained was weighed and the ACI calculated.

In Step 2, the other nitrogen containing compound was added to the gel from Step 1. The gel was filtered from any free water present . The gel was then weighed and the ACI calculated. Subsequently, the gel was dried at about 770°C.

Measurement of the nitrogen content of the dried gel was made using the Dumas test. Rate of reswelling in water was measured using the ACI test procedure described above .

Further details and the results obtained are summarized in Table 5 and Figure 2. The data in Figure 2 was obtained in the same manner as that in Figure 1 Note that Runs 85-88 correspond to Runs 15-18 of Table 1.

Table 7

Run No . 85 86 87 88

SAP, g 10 10 10 10

1st nitrogen

Compound (g) U* 20 U 40 CaN* 6 CaN 15

gel ACI, first treatment 152 154 26

2^nd nitrogen Compound (g) CaN, 6 CaN, 15 urea, 20 urea, 21**

gel ACI, 2^nd treatment 24 27

Nitrogen content , dried gel % 13.4 NM*¹ 30,4 29.1 * CaN = calcium nitrate

U = Urea ** This is the maximum quantity of urea that would dissolve in the gel SAP. *** Not measured.

These results show that gel deswelling only occurs when the calcium ion is added. With Samples 87 and 88, there was no water loss in the second treatment, so the gel weight increased by the weight of the urea added, and therefore the ACI value increased.

As urea does not dewater the gel SAP in the second processing step, all of the urea remains in the SAP after it is dried. Samples 87 and 88 showed retained nitrogen levels of about 30% which indicates urea concentration in the dried final product of 65% by weight.

The reswellability of sample 88, (see Figure 2) is in contrast to Sample 86 when its low ACI after the second treatment is considered, ACI=5 c.f. ACI=3. It is believed that Sample 88 contained so much urea that in the final dry state of the SAP the calcium ions were not as effective crosslinkers as in earlier tests where the anion concentration within the deswollen gel SAP was lower. Sample 88 has the added feature, vs sample 87, of lower drying costs because there is less water to evaporate from the gel .

Claims

CLAIMS :

1. A soil additive comprising a super absorbent polymer and a growth-promoting additive, said super absorbent polymer being a polyacrylate and being in the form of a particulate and said growth-promoting additive being absorbed into the super absorbent polymer particulate, said super absorbent polymer containing growth-promoting additive having an absorption capacity index in the range of about 4 to 50, where absorption capacity index is defined as: (wt of water saturated polymer - polymer dry wt) /polymer dry wt .

2. The soil additive of Claim 1 in which said super absorbent polymer has been treated, when it is in a swollen aqueous gel state, with a composition, a major portion of said composition being inorganic compounds and at least part of said composition being growth-promoting additive, said treatment with said composition effecting shrinkage of the super absorbent polymer such that the absorption capacity index of said super absorbent polymer containing growth-promoting additive is in the range of about 4 to 50, where absorption capacity index is defined as : (wt of water saturated polymer - polymer dry wt) /polymer dry wt .

3. The soil additive of Claim 1 or Claim 2 in which growth-promoting additive is urea or a nitrate.

4. The soil additive of Claim 1 or Claim 2 in which the growth-promoting additive contains at least one of nitrogen, phosphorus and potassium.

5. The soil additive of Claim 1 or Claim 2 in which the growth-promoting additive contains at least one of nitrate, phosphate and potassium.

6. The soil additive of Claim 1 or Claim 2 in which the inorganic compound is calcium nitrate, the growth promoting additive is urea, the ratio of urea to super absorbent polymer is 0.5-3 by weight and the ACI of the super absorbent polymer containing urea is in the range of 2-30.

7. The soil additive of any one of Claims 1-6 in which the super absorbent polymer is sodium polyacrylate.

8. The soil additive of any one of Claims 1-7 in which the super absorbent polymer is recycled super absorbent polymer.

9. The soil additive of any one of Claims 1-8 in which the super absorbent polymer has been separated from a process for recovery of components from personal care products .

10. The soil additive of any one of Claims 1-9 in which the absorption capacity index of the superabsorbent polymer is in the range of 10-45.

11. The soil additive of any one of Claims 1-10 in which the superabsorbent polymer has been treated with a soluble salt of at least one of an alkaline metal, an alkaline earth metal, aluminum, copper (II) , iron (III) and zinc.

12. The soil additive of any one of Claims 1-11 in which the superabsorbent polymer has been treated with a composition comprising salts of ammonium ions.

13. The soil additive of any one of Claims 1-12 in which the superabsorbent polymer has been treated with a composition comprising at least one cation selected from potassium, calcium and ammonium, with nitrate as anion.

14. The soil additive of any one of Claims 1-13 in which the additive is added to soil in an amount of 0.01- 0.5 percent by weight.

15. A method of forming a soil additive comprising a super absorbent polymer and a growth-promoting additive, said super absorbent polymer being a polyacrylate and being in the form of a particulate and said growth-promoting additive being absorbed into the super absorbent polymer particulate, said method comprising the steps of treating super absorbent polymer in a swollen aqueous gel state with a composition, a major portion of said composition being inorganic compounds and at least part of said composition being growth-promoting additive, said treatment with said composition effecting shrinkage of the super absorbent polymer such that the absorption capacity index of said super absorbent polymer containing growth-promoting additive is in the range of about 4 to 50, where absorption capacity index is defined as: (wt of water saturated polymer polymer dry wt) /polymer dry wt , and separating said soil additive.

16. The method of Claim 15 in which super absorbent polymer is treated sequentially with said inorganic compound and said growth-promo ing additive.

17. The method of Claim 15 or Claim 16 in which the super absorbent polymer is sodium polyacrylate.

18. The method of any one of Claims 15-17 in which the super absorbent polymer is recycled super absorbent polymer.

19. The method of any one of Claims 15-18 in which the super absorbent polymer has been separated from a process for recovery of components from personal care products.

20. The method of any one of Claims 15-19 in which the absorption capacity index of the superabsorbent polymer is in the range of 10-45.

21. The method of any one of Claims 15-20 in which the superabsorbent polymer has been treated with a soluble salt of at least one of an alkaline metal, an alkaline earth metal, aluminum, copper (II) , iron (III) and zinc.

22. The method of any one of Claims 15-21 in which the superabsorbent polymer has been treated with a composition comprising at least one cation selected from potassium, calcium and ammonium, with nitrate as anion.

23. The method of Claim 22 in which the inorganic compound is calcium nitrate and the growth-promoting additive is urea.