USE OF CROSS-LINKED POLYACRYLAMIDE IN AGRICULTURE FOR
INCREASING YIELD OR REDUCING IRRIGATION

Daniel J. Wofford, Jr.
Western Polyacrylamide, Inc.
Click Here For Current Contact Information

November 8, 1989
AgFresno Farm Show, Fresno, California

Abstract

This paper summarizes Western Polyacrylamide, Inc.'s (WPI) (Hydrosource™) research in progress on agricultural uses of "gel-forming cross-linked polyacrylamide. Hydrosource absorbs up to 4OO times its weight in deionized water, and has a known life span of over seven years." It is an update of a paper presented at Penn State on 29 April 1988 for the International Conference on Stand Establishment for Horticultural Crops. What has been learned in the ensuing year and a half is described. Cross-linked polyacrylamide is shown to carry out its earlier promise of cost-effective reduction of watering requirements for irrigated crops and increase of yields with the same water application.

Researchers and farmers are slowly sorting out the proper application rates and methods by time-consuming trial and error. Major advances probably will not occur until researchers give us a much clearer picture of how polymer interacts with soil, water, minerals, fertilizer and the plant. Breakthroughs will likely occur on a crop by crop and on an area by area basis as farmers learn to adapt the polymer to their varying water, soil and crop conditions. Based on 1988 and 1989 WPI results, it appears that cross-linked polyacrylamide holds tremendous potential for American agriculture.

Research Overview

Western Polyacrylamide, Inc. is a small, low cost, high volume firm with a strong commitment to promoting research into agricultural, landscaping and other horticultural uses of cross-linked polyacrylamide. Currently WPI is involved with more than 70 research projects at 26 universities. Several are significant in size (i.e., dryland agriculture and turf in Colorado), but most are small projects designed to solve specific problems identified by researchers. Because of the wide scope of cross-linked polyacrylamide application possibilities, WPI has taken a shotgun research approach to identify as quickly as possible the most promising areas of application. When identified, we then focus on the area of application with a combination of university research and field trials by interested farmers. We are aware of several outstanding successes, and a few outright failures.

It is difficult to generalize, but it appears that cross-linked polyacrylamide may work better on irrigated crops more prone to stress damage. Application methods are crucial to success. In summary, use of cross-linked polyacrylamide is seemingly simple, but, in reality, is incredibly complex in its mechanisms. Yet, it seems destined to play an important role in the future of American agriculture as farmers are called upon to produce more with dwindling supplies of water. We are also learning not to consider "failures" as failures, but to judge them as "application and rate errors." Witness the cantaloupe paragraph below.

Banding vs. Incorporating Uniformly into the Bed

Most of our early failures in the Western United States have come from banding the cross-linked polyacrylamide into seedrows or beside the seedrow of crops with far ranging root systems (corn, soybeans, dry beans, wheat, etc.) whether irrigated or dryland. The banding method also appears to have failed at rates up to 160 lbs. of polymer per acre with canner tomatoes. We have had high success with low rates (15 lbs. per acre) when the polymer is flown on and worked into the beds. Based on testing here in the West, it appears that banding in most crops doesn't work while incorporating the polymer uniformly into the beds gives excellent yields. Still, recent reports from Penn State show that banding into cool season vegetable crops may provide excellent yield increases. The following is a review of some tests with cross-linked polyacrylamide.

Cauliflower: Dr. Michael Orzolek of Penn State reports 20-40% yield increases with fall cauliflower (Majestic variety from Sakata Seed Co.) with banded cross-linked polyacrylamide rates between 15 and 30 lbs. per acre. The six week old plants were transplanted 10 August 1989, and Dr. Orzolek notes increased growth, increased vigor, better stands, and 7-10 day early maturity.

NOTE: This is our best example of banding success in a crop with a smaller, bunched root system.

Cantaloupes: A 1988 irrigated cantaloupe test at several rates up to 100 lbs. per acre of polymer didn't provide any appreciable yield increases at Colorado State University (Rocky Ford). The various rates of polymer were applied via banding and this failure initially caused us to believe that the polymer simply did not work on cantaloupes. Yet, in 1989, a Texas A&M researcher at Uvalde obtained an approximate 2300-lb. yield increase per acre by incorporating 45 lbs. of cross-linked polyacrylamide per acre into "twin-furrow" dryland cantaloupe beds, with most of the approximate 25% yield increase being in the medium fruit.

NOTE: Another test was done at Uvalde using polymer under impervious black plastic that did not work. Yield was about the same, but the amount of small fruit increased. As an educated guess, it appears that the polymer and the black poly may do a related "capping" function in the soil.

Onions: The following results are from a farmer-monitored Spring 1989 onion test conducted near Huron, California. The six acre tract was treated with 15 lbs. per acre flown on and incorporated (disked) into the soil:

Factor Treated Untreated
Acreage 6 acres 6 acres
Yield 6166 bags 6184 bags
% Large 76% 63%
% Medium 24% 37%
Total Sales $76,517 $73,176

The treated six acres emerged early, and an estimated 15% of the total treated onions were killed by a cold spell that saw temperatures dip below freezing for five nights in a row. Since the untreated onions had not emerged, the control field did not suffer any freeze loss. Because the price per bag for large onions at time of harvest was $l4 as opposed to medium at $6, the treated field yielded a profit increase of $446 per acre (!) despite being outproduced in total numbers of bags. Here the 5-day freeze complicates the assessment, as one could make the case that the thinning by the freeze contributed in some way to the increase in percentage of large onions from 63% to 76%. Still, since we see larger fruit because of cross-linked polyacrylamide use in most vegetables, it is possible that most of the credit for the size increase is due the polymer.

Lettuce: A Rio Grande Valley lettuce test in Spring 1989 with 15 lbs. cross-linked polyacrylamide per acre rate incorporated into the beds produced 450 cartons per acre as opposed to 402 cartons for the untreated. The cartons from the treated field weighed an average of 50 lbs. and from the untreated field, 44 lbs. The treated field produced 76% USDA No. 1 lettuce as opposed to 65% USDA No. 1 from the untreated field. Given the prices in effect at the time, the treated acreage would have yielded a net profit of $186 per acre over the untreated control.

Bell Peppers: Twenty-five acres of San Joaquin Valley bell peppers treated with 15 lbs. of polymer incorporated before transplant produced 913 cartons per acre as opposed to 868 per acre for the 125 acre control. Net profit increase per acre for the treated acreage was $385 (at the $16 per carton price in effect at the time.)

Tomatoes: Incorporation of cross-linked polyacrylamide at 15 lbs. and higher per acre uniformly into tomato beds appears to result consistently in yield increases in the 15-35% range or even higher. With higher rates in gardens (i.e., 100 lbs. per acre), it appears possible sometimes to push yields beyond 50%. Despite some setbacks in recording harvests, we feel confident of both the application method and the 15-30 lbs. per acre rate for field crop tomatoes:

1.) A Fresno County, California, farmer flew on and incorporated 20 lbs. per acre (1989) on 25 sandy acres of a 160-acre canner tomato field. In previous years the farmer had never gotten full yields from the sandy field, but this year he could not differentiate between the sandy and non-sandy fields. Yields for both were in the 40 ton range; the sandy field had never before produced more than 27 tons per acre.

2.) Another farmer in the Valley applied 15 lbs. per acre on 20 of 160 acres (1989), (by air, incorporated) and produced the same yield despite a cutback of two full irrigations.

3.) A 1988 test with 15 lbs. (Gandy machines, and then incorporated) supplied a yield increase figure of 4.7 tons over the control (26 to 30.7), while observers counting length of rows to fill the 25-ton trailers calculated a yield increase of about 9 tons. Despite the yield controversy, the field yielded a wealth of observation data.

a.) The treated tomatoes were 15-20% larger with an average of one extra tomato per set.

b.) The treated field ripened 10-14 days early, (such early maturity is frequently observed with the polymer.)

c.) A petiole test done 5 weeks before harvest showed the treated field running low on fertilizers because of a failure to compensate for the extra 25-35% increase in foliage and fruit.

d.) Both fields were inadvertently stressed two weeks before harvest, with a resulting loss of about 7 acres of foliage from the control but none from the treated field.

e.) After the 1988 harvest the 40-acre field and its companion control were plowed under and leased to an unwitting lettuce grower for a January 1989 crop. When the treated lettuce reached 5", it was observed to be approximately 30% larger.

NOTE: Unfortunately, harvest yields were not recorded.

4.) The Soil Conservation District in Cimarron County, Oklahoma, reports that an 81 year old woman gardener and her son harvested 1142 lbs. from 67 tomato plants (six varieties) with cross-linked polyacrylamide.

5.) In contrast to the above string of successes with the incorporation method, two University of California/Davis Farm Advisors reported near total failures in efforts to increase yields using the banding method at rates of 0, 10, 15, 20, 30, 40, 80 and 160 lbs. of polymer per acre. But, Fresno Farm Advisor Don May got 4-ton yield increases with both 5- and 10-lb. rates banded with the fertilizer 2-4" directly under tomato seeds. He speculates that increased fertilizer storage by the polymer may be primarily responsible for the 4-ton yield increase.

Wheat, Corn, Soybeans and Dry Beans: We have several examples of the failure of the banding method for wheat, corn, soybeans and dry beans in Colorado in both irrigated and dryland tests. The tests were conducted by Colorado State University's Crop Testing Team, replicated four times, at various farms throughout the state. Rates used were 0, 5, 10, 15 and 30 lbs. of polymer per acre. There were six wheat tests (three of the original nine on six farms were lost to a severe spring freeze that saw temperatures dip below minus 25 degrees Fahrenheit for five days in a row!), three corn tests, two soybean and three dry bean tests. Three sunflower tests have not yet been harvested this fall.

All our field corn, wheat, soybean and dry bean tests that didn't show significant yield increases were banded, with the various 5-30 lb. per acre polymer applications applied directly into the seedrow. The exception was a single 65-acre (wheel line) irrigated Pinto bean field in northern Wyoming that was treated with 10 lbs. of polymer broadcasted and disked into the ground. Comparison of treated and adjacent 110-acre (pipe irrigated) fields on a total yield basis was impossible because of heavy ($8,000) antelope damage on the 65-acre treated field, but 25 treated and 25 untreated individual bean plants were selected from rows at 100' intervals across both fields to obtain representative samples. Several plants from the treated field produced 1095 beans as opposed to only 740 beans from the same number of untreated bean plants. The cost significant difference was in the number of bean pods that produced five beans per pod. For example, seven treated bean plants, selected randomly produced 104 pods with five beans each, 77 with three beans each, and 26 with two beans each (for a total of 804 individual beans). In contrast seven untreated bean plants taken from a similar soil area produced only 51 pods with five beans each, 46 with three beans and 2 with two beans each (for a total of only 397 beans.) Despite all the variables, irrigated beans appear promising, and we plan 2-3 university tests in 1990 to determine yield increases with this technique.

NOTE: On 16 January 1990 Dr. John Keenan at New Mexico State Science Center (Navajo Indian Reservation) reported a 25% yield increase in pinto beans at 30# Hydrosource/acre (none with 15#), incorporated into the beds.

Given the small Wyoming sample and what we are seeing with beans and corn grown in small gardens, we intend to duplicate all the CSU tests above in 1990, using the same parameter test rates, but incorporating the polymer uniformly into the test fields.

Combining the French Intensive Method of Gardening with Cross-Linked Polyacrylamide

Working with a large international refugee organization, we are attempting to combine the French Intensive Method of Gardening with the cross-linked polyacrylamide to produce small, inexpensive, high yield, 1000 square foot garden plots for refugees. Sample garden plots grown in the Denver area this past summer show that 50-100% yield increases may be possible with 2-5 lbs. of polymer incorporated per 1000 square feet (85-215 lbs. per acre). These estimated yields are being gotten with significant decreases in watering, (i.e., once every four days instead of two days.) Crops showing the most obvious yield increases are tomatoes, broccoli and sweet corn. Because of unusual growth activity from sweet corn treated with very high polymer rates, we are describing our observations in detail below:

Dicotal vs. Monocotal Plants

The dicotal plants in the garden (tomatoes, broccoli, etc.,) get larger with correspondingly greater yields without obvious basic changes in plant structure. Still, we are observing unusual changes in sweet corn, the only monocot being grown in these small gardens. In a 1988 garden test without controls, 305 sweet corn plants (variety not known, but believed to be a hybrid) were planted with polymer at a rate of 100 lbs. per acre spread directly into the 1 1/2"-deep furrow with the seed at time of planting. The corn produced 3 1/2'-tall stalks with an average of almost four full ears per stalk in a garden where 6' stalks with 1-2 ears are the norm. Less than 1% of the corn had suckers and these were too short and immature to produce corn.

In 1989 no new polymer was added, but the garden was rototilled and corn seed saved from the 1988 ears for planting. This time considerable suckering occurred, with most plants producing 3-4 suckers per plant to the slightly taller primary stalk. Yields were equal or better than last year's average of almost 4 ears per stalk because each sucker produced a full ear.

To explain the high yield from the suckering, one horticulture expert suggested the sweet corn might be losing its apical dominance because of the following:

1.) The apical buds no longer produce sufficient growth hormones that prevent the sucker production.

2.) The effect of the growth hormones is being nullified by some factor at the sucker buds.

He also suggested that the expanded water supply available from the polymer to the corn plant could be causing significant increases in mycorrhizal activity, a phenomenon already observed in polymer treated tree seedlings (bare root dipped only) by the U.S. Forest Service's Institute of Mycorrhizal Research. With the resulting large increase in water and nutrient availability, it is possible that this somehow aids the basal suckers in breaking the apical dominance.

We know that corn flooded excessively will sometimes begin suckering due to the production of ethylene during anaerobic respiration, so we also could be creating the same effect due to the unusual amount of water made readily available by the polymer to the hair and side roots.

We witnessed the same suckering with high yields in a garden some 17 miles to the west that was planted circa 1 April 1989 with two rows of honeycomb hybrid sweet corn (Northup King.) Polymer was applied directly into the seed row at the rate of perhaps 50-100 lbs. per acre (rough estimate) at time of planting. The gardeners (Ron and Lougene Baird) reported the following results and the test plots were photographed:

"The [Honeycomb] corn planted with the Hydrosource cross-linked polyacrylamide suckered heavily; three to four producing suckers were not uncommon. The control crop had very few immature suckers falling primarily into the 2-3' range; while the control crop exceeded six feet, its normal height. Wind damage was suffered by the taller control crop, but no wind damage occurred in the shorter treated row. The control crop had its traditional one ear per stalk of corn--with only a few immature suckers, none of which had mature corn.

Still, the polymer treated corn regularly produced two ears per main stalk and sometimes three. Each sucker produced at least one mature ear of corn the size of those on the main stalks. Therefore, considering that the entire plant--averaging 3-4 stalks-- came from one seed, the yield from the polymer-treated rows averaged 4-6 ears of corn against one ear produced by the untreated control corn plants. One treated corn plant produced a total of eight ears--two on the main stalk and one ear from each of six suckers!"

The Bairds also reported significant decreases in watering at the 6400' elevation garden site 20 miles SW of Denver. Watering was identical for both treated and control. The treated rows were planted during the first week of April and the control on 1 May 1989. The Bairds have grown Honeycomb sweet corn at the same garden site for 6-7 years. (Author's note: Sweet corn often puts out one or two suckers at Eastern Colorado garden sites, but the suckers seldom produce mature ears.)

Tree Planting for Agriculture

Three basic types of tree planting techniques are emerging:

1.) Survival planting with 1/2Oth to 1/lOth of an ounce, plus bare root dipping. Cost per seedling is 2-5 cents.

2.) Accelerated first year growth with 4-8 ounces per seedling, sometimes eliminating drip irrigation. Cost per seedling is approximately $1-2.

3.) Significant long-term water storage where polymer is used to store 25-250 gallons of water, both as a primary system and as a large reserve for drought periods. Cost is roughly $1 per 6-8 gallons of water storage capacity.

Survival Plantings: Survival planting with modern cross-linked polyacrylamide is now entering its sixth year in the Rocky Mountains, and an estimated 500,000-1,000,000 seedlings are planted annually in Colorado alone. Spreading primarily by word of mouth from satisfied Forest Service, Soil Conservation and private landowner customers, this type of inexpensive survival planting costing only a few cents per tree is rapidly expanding throughout the Rocky Mountains and into the Great Plains. Typical plantings are done with one cup to one pint of hydrated crystals mixed into the backfill, plus the bare root dipping with a slurry using a fine, pulverized version of the crystals. Since many sites may not receive enough rainfall to hydrate the crystals for several months, the Colorado State Forest Service normally recommends planting with pre-hydrated polymer.

Accelerated First Year Growth: Based on preliminary results from a few farmers, nurserymen and a researcher who have used this technique, we expect the use of 4-6 ounces of dry polymer per planting hole (where subsequent watering is possible) to become part of standard planting procedure for fruit, nut and ornamental trees in the future. In addition, this technique will sometimes allow seedling planters to eliminate drip irrigation, a prerequisite for survival in marginal areas (especially for windbreaks, ornamental trees, etc.) The following three examples illustrate this technique:

A.) Sunrise Nursery: In April 1988 Keith Rule of Sunrise Nursery (Castle Rock, Colorado) planted 1050 seedlings on his tree farm located on a 7000' elevation plateau in a 14" rainfall zone just south of Denver. Using a 24" tractor mounted augur going 24" deep, he mixed 5-6 ozs. of cross-linked polyacrylamide (enough to hydrate 6 gallons of water at his site) into the 4 cubic feet of backfill. The 1.25 cost per tree of the polymer nearly cancelled the initial cost of $1 per tree for drip emitters and hoses, and will eliminate most of the $5-8 per tree irrigation costs over the next 5-6 years from an already overtaxed 800' irrigation well. After 18 months with little more than natural rainfall, the entire 1050 trees appear to have normal or better growth than those irrigated with a drip system.

B.) Central Arizona Project: Our 1 December 1988 Bureau of Reclamation planting of 125 mesquite, false mesquite, acacia, Palo Verde and caliandra has been successful despite the worst winter in Arizona history and "a very dry summer." The polymer showed evidence that it worked better with certain species (false mesquite, acacia and caliandra.) General success rate was 60-70% as opposed to 40-50% for the untreated, but the major lesson learned was that 2 ounces of cross-linked polyacrylamide in each 18-24" deep, 6-9" augured hole is the most successful rate. The one ounce rate did not work, and the 8-, 12- and 16-ounce rates per planting hole not only swelled polymer out of the ground but also had reduced survival rates. The 4-ounce rate also worked well, but not as well as the 2 ounce trial.

AUTHOR'S NOTE: We feel the results above possibly had more to do with the ideal polymer/soil volume ratio described below than the net amount of polymer. The holes were simply not big enough for the amount of polymer used.

C.) Almonds (Kern County, California): A late January 1989 new (irrigated) almond planting treated with polymer has resulted in an estimated 20-30% increase in foliage by June 1989, but decreasing to an approximate 15% increase by November 1989. A total of 30 of the 66 treated seedlings as of 11 November 1989 showed caliper diameters more than 2" as opposed to only 17 from the 66 control seedlings. Augered planting holes were 10" in diameter and 18" deep, and rates of 4, 8 and 12 ounces of polymer per hole were used. The 12-ounce treatments are doing the best, possibly because of better polymer distribution or limited watering. Six ounces were mixed into the fluff at the bottom of the hole, and the remaining 6 ounces mixed into the backfill.

NOTE: Since the 12 ounces can hydrate more than 20 gallons at that Kern County site, it appears the irrigation of the field was not sufficient. Normally we would not recommend this high rate for such a small hole.

For harsh site plantings using this technique, we expect significant increases in polymer capability with the use of simple water catchment systems that will insure recharging of the entire 4-6 ounces of cross-linked polyacrylamide with each rain (even light rains.) Because of the limited water-holding capacity of this type of planting (4-10 gallons) and planting hole digging limitations, this is a 1-2-year system designed to promote survival and accelerated early growth depending on the type of tree.

Large Water Storage Systems: With the cross-linked polyacrylamide routinely storing in the 16-32 gallon range of water per pound of polymer, it is now possible to store almost any desired amount of water underneath a single tree or vine at an estimated cost of $1 per 6-8 gallons of storage capacity. In the past, applications were primarily limited to dug and augered holes, but the arrival of compressed-air and mud-pump injection systems has now made it possible to store large amounts of water in hydrated polymer under existing trees.

Chrysanthemums: We have evidence from a chrysanthemum test in Europe with Aquastore cross-linked polyacrylamide that a 10% hydrated polymer to 90% soil mixture produced the highest number of blooms, but the 20% hydrated polymer to 80% soil mixture produced the tallest chrysanthemum plants. Because we are seeing the same results from a wide range of tree seedling plantings in Colorado and elsewhere, we are recommending that the 10/20% to 80/90% hydrated polymer to soil mixture rule is followed as a rule of thumb until further research either verifies it for tree seedlings or results in some modification. Thus the actual rate of polymer depends on the amount of backfill in the tree planting hole. Please note that rates more than approximately 35% hydrated polymer decrease both the numbers of flowers and the height of the plants, a phenomenon we also see in tree seedlings. Because of the importance of this information, we are reproducing the chrysanthemum charts below.

Grapes: A March 1989 planting of 48 Harmony grape cuttings with 2 ounces of cross-linked polyacrylamide in shovel size holes showed 20-30% more canopy foliage, before the August 1989 cutback, but the difference between treated and untreated vines by early November 1989 had almost disappeared. Obviously the treated vines had outstripped the usefulness of the small amount (2 oz.) of polymer. A companion treatment of 48 Harmony grape cuttings with one ounce of Polymer in each hole showed little difference with the surrounding 80 acre control. The entire field received identical irrigation water and fertilizer.

Based on the above (admittedly brief) experiment, it appears that the growth pattern of grapes with cross-linked polyacrylamide may be similar to that of seedling trees for the first growing season. If the patterns are similar, then it may be possible to design heavier cross-linked polyacrylamide applications (in the 6 to 24 ounce range?) to maintain the accelerated growth to earlier maturity and higher long-term yields.

Retrofitting grape vineyards, both with rippers and large compressed-air machines, appears promising, with compressed-air injection likely to emerge as the dominant application method because it allows deeper root zone placement without adversely pruning the root system. In addition, the ability of the compressed-air injection to aerate and break up tightly compacted soil up to 2-3' deep has already been proven in the tree care industry with the Terralift, Olathe, Grow Gun and Aqualife injection equipment.

Pitts Carbonic and Ag Services, Inc. (Fowler, California) has already developed the prototype of a higher speed, tractor mounted compressed-air injection system that fractures the ground up to 3' deep and 10-15' in diameter at two locations at the same time while simultaneously injecting measured amounts of dry cross-linked polyacrylamide crystals into the underground fracture zones. Because of severe weight limitations created by pre-hydrated polymer, this compressed-air system for dry polymer injection will likely prove superior over pre-hydrated for large-scale irrigated vineyard and orchard agriculture.

Loading the crystals with nematocides, micronutrients, fertilizers, (non-toxic) insect repellents, etc., before injection also appears promising. Since each pound of dry crystals contains 65,000-75,000 crystals, a slow release mechanism is created underground to help store the desired material until it is extracted by the plant on a crystal by crystal basis.

In summary, accelerated first year tree or vine growth in the 15-40% range appears possible for carefully designed plantings with 2-3 oz. of polymer in augered or dug holes, but the plant root system generally outstrips the useful benefit of the polymer placed in the planting hole after several months or the first growing season. A major focus of future research will be to figure out whether we can maintain the accelerated growth through the 2nd., 3rd. and 4th. years, etc., theoretically translating into bigger trees with increased fruit, nuts or shade.

We propose the following test, with polymer to be injected with one of the compressed-air injection guns. By increasing the depth of the blast in measured increments, we can inject larger amounts of polymer into increasingly larger diameter areas. To get faster results we propose this test using almonds or another fast growing fruit or nut tree.

If this test works as predicted, the growth of almond seedlings with polymer only in the planting hole should peak at 6-8 months, (based on our 1989 Kern County, California almond test), and the 2' diameter area might see rapid growth continued into the 12-20-month period.

In general, the compressed-air guns for injecting polymer will fracture about 3-4' in diameter for each foot of depth the probe is inserted. At a 2' depth the fracture diameter will be 6-8' and at a 3' depth, 9-12'. The injection depth would depend on the root system of the tree being tested. Since fracturing of the soil may have considerable impact on growth, two types of controls should be included: A.) fracturing without polymer injection, and B.) no fracturing, no polymer.

The following sketch shows the fracture pattern of a typical compressed-air injection gun:

Speculation on Reasons for Polymer Success

Understanding why the cross-linked polyacrylamide gets outstanding successes sometimes but fails in others is a time-consuming process of sorting out. Meanwhile, many researchers and farmers are coming up with theories to explain their successes:

1.) Simple Water Storage: With one pound of dehydrated crystals typically absorbing 15-30 gallons of water during each rain or irrigation watering, the extra water storage undoubtedly plays a role in increased yields.

2.) "Capping effect": It appears that even distribution of the crystals (which store extra water) into 3-6" deep beds will create some type of "capping effect" that reduces the normal upward capillary action of water lost to wind and heat evaporation. This theory was initially proposed by Bill Carney, a veteran San Joaquin Valley farmer who possibly knows more about polymer use in agriculture than any other person.

Mr. Carney also suggests that polymer distributed uniformly throughout the soil in field crop beds also may have a cooling effect, a phenomenon known to occur when polymer is used at much higher rates in nursery beds and potted plants.

NOTE: The polymer used in this manner may help regulate variations in soil temperature.

3.) Fertilizer Retention: The uniform distribution of the polymer in the bed appears to stimulate root growth, thus giving the roots more opportunity to collect minerals and nutrients needed for accelerated growth. Temporary storage of NPK fertilizers in the anionic crystals also appears to retard fertilizer leaching, and the loading of NPK fertilizers into cross-linked polyacrylamide gels both to reduce fertilizer leaching and increase fertilizer efficiency has become a promising research effort at the National Fertilizer Development Center at Muscle Shoals, Alabama. (Dr. Robert Mikkelson, Ph.: 205-385-3625.)

4.) Stress Reduction: During hot days, the hair root system of a plant pulls out and depletes most of the water from the area close to the root system, thus causing the plant to go into stress. But, it appears that if the hair root system is hooked into a certain (unknown) number of crystals, the plant stress in significantly reduced.

This may explain the remarkable ability of tomato plants to use cross-linked polyacrylamide to their benefit. One researcher noted that in 1960's polymer research it was found that when tomato plants go into water stress they permanently lose 50% of their photosynthetic capability. This suggests that photosynthesis tests may be a strong indicator of which plants can best use cross-linked polyacrylamide to grow larger plants with more fruit.

The ability of cross-linked polyacrylamide to reduce stress in plants is a recurring theme in the comments of many researchers and observers. Very uniform growth with no bare patches around the edges of the field was seen in a California alfalfa field (Also yield increased at least 10% with 30 lbs. of polymer per acre flown on and disked in.)

Ron Baird (garden corn discussed above) could omit the usual treatments for corn diseases because his plants were much healthier, apparently from having avoided water stress, as well as surviving a severe frost.

Cross-Linked Polyacrylamide as a Carrier for Insect and Animal Repellents

The anionic cross-linked polyacrylamide will hydrate and temporarily hold a variety of soluble liquids, and appears to hold high promise as a carrier for nematocides, fungicides, (plant derived) insect repellents, insecticides, animal repellents and possibly even herbicides. The polymer contains 65,000-75,000 crystals per pound, and placement pattern in the general plant area governs time of release. For quick release, place the crystals in the seed row; for slow release spread the crystals throughout the seed bed to force the hair root system to seek them out one by one. The following are some examples:

A.) A Colorado State University entomologist diluted liquid garlic 50%, and hydrated the garlic solution with cross-linked polyacrylamide crystals for injection into greenhouse wheat rows. This resulted in a 50% reduction in Russian wheat aphid damage to the wheat plants. This search for plant derived insect repellents is continuing.

B.) Ani-pel is a systemic, biodegradable game repellent, containing "one of the world's bitterest substances." When pellets are placed into a tree root system, uptake is quick (3-4 weeks), and turns the entire plant bitter for up to 4 years as protection against above and below ground animal damage. We have several tests underway to figure out if the Ani-pel sufficiently alters plant taste to serve as an insect repellent as well. We're testing with Ani-pel against white flies in a Texas greenhouse, chinchbugs in Pennsylvania turf, honeysuckle aphids in Iowa, bluegrass billbugs in Colorado, pine tip moth in Oklahoma, Russian wheat aphids in Colorado, etc.

Some of these tests involve hydrating cross-linked polyacrylamide with Ani-Spray (liquid version of Ani-pel) and injecting the crystals into the roots. If this systemic, biodegradable "bittering agent" works against selected insects, then the search will begin for plant derived insect repellents that will repel insects but not alter the taste of food crops. If the Ani-pel/Ani-Spray works, then it could be retained for non-food crops.

C.) The United States Forest Service is beginning to test Ani-Spray-loaded cross-linked polyacrylamide for the growing of tree seedlings in a nursery. We do not know how long the bittering agent will remain in the polymer crystals, but the goal is to grow nursery seedlings that will be immune for at least 1-2 years to all above and below ground animal damage. Since the bitter taste from the Ani-pel tablets will last up to four years (and that loaded on crystals less time), it may prove possible to use the Ani-pel on young fruit or nut trees in areas where animal damage is a serious threat to orchard establishment.

In another development that holds high promise, Dr. Lance Meinke (Dept. of Entomology, University of Nebraska at Lincoln) is loading insect attractants and much smaller amounts of insecticides onto starch polymers (used because of quicker bio-degradation) to get crop insect kills with very significant decreases in insecticide amount uses. Kills of beneficial insects in initial tests were also reduced over spraying, causing researchers to focus on selective attractants. The USDA/ARS in Brookings, South Dakota, and Dr. Robert Metcalf (University of Illinois) are also engaged in this promising research.

NOTE: This type of research will likely split into use of two polymers: cross-linked polyacrylamide for long-term (multi-growing seasons); and starch polymers for short-term (one growing season.)

Grind Size

Time and again, we have discovered that crystal size and uniformity are very important to success. For example, "standard" (500-3000-micron crystals) and "fine" (pulverized into 0-500-micron size) are identical products, yet their properties are so different in use that we should consider them as dissimilar products. The "standard" crystals hydrate to form large, individual lumps of gel with resulting high pore space, while the "fine" material forms an amorphous, sticky mass popular as a bare root dip. The "standard" crystals are a near total failure as a bare root dip, and the "fine" can cause failure in situations where high pore space is important. We have seen several applications of "fine" or "standard" with high powder concentrations where the suspected cause of failure was the high amount of powder.

Proper Identification of Polymer in Scientific Papers

Even for a polymer expert, attempting to review the results of scientific literature published in the U.S. and abroad on the so-called "hydrophilic gels" is a very confusing, difficult issue. Seldom are the polymer products properly identified about type, and less often is screen size (a very important factor) even mentioned. A major purpose of scientific research is to create "building blocks" of data that may be used by future researchers, but this is proving difficult in the polymer field due to the confusion in analyzing past data. We suggest that several key university researchers with polymer experience devise and publish an identification code on all polymers currently used with plants. For example, the simple identification for a particular cross-linked polyacrylamide might read: "Gel-forming, cross-linked polyacrylamide. Absorbs 410 times its weight of deionized water, and X-times in a standard 'XYZ' saline water solution. Granule size: 500-1500 microns (or appropriate screen sizes)."

Working with polymer experts, the different types of polymers could be divided into basic categories or generic classes as a further identification aid both to researchers and users.

The "Eaker Beaker," a Device for Measuring Cross-Linked Polyacrylamide Performance

Salt content of both soil and water is the primary factor affecting cross-linked polyacrylamide performance in the soil. For example, one pound of dry crystals will hydrate 48 gallons of deionized water, 32 gallons in Fresno tap water, and as low as 20 gallons in certain parts of Colorado. If these figures roughly correspond to the salt content of the soil in the respective areas, this means that achievement of a specific Fresno tomato yield with 15 lbs. of cross-linked polyacrylamide per acre would require about 25 lbs. of polymer at the Colorado site. Thus a simple, easy to use, standardized field test for comparison of cross-linked polyacrylamide performance is needed.

To remedy this problem, an O'Donnell (Texas) High School researcher, Melvin Eaker (pronounced "Aker") is allowing us to take poetic license with his name as he designs the "Eaker Beaker" (pronounced "Eker Beaker"), a graduated, one quart jar and tiny "thimble" measuring system. The thimble full of polymer will fully hydrate one quart of deionized water within one hour, and varying amounts depending on the salt content of the water being tested. Fill the quart jar with water from the test site, then pour in the thimble full of polymer and let it sit for one hour. Pour off the excess water, and the remaining hydrated polymer will give you a percentage performance figure from the tested site. Since the salt content of the soil may differ considerably, we are attempting to figure out if some predictable correlation exists. If not, we may have to develop some type of leaching bucket to measure water after it has been leached through the soil from a specific site. Since many sites receive both irrigation water and natural precipitation we also will attempt to correlate the difference in the same field after these two types of watering.

Since initial laboratory research shows that higher concentrations of water soluble calcium, magnesium and iron may damage the rehydration capability of the cross-linked polyacrylamide, this is another factor that must be considered in developing the measuring device. Yet, it appears that cross-linked polyacrylamides are less susceptible to this type of damage than most other polymers.

When the "Eaker Beaker" is completed, both farmers and researchers will have an easy-to-use test by which to correlate cross-linked polyacrylamide performance between any two sites in the world. (Editor's request: if any reader has studies showing the relationship between source water salt content and water leached from the same soil, please mail a copy to: Melvin Eaker, O'Donnell High School, O'Donnell, Texas, 79351. Reader assistance will help accelerate his development of this valuable testing device.)

NOTE: The use of factory-sized small cubes of cross-linked polyacrylamide for insertion into the ground has been studied and rejected as a measuring tool, because of the continuing uncertainty in determining whether full hydration capability had been reached in the soil.

SUMMARY

The 400 X's modern cross-linked polyacrylamide was developed in 1982, but gained little popularity during its first five years due to high prices and lack of research. But, the combination of retail prices dropping below $5 per pound (in 50-lb. bags) and recent extensive research generated by WPI and others are creating the momentum needed to make this fine product practical and affordable for a wide range of uses.

AUTHOR'S NOTE: Since most of the data collected outside Colorado was gathered from researchers by telephone, the author wishes to take full responsibility for any errors in this draft paper.

Copyright 1989 by Daniel J. Wofford, Jr, and Dale Greenwood.