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Corn Starch as a Soil Amendment: Deal or no Deal?

 

By Eric Simonne, Bob Hochmuth, Lani Davis, April Warner, Aparna Gazula, Debbie Gast, and Audrey Simonne

 

Because of high water holding capacity, corn starch may become a Best Management Practice that would help reduce nutrient movement below the rootzone of vegetables. Amending a Blanton-Foxworth-Alpin complex fine sandy soil with a commercially available hydrolyzed starch-polyacrylonile graft copolymers product at rates of 0 to 0.5 and 0 to 6 g/12-kg pot did not significantly increase soil water retention.  Leachate volume and electrical conductivity were not significantly reduced by these rates of cornstarch when summer squash was planted in the pots.  Based on the manufacturer’s information, the recommended rate of 9 kg/ha (0.5 g/12-kg pot; $110/ha) banded-applied would only hold the equivalent of water delivered by a 13 min. drip irrigation event.  These results suggest that current manufacturer’s rate is too low to practically affect irrigation management of drip-irrigated vegetables.  Increasing cornstarch rate twenty fold would allow for a theoretical increase in water storage equivalent to 4 hrs of drip irrigation (greatest daily volume applied), but would require a change in pricing structure.

 

 

 

 

 

 

Corn starch in water. Note colloids float when fully hydrated

 

 

 

 

 

A buried clump of corn starch “lifted” the soil during hydration

 

 

 

 

 

 

Low rates of corn starch had no visible effect on squash growth

 

Introduction

            Increasing soil water holding capacity would help keep water in the rootzone of vegetables which could result in reduced nutrient leaching.  Provided it is economical, this strategy is particularly attractive to vegetable growers because (1) most vegetables are grown on sandy soils with water holding capacity of 8 to 10%, and (2) the development of Best Management Practices (BMP) encourages cultural methods that increase nutrient use efficiency (www.floridaagwaterpolicy.com).  Because of its low price, high water holding capacity, and biodegradability, corn starch may become a potential BMP.

            According to the company’s web site (www.zeba.com), Zeba is formed by hydrolyzed starch-polyacrylonile graft copolymers. Zeba copolymers are highly absorbent, but are water insoluble.  A starch-based polymer, Zeba is made of glucose molecules which are linked together chemically to become a water insoluble, net-like matrix in the form of a hydrogel that holds and releases water and nutrients in a reversible manner. These pH neutral, anionic units are able to absorb water, swelling up to 400 times their original weight, until reaching maximum absorbency. Zeba is formulated from natural cornstarch, but it is not organic-certified. The company has several reports documenting the positive effect of Zeba on vegetable growth and productivity. Zeba is typically dropped in seed furrows below the seed or the sidededress as a soil amendment at a rate of 8 lbs/acre.

            The goal of this project was to assess the feasibility of using corn starch as a nutrient Best Management Practice.  The objectives of this project were to (1) determine the effect of increasing corn starch rates on the water holding capacity of a typical sandy soil, and (2) determine the effect of corn starch rates on drainage and nutrient availability in a pot study.

Materials and Methods

Corn starch. The material provided by the manufacturer was a 1-lb sealed bag of “farm” grade cornstarch.

Calculation of the recommended rate of corn starch for plasticulture production.  For agronomic crops, the manufacturer recommended rate was 8 lbs/acre of corn starch broadcast applied and incorporated, which corresponds to a 4 ppm rate (Table 1).  If the 8lbs of corn starch are applied in the 16-inch wide wetted zone of a drip irrigated field, the amount of corn starch applied now becomes 50 g of corn starch/100 lbf (8 x 454/72.60).  Because planting distance for summer squash is 1ft, 0.5g/plant (and per pot) was the recommended rate (X) in this study (Table 2).

Soil study: water holding capacity determination.  Because limited information was available at the beginning of this project, the effect of corn starch on soil water holding capacity was determined on 3 consecutive experiments.  In all experiments, approximately 1-inch of pine bark was placed at the bottom of 3 gallon plastic pots to prevent soil loss through the wholes and sun-dried Blanton-Foxworth-Alpin complex fine sandy soil from the 0-12 inch depth (exactly 25 lbs/pot) was used.  Each treatment was repeated 10 times. Experiments 1 and 2 were conducted on 10 Oct. with broadcast corn starch rates ranging from 0 to X (0, 0.06, 0.12, 0.18, 0.24, 0.36, 0.48 g/pot) and clumped broadcast corn starch rates ranging from 1 to 40X (0.5, 1, 5, 10, 20 g/plot), respectively.  Experiment 3 was conducted on 22 Oct. with broadcast corn starch rates ranging from 0 to 12X (0, 2, 4, and 6 g/pot).   Individual pots were experimental units and each treatment was repeated 8 times.  All pots also received 27g of 10-10-10 broadcast in the pot.  Pots were watered sequentially with 1 pint of water every 10 minutes until the point of drainage.  The next day, water was added again at a rate of 1 pint every 10 minutes until the point of drainage. Pots were weighed after drainage stopped, and water holding capacity was calculated as 100 times the ratio dry weight divided by fresh weight.

Plant study: soil moisture, leaching and plant sap petiole analysis.  Pots were then taken into a greenhouse with benches oriented north-south, divided into two groups of identical size (5 single-pot replications in each group), and three ‘Gentry’ summer squash seeds were planted in each pot.  After emergence, plants were thinned to one plant per plot. 

            Irrigation schedules used in the east and west side groups of the greenhouse were planned to be based on the results of the water-holding capacity part of the project. It was planned to irrigate the pots in group 1 (west side) based on the wilting of the non-amended pots and to allow plants in group 2 (east side) to grow without irrigation until the reference starch rate (X) started wilting.  Water would have then been applied in the amount corresponding to the amount of water held by the 0 (native soil) and X corn starch rate used in the first part of the project for the west and east groups, respectively.  Because no effect of corn starch rate was found in the first part of the project, it was decided to apply the same seasonal volume of water to both groups, but in different application volumes (Table 3).  In short, the west side received less frequent, larger irrigation volumes, whereas the east side received more frequent, smaller volumes.

            Pots in the west side were placed above saucers for leachate collection and measurement of volume and electrical conductivity (EC) of the leachate.  Petiole NO₃-N and K sap concentrations were measured individually on each plant in the west side group on 3 Dec. using ion specific electrodes (Cardi meters).  Soil moisture was determined on the east-side group three times weekly in the morning before irrigation on Mondays, Wednesdays and Fridays in each pot using a portable TDR (Hydrosense, 20-cm long probes) for approximately 3 weeks.

            The responses of soil moisture content, leachate volume and EC, and NO₃-N and K sap petiole concentrations to corn starch rates was determined using regression analysis (SAS).

Results and Discussion

            Corn starch rates for vegetables and theoretical impact on drip irrigation schedules.  Because vegetables are grown at bed spacings ranging from 4 (strawberry) to 8 (watermelon), the rate on a per linear bed foot basis should remain the same for other crops.  The amount of corn starch needed on a real estate basis is then 12 and 6 lbs/acre for strawberry and watermelon, respectively. This modified broadcast approach to corn starch application increased the rate to 16 ppm (Table 1).  Assuming that the corn starch can hold 200 to 400 times its weight in water, it was calculated that a rate of 0.5 g/lbf of corn starch applied near a medium-flow drip tape can hold water applied in 13 and 26 minutes, respectively.  In comparison, most vegetable growers schedule drip irrigation in half-hour increments when the plants are small and in 1 hour increments thereafter.

            The effect of corn starch rates on water holding capacity and soil moisture content was not significant (Figs. 1, 2, 3, 4).  Overall, squash growth vas uniform across all pots and treatments in the greenhouse.  Soil moisture was determined with a portable TDR on 13, 16, 20 and 28 Nov, and no significant effect of cornstarch rate on soil moisture was found (Table 3). However, replication effect (pot-to-pot variability) was significant on most dates.  The lack of pot-to-pot variability in composition as shown by the similarities in moisture content and plant growth suggest that the pot-to-pot differences in soil moisture were due to random effect on whether or not the TDR probe touched more or fewer cornstarch particles.  This observation has two practical consequences. First, the rates of cornstarch used were likely to result in uneven moisture in the pots. Second, this indicates a limitation in this rate of cornstarch to intercept water moving through the pots.  In other words, it is likely that most of the water will “miss” the cornstarch particles and will freely move through the pot, thereby not producing the expected effect.

            Leaching events occurred on 20 and 28 Nov. after the large irrigation events (Table 3).  Regression analysis showed that the response of leachate volume and EC was not significant on either date (Table 4).  These results are in agreement with the TDR readings and the observation of wilting patterns.

Conclusion

            At the currently recommended manufacturer rate, it is unlikely that cornstarch will result in reduced nutrient leaching.  The highest rate tested (6g/pot) is 12 times greater than that rate, and was still not enough to witness and document positive effects of cornstarch on water relations in the pots.  A broader range of rates, more targeted application method, and an economical analysis should be tested next in the field.  Based on these results, the promises of using cornstarch as a method to reduce leaching have not been met, and cornstarch cannot be yet considered an efficient nutrient BMP.

Table 1. Calculation of cornstarch rate for plasticulture for a crop grown on a sandy soil with raised beds on 6-ft centers.

Application method name	Description and calculation	Rate in amended area (ppm cornstarch/soil)
Broadcast	The target rate from the Zeba manufacturer is 8 lbs/acre broadcast which when you look at it is 8 lbs of starch for 2,000,000 lbs of soil or 4/1,000,000	4
Modified broadcast – in bed	If we try to apply this to mulched culture on 6-ft centers, now we need to apply 8 lbs of Zeba to and area of 7,260 lbf x 2.5-ft wide beds  = 18,150 sq-ft = 0.42 acres under mulch.  So, our 2,000,000 lbs of soil now are reduced to 2,000,000 x 0.42 = 840,000 lbs of soil into which we will mix Zeba.  Now, we are putting our 8 lbs of Zeba in 840,000 lbs of soil which makes 8/840,000	9.5
Modified broadcast – wetted zone	If we now consider that drip irrigation only wets 16 inches of the bed (based on our dye tests) (16 inch = 1.33 ft), and we want to apply Zeba only where it will be wetted, we are now putting 8 lbs of Zeba in 1.33 x 7,260 sq-ft = 0.22 acres.  This contains 2,000,000 x 0.22 = 444,444 lbs of soil. We are here putting our 8 lbs of Zeba  in 444,444 lbs of soil which makes 8/444,444	18

Table 4. Soil moisture (%) response to cornstarch rates on four dates with pot-grown summer squash.

Cornstarch Rate (g/pot)	TDR reading (soil moisture, %)
	11/13	11/16	11/20	11/26
Experiment 1 – Low rates
0	4	6	9	11
0.06	5	8	14	14
0.12	6	8	11	15
0.18	4	6	11	10
0.24	4	6	9	11
0.36	6	7	12	14
0.48	5	7	13	15
Significance	0.73	0.45	0.42	0.45
R2	0.50	0.33	0.29	0.35
CV	42	24	38	30
Regression	NS	NS	NS	NS
Experiment 3  - High rates
0	23	20	32	36
2	27	19	34	18
4	31	18	43	32
6	21	25	46	18
Significance	0.60	0.74	0.74	0.44
R2	0.69	0.55	0.66	0.35
CV	44	53	54	72
Regression	NS	NS	NS	NS

Table 5. Leaching volume, electrical conductivity and petiole sap concentration responses to cornstarch rates.

Corn starch Rate (g/pot)	Leaching Event 11/20/2007		Leaching Event 11/28/2007		Petiole Sap Concentration 12/03/2007
	Volume (mL)	EC (dS/m)	Volume (mL)	EC (dS/m)	NO3-N (mg/L)	K (mg/L)
Experiment 1 – Low rates
0	17	8.0	27	5.3	1,540	4,080
0.06	20	10.9	33	6.6	2,240	3,360
0.12	21	9.4	35	5.8	2,300	3,360
0.18	16	8.1	23	5.6	1,580	2,960
0.24	17	6.8	26	4.5	580	3,220
0.36	17	8.5	28	5.5	1,420	3,220
0.48	19	9.3	27	6.1	1,670	3,120
Significance	0.90	0.03	0.08	0.27	0.01	0.04
R2	0.55	0.48	0.48	0.37	0.54	0.43
CV	37	19	22	22	40	15
Regression	NS	NS	NS	NS	NS	L,Q
Experiment 3  - High rates
0	51	9.1	56	6.7	2,630	3,080
2	31	14.2	40	7.1	1,600	3,630
4	36	12.1	44	7.4	1,330	3,500
6	36	12.2	37	5.7	1,040	3,250
Significance	0.38	0.03	0.04	0.77	0.01	0.05
R2	0.59	0.69	0.68	0.29	0.90	0.66
CV	40	16	19	36	18	8
Regression	NS	NS	NS	NS	Q	L,Q

 

Figure 1.  

 

 

Figure 2.

 

Figure 3.