A comparison of traditional nitrogen and nutrient-dense fertilization regimes in organic potato production

A comparison of traditional nitrogen and nutrient-dense fertilization regimes in organic potato production

Summary

The goal of this project was to compare two different organic fertility programs on an annual vegetable crop, potatoes. The first program uses fertilizer recommendations based on a University of Massachusetts soil test, followed by side-dressing of nitrogen based on the Pre-Sidedress Nitrate Test (PSNT). The second program follows recommendations of Advancing Eco-Agriculture (AEA), which emphasizes calcium, potassium, and micro-nutrient fertilization over nitrogen. The AEA program uses a plant-tissue sap test to determine fertilization during the crop season. This program claims to increase crop yields and avoid pest problems by supporting optimum plant health.

            This project evaluated the effectiveness of fertilization strategies that are being promoted to organic farmers under the name “Nutrient Density”.   While the claims of these programs are compelling and cohesive, I was not aware of rigorous scientific testing of any of these programs. Some of the recommendations of the AEA program are consistent with a more conventional approach to soil fertility, while others depart widely. One of the clearest contrasts between the AEA approach and a more conventional program is in the emphasis on the role of nitrogen fertilization. The conventional University recommendations target nitrogen fertilization during the season based on the results of a PSNT—if nitrate levels in the soil are below a certain point at a critical growth point in the season, then nitrogen fertilization is recommended. The AEA program relies on testing of a wide range of nutrients in both the crop tissue and the soil, and aims for a balanced level of all nutrients. In the AEA program nitrogen is rarely the most limiting nutrient. In fact, the AEA program typically calls for fertilizing with other nutrients because over-fertilizing with nitrogen tends to encourage pest problems by increasing the levels of nitrates in the plant, making the plant more attractive to insects. I have seen positive results of feeding liquid fertilizers from AEA through drip tape in fruiting crops. I feel that in recent years, I haven’t been getting the yields I would like from heavy-feeding crops that I grow on a two-row system such as potatoes and brassicas. My hunch is that these crops would yield better with better in-season fertilization, and I wanted to compare these two fertilization programs within the season. I chose potatoes as the test crop because I grow potatoes with a large enough block of a single variety that it will be more convenient to make a replicated trial with large enough plots that treatments can be applied with tractor-mounted implements.

            The “Nutrient Density” programs are being heavily promoted by the Northeast Organic Farming Association and other associated organizations, but there is a great deal of confusion among farmers about how to implement these programs, as well as strong skepticism from the University community and some segments of the organic farming community. The genesis of this project occurred in 2012, when I had mediocre growth in my fall broccoli plantings. My farm is next door to Extension Vegetable Specialist, Ruth Hazzard, and she is also a member of my CSA. We both saw the lackluster growth of my fall broccoli plantings, and had a strong interest in improving my broccoli yields. Ruth and I took a PSNT and a plant tissue test in my broccoli plantings, and found that while the PSNT called for nitrogen fertilization, the Logan Labs test plant tissue test found an excess of nitrogen, and instead called for additions of calcium and potassium to bring those nutrients into balance with nitrogen. This project tested these two contrasting fertility programs.

            The results showed no significant differences in yield or in insect or disease pressure. The AEA program was able to support adequate nitrogen levels in the plant with minimal added nitrogen, and showed higher levels of potassium and micronutrients in the sap test results. The fertility program cost was higher for the UMass program, though the UMass program was much less costly if lower-cost fertilizer sources were used.

Objectives/Performance Targets

 What we did

The field was separated into 6 plots, 3 of which were randomly selected to receive the AEA treatment, and three of which received the UMASS treatment. The plots were each 2 beds (4 rows or 12 feet) by 150 feet. The whole field was planted on May 9th, with Keuka Gold potatoes planted about 18 inches apart in 36” rows, by hand into furrows made by an offset potato hiller, and then the furrows were closed with a potato hiller. Potatoes were cultivated and hilled as we usually do. We sprayed the whole field with Pyganic approximately weekly for Potato Leafhopper starting on June 12^th (the first two sprays also included Grandevo) and ending on July 16^th for a total of four sprays. The first two sprays included Grandevo for additional leafhopper control, and the second two included Entrust for Colorado Potato Beetle. On July 30^th, late blight was confirmed in the field, and the whole field was mowed to prevent the spread of the disease to the tubers and to tomato plots nearby. On August 20^th, we took (2) 5-foot subsamples from the inner rows of each plot and measured the total marketable yield (in both weight and number) of potatoes from each plot, along with the total weight and number of potatoes in each of four size classes (creamer, small, medium and large).

            The fields were scouted weekly, and data was kept on the incidence of pests and disease in each treatment. We also took separate PSNT and plant sap samples from each plot so as to measure the nutrient status of each treatment by the measurements of the opposite program. Yield samples were assessed for quality and samples were analyzed for nutrient composition.

UMass plots

Soil from the UMass plots was sampled April 15th, and sent to the UMass Soil Testing Lab. Based on the recommendations of this soil sample, we applied blood meal and sulfate of potash for a total of 100 pounds of actual nitrogen and 40 pounds of actual potassium. We took soil samples from within the potato hills when the potatoes were approximately six inches tall to determine if more nitrogen was needed (a PSNT or Pre-Side-dress Nitrogen Test). This test determined that there was no need for further nitrogen, so none was applied. The UMass plots then received the same treatment and sampling that the rest of the field received.

AEA Plots

The program in the AEA plots was a generalized program that was modified only by testing of the plants in the season. Soil from the AEA plots was sampled on April 15th, and sent to Logan Labs for testing. The only recommendation from that soil test was to apply 300 pounds per acre of gypsum, but we received that recommendation too late to apply gypsum. The AEA plots were sprayed with 2 gal/A of Rejuvenate (a microbial stimulant) and 50 g/A of Spectrum (a microbial inoculant) after the winter rye cover crop was plowed in but before disking of the field. Then a mixture of planter’s solution and Sea-shield (nutrient blends), and Bio-Coat Gold, Pepzyme, and Biogenesis (microbial inoculants and stimulants) was sprayed on the potato seed in the furrow before the furrow was closed. The recommended rate of these products was 3 gallons per acre of Planter’s solution, one pound per acre of Biogenesis, 12.5 oz./A Pepzyme, 2 qt./A Sea-Shield, and 4 oz./A Bio-Coat Gold. We had trouble with the sprayer and ended up spraying more than the recommended rate. Starting on June 5th, we applied a liquid side-dress to the soil every 3 weeks, consisting of 6 qt./A of Planter’s Solution and 2 qt./A each of Sea-shield, PHT Potassium and PHT Calcium.

We also applied a weekly foliar feed mixture of 0.25 acres Micro 5000 at 2.66 oz. per acre, 0.25 acres Pepzyme Clear at 12.5 oz. per acre, 3 qt. Forage Foliar Blend, 2 qt. Sea Shield, 1 qt. PHT Potassium, 1 qt. PHT Calcium. We sampled leaves from the AEA plots and sent them to Crop Health Laboratories for sap testing on XXXdates. Based on results of these tests, starting June 23^rd, the weekly foliar feed program was changed: the rate of Forage Foliar Blend was increased to 4 qt./A, and the PHT Calcium was increased to 3 qts./A. On July 18^th the weekly foliar feed program was modified again based on sap testing to add 8 oz./A of MnS.

Accomplishments/Milestones

Results

Interestingly enough, there were no differences between the treatments in disease or insect pressure, yield, size, or nutrient composition in the tubers. The only difference that we measured was in the plant sap tests: The AEA plots were higher in potassium, boron, and manganese. The higher boron level in the AEA plots was also found in the tissue tests that we sent to the UMass lab. The Umass plots had higher magnesium in the older leaves than in the younger leaves, which is a sign of excess magnesium. The AEA program tends to associate excess magnesium uptake with lower potassium uptake, which is supported by this data.

 Discussion

 We started out this project to test some of the core assumptions of AEA’s soil fertility program. Though there were no significant differences between the plots in the core measures that we sought to influence, yield, and insect and disease pressure, there were still some interesting results that support some assumptions of this program but not others.

 AEA assumption #1: Nitrogen fertilization is not necessary if other methods are used to support microbial populations that can supply nitrogen to the crop plant. This assumption was well supported by this project. We did not measure the microbial populations, so we cannot be sure of the mechanism by which nitrogen was supplied to the plant in the AEA plots. But though we barely applied any nitrogen fertilizer to the AEA plots, the sap tests show adequate levels of nitrogen in the plant. There was some nitrogen applied in the Sea Shield, but at 2% nitrogen, that only amounts to about 1 pound of nitrogen per acre, as opposed to 100 pounds per acre in the UMass plots. With similar yield numbers between the two treatments, the yield per applied nitrogen is astronomically higher in the AEA plots. This result is a startling contradiction to established wisdom about nitrogen fertilization.

AEA assumption #2: Applying nutrients through foliar feeding and adjusting the foliar feed program in response to testing of plant nutrient levels results in a more balanced nutrient profile in the plant and crop. This assumption was somewhat supported by this project. The AEA program supported higher levels of potassium and micronutrients in the potato plants. The higher levels of Mg in the old leaves in the UMass plots is associated in the AEA paradigm with excess Mg, and with lower uptake of potassium.

AEA assumption #3: More balanced plant nutrition can lead to crops which are resistant to insect and disease pressure. This assumption was not supported by this project. There was no observable difference in insect and disease pressure between the treatments. Levels of Colorado Potato Beetle, Potato Leafhopper, and Late Blight were all quite high across all plots.

AEA assumption #4: More balanced plant nutrition can lead to higher yields. This assumption was not supported by this project. Yields were not statistically different between the treatments. The presence of late blight caused us to mow the potatoes during tuber sizing. There is some chance that the higher potassium levels in the AEA plots could have led to higher yields since potassium is needed by the plant during tuber sizing. However that is quite speculative, and no yield difference was observed.

Economic impact of the two different systems

Since the only difference in the two systems was in the fertility program, I estimated the cost of fertility inputs in the two systems, and assumed that all other costs were the same. I also estimated that there was more labor involved in the AEA system due to extra tractor passes involved in the foliar feeding and side-dressing.   I estimated that there were an additional 4 tractor passes, since some of the foliar feed would have been mixed in with the pest control inputs (tractor passes were calculated at one hour per acre and $18/hour).

The UMass program was much costlier, since it used blood meal, which is one of the more expensive organic nitrogen sources. Blood meal made sense in a small scientific plot where I wanted to apply only nitrogen. In a more typical farm field situation I would have used a pelleted chicken manure that is available in a high-nitrogen form (analysis 8-3-3), so I also calculated the fertility cost of following the UMass recommendations with that fertilizer. At 1250 pounds per acre, I would have provided the same amount of nitrogen as with the blood meal, 37.5 pounds of potassium, and 37.5 pounds of phosphorus that was not called for by the soil test. In that case, the fertility cost would have been much lower than the AEA program.

  Conclusion

Should I use the AEA program again? The results of this project were fairly inconclusive as regards the AEA program. The yields, crop quality, and pest pressure were similar for the two treatments, and the AEA program was more costly than the UMass program with pelleted chicken manure fertilizer. I wouldn’t expect anyone to read the results of this project and start using a “nutrient density” program. For my part, I still find the system intriguing and intend to use some aspects of it on my farm. I was quite happy with my yields and quality in 2014 where I used pelleted chicken manure as a pre-plant fertilizer and then used foliar feeds to supplement plant nutrition. I also used sap testing to modify my foliar feed program in 2014

Impacts and Contributions/Outcomes

We featured this project at two field days in 2014. One was during the NOFA Summer Conference, and the other was in conjunction with the UMass IPM program.  There was considerable interest at both events, though no results were available at that time.  The final report will be worked into a more readable and interesting document sometime after the New Year, and submitted to The Natural Farmer, Small Farmer’s Journal, Growing for Market and the UMass VegNotes publication.

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