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

View the project final report

• 2014 annual report

Commodities

• Agronomic: potatoes

Practices

• Crop Production: foliar feeding, organic fertilizers, tissue analysis
• Production Systems: organic agriculture
• Soil Management: soil analysis, nutrient mineralization, soil quality/health

The goal of this project is to compare two different organic fertility programs. 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 saturated-paste soil test and tissue testing 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 will evaluate 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 am 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 would like to compare these two fertilization programs within the season. I have chosen 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 will test the consequences of following those two contrasting fertilizer programs. This project will test one of the more controversial claims of the ”Nutrient Density” program (questioning the primacy of nitrogen as a limiting element), and will help me to determine which course I should take to increase yields of heavy-feeding crops such as potatoes and brassicas.

           Outreach will be through a field day hosted by UMass Extension, a tour during the NOFA Summer Conference, and presentation of results at at least one winter grower's conference to be determined.  The final report will also be written in the form of an article that will be submitted to Growing for Market, the Natural Farmer, and possibly other publications.

Potatoes will be planted in April or early May. We grow potatoes in 7-bed sections, and typically grow more than one section of Keuka Gold potatoes. One section of potatoes will be used as a randomized complete block design with 3 replicates of two different treatments. Each plot will consist of two 2-row beds, half the length of the field (approximately 150 feet). The PSNT treatment will receive pre-plant organic fertilizer based on recommendations of a UMass soil test, and the AEA treatment will receive pre-plant fertilization based on recommendations of a soil test from Logan Labs, which is in the AEA program. When the plants are about 6 inches high (just before tuber initiation), we will take PSNT, Saturated Paste soil test, and Logan Labs tissue tests from all plots. In the PSNT plots, we will apply organic nitrogen fertilizer (likely blood meal, since that is one of the few organic fertilizers that contain mostly nitrogen) based on the PSNT results, and in the AEA plots, we will sidedress and/or foliar-feed organic fertilizer based on the Logan Labs tests. Pest control (likely Pyganic for potato leafhopper and Entrust for Colorado Potato Beetle) will be applied uniformly across all treatments with our boom
sprayer, and then a separate pass will be used to foliar feed in the AEA plots. We will scout weekly for insect pressure, recording levels of pest pressure, along with measurement of plant sap brix, in all plots. Our Technical Advisor will provide training for the insect scouting. We don't usually have major disease problems in potatoes, but if we notice potato disease in the course of scouting, we will also record disease incidence and severity levels.

In August, or whenever crop growth seems to be finished, we will take a representative subsample of potato plots and measure total and marketable yield. Insect scouting and yield sampling will be conducted on the two interior rows of each plot in order to minimize edge effects. We will also send a sample of the potatoes to be tested for nutritional quality, since improved nutritional quality is one of the claims of the “Nutrient Density” school of thought. This plant tissue testing will be done at the UMass Soil Testing Lab and will determine major and micronutrient content in the tuber tissue. The two treatments will also be extrapolated out to an acre for an enterprise analysis of which system is more profitable, along with comparisons with average potato yields. Our technical advisor will assist with designing and setting up the trial, provide training on scouting for insects and diseases, assist with sampling and harvest, and will provide assistance with statistical analysis.

Insect and disease incidence will be assessed visually according to UMass Vegetable Extension guidelines and scouting training and assistance will be provided by the technical advisor. Plant sap brix will be measured throughout the growing season to assess the effects of the two treatments on plant health, and to determine if plant nutrient status or plant health is correlated with insect pressure. Pest control may mask potential effects of treatment on insect feeding or disease incidence, but the organic materials provide only partial control so differences may be observed. Finally, the ultimate goal is to improve crop yield and/or quality, and this will be measured by sampling each plot. We will extrapolate the results out to an acre for both.

The plant tissue tests and the two soil tests taken from all plots will allow us to track soil nutrients and plant nutrient status in all treatments. A sampling area will be designated within each plot consisting of the two innermost rows of a 50 foot section at the center of each plot. As mentioned above, soil samples will be taken from each of the six plots before potato seed is planted and sent to either the UMass Soil Testing Lab for a routine soil plus organic matter analysis or to Logan Labs for a standard soil test. Fertilization will be applied to each plot based on the recomendations of those tests. At tuber initiation or when plants reach 6 inches in height, soil and tissue samples (at least 25 young, mature leaves per plot) will be taken from each plot. Soil samples will be sent to the UMass Soil Testing Lab for PSNT and plant tissue tests (total elemental composition plus nitrogen) as well as to Logan Labs for plant tissue test testing and soil samples will be assessed by saturated base test. At harvest, total and marketable weight per sample area per plot will be assessed. Tuber tissue will be submitted to the UMass Soil Testing Lab for total elemental analysis in order to characterize nutritional differences. Since we are using both the UMass and Logan Labs testing methods, we will be assessing the success of both treatments using the measurements of the other program.

Insect and disease incidence will be assessed visually according to UMass Vegetable Extension guidelines and scouting training and assistance will be provided by the technical advisor. Plant sap brix will be measured throughout the growing season to assess the effects of the two treatments on plant health, and to determine if plant nutrient status or plant health is correlated with insect pressure. Pest control may mask potential effects of treatment on insect feeding or disease incidence, but the organic materials provide only partial control so differences may be observed. Finally, the ultimate goal is to improve crop yield and/or quality, and this will be measured by sampling each plot. We will extrapolate the results out to an acre for both treatments, and come up with a partial enterprise budget to assess the effect of the treatments on profitability.

The PSNT soil test is an extremely well-developed and tested technique for assessing the need for nitrogen side-dressing (Heckman, 2002). Soil nitrate levels have been established for a wide range of vegetable crops, and reams of scholarly data have been devoted to various means of assessing the nitrogen needs of crops and the effect of nitrogen fertilization (Belanger et al., 2001). On the other hand, I have been very frustrated by the lack of good literature for the “nutrient density” school of thought. The system appears to have been developed empirically, and relying on science, but have not to my knowledge been tested in rigorous, replicated trials. The proposition that nitrogen fertilization is less important than fertilizing with other nutrients would be a true paradigm shift. It makes good intuitive sense that tissue and soil testing to determine the complete nutrient status of the plant would help to enhance the plant's health, and that improved plant health will lead to lower levels of insect and disease pressure. I have seen positive results of using AEA's materials on my farm, and know of other farms who have had similar results, but would like to see the program tested against UMass recommendations. I found no projects that seemed at all like this one after a search of SARE's and OFRF's databases, as well as a Google search under the search terms “nutrient density,” “biological agriculture,” “nitrogen fertilization,” or “advancing eco-agriculture.” The technical advisor, after consulting with UMass scientists studying crop physiology, also could not find any related literature in scholarly journals.

Belanger, G. et al. 2001. Predicting nitrogen fertilizer requirements of potatoes in Atlantic Canada with soil nitrate determinations. Canadian Journal of Soil Science. 81(5):535-544.

Heckman, J.R. 2002. In-season soil nitrate testing as a guide to nitrogen management for annual crops. HortTechnology. 12(4): 706-710.