There are many challenges to predict the soil available nitrogen (N) for vegetable growers in a timely manner. Soil testing and yield are used routinely to guide agricultural applications of N. Under or over fertilization is apt to occur in any given growing season.
Insufficient application of N can have serious economic consequences for the growers, whereas excessive fertilization increases the risk of environmental pollution. This is true for both conventional and sustainable farmers using organic sources of nitrogen such as cover crops or compost. The organic sources mineralize at different rates under various temperature and moisture conditions, so it is are harder to predict how much N is available and when.
The objective of this study is to estimate the soil N availability for tomato production in high tunnel vs. field under organic and conventional system using the Illinois Soil Nitrogen Test (ISNT). The ISNT estimates the potentially mineralizable fraction of soil organic N, specifically it measures the amino sugar N. Another objective is to determine if there is any relationship between N mineralization from incubation as potentially mineralized nitrogen (PMN) and the ISNT.
This approach has a potential advantage over the other testing techniques and has value for improving N-fertilizer use efficiency. We will develop fertility recommendations based on the results. If adopted, this could increase the profitability of tomato production and reduce the adverse environmental effects of excessive N and phosphorus P fertilization, especially for sustainable specialty crop producers using organic soil fertility amendments.
Limited soil N availability is a common problem in organic vegetable production that often necessitates in-season fertilization. Growing legume cover crops is generally the most economical way to provide plant available N in organic systems through crop rotation. However, the actual N contribution can vary widely depending on field specific conditions. Another practice in organic culture is application of composted manure. However the slow rate of N mineralization from composts makes it difficult to predict the effective N contribution. Pre- season mineral nitrogen (nitrate and ammonia) tests do not measure the amount of nitrogen that will be available later in the season.
Estimation of plant available N is complicated enormously by the dynamic nature of soil N owing largely to the effects of temperature and moisture supply on the N cycle processes. Soil tests for N that would estimate the labile organic fraction that supplies the plant through mineralization would be of great value to specialty crop growers. One test that shows much promise for organic farmers is the ISNT which analyzes the soil for amino sugar N, a fraction of soil that is easily mineralizble and that may become available for the crop during the growing season (Khan et al., 2001). This test has been shown to predict N responsive sites where liquid or composted dairy manure is applied to corn agroecosystems (Klapwyk et al., 2006). Because of its ability to predict readily mineralizable N, it may have great utility for organic fields which have received compost or manure for several years to decades, such as our research center in Olathe, KS which has been managed under these conditions for the last 10 years. The ISNT is offered commercially in some states, but the interpretation provided for the test is limited to corn growers in the region.
A second test that has been used historically is potentially mineralizeable N (PMN). In this test, soil is incubated at a controlled temperature and moisture level for a period of time, and repeatedly sampled for the mineral N fraction that is released during the incubation. Unfortunately, this test is time consuming and is not commercially available to growers. However, we will use in our trials to compare the ISNT to the PMN test on the same soil samples. Results will be compared to previously published literature on PMN and compost applications to specialty crops.
The objective of this study is 1. to estimate the soil N availability for tomato production in high tunnel vs. field under organic and conventional system using the Illinois Soil Nitrogen Test (ISNT) 2. to explore the relationship between N mineralization from incubation as potentially mineralized nitrogen (PMN) and the ISNT 3. to determine the impact of long term fertility management practices of organic and conventional on soil amino sugar-N 4. to explore the relationship between ISNT and organic matter, total carbon and total nitrogen.
Trials were conducted on replicated experimental plots set up for the comparison of crops grown under organic and conventional production systems in high tunnels and open field plots. The soil was a Kennebec silt loam. Six 9.8 m x 6.1 m high tunnels with 1.5m sidewalls (Stuppy, North Kansas City, MO) and six adjacent 9.8 m x 6.1 m field plots were used for this study. High tunnels were covered with single layer 6-mil (0.153mm) K-50 polyethylene (Klerk’s Plastic Product Manufacturing, Inc., Richburg, SC). Since 2002, three of the six field plots have been managed with organic amendments and the other three with conventional amendments. In the high tunnels, three have been managed with conventional and three with organic amendments. Organic plots were managed in compliance with USDA National Organic Program standards, and were inspected and certified in 2003, 2006, 2007, 2008 and 2010
In 2007, each high tunnel or field plot was subdivided into three 3.2×6.1 m plots to which one of three fertilizer levels was assigned (control, low, and high) following a latin square design to account for the gradient effect of light in the high-tunnels. Fertilizers rates were determined based on soil analysis at the beginning of the study in 2007, and recommendations for vegetable crops in Kansas (Marr et al., 1988) with compost applied to organic plots and synthetic fertilizer applied to conventional plots. Compost application rates were based on the assumption that 50% of the nitrogen from compost would be available to plants during the growing season, while 100% would be available from conventional fertilizers (Warman and Havard, 1997). Low and high fertility plots were fertilized with equal amounts of compost or synthetic fertilizer at the beginning of the growing season, and high fertility plots received additional fertilization during the growing season by liquid application through the drip irrigation system.
Two crops were grown in this experiment; pac choi and tomato. The crops were each grown in one half of each open field or high tunnel plots with a rotation between pac choi and tomato crops each year to meet organic certification criteria. In this system, a spring and a fall crop of pac choi was grown each year, while a single crop of tomato was grown during the summer months. Between the spring and fall pac choi crops, the plots were seeded with a summer cover crop of buckwheat (Fagopyrum sagittatum) (Albert Lea Seed, Albert Lea, MN, U.S.A.) at a rate of 134 kg/ha. In the late fall, all plots were seeded with a cover crop of annual rye grain (Secale cereale) (Albert Lea Seed, Albert Lea, MN, U.S.A.) at a rate of 229 kg/ha.
For the ISNT study, only two fertility rates were used (control and low). Jack’s Peat-Lite 20N:4.4P:16.6K J. R. Peters, Inc., (Allentown, MO) at a rate of 87 lb N/hectare was applied for conventional plots and a mixed-source compost (Microleverage 0.6N: 0.4P: 4.4K, Hughesville, MO.) at a rate of 176 lb N/hectare was applied for organic plots.
In 2010 soil samples were collected at 0-15 cm depth from the tomato plots, as 6 cores taken at random within each fertility rate, then mixed together as a bulk sample. . The samples were stored in a cooler room (2-5oC) at Kansas state university, then sieved through a mesh-screen (4mm) and divided into 7 subsamples. One subsample was used to measure the (NH4+amino sugar)-N using the ISNT through a commercial lab in Wisconsin (VH consulting Inc. Hudson, WI). The second subsample was used to determine the gravimetric soil water content. The third subsample was used to analyze the mineral nitrogen at time “zero”. The four subsamples left were used for incubation in controlled laboratory conditions at Kansas State university with temperature of 30oC and soil moisture adjusted to 50-60% of water filled pore space. All incubated subsample flasks were covered with parafilm to allow air exchange. Subsamples were extracted at week 1, week 2, week 4 and week 8 with 2 µ KCL and used to determine the nitrate and ammonia levels. These are added together to obtain the mineralizable N, which is reported as net cumulative mineral N after 8 weeks of incubation.
Statistics were performed to determine if the effect of management system (organic vs. conventional) is significant, and also whether the fertilizer treatment rates (control vs. low) were different. Due to the design and layout of the plots within the experiment, we can’t compare the field plots and high tunnel plots statistically. We also performed the Pearson regression analysis on the variables in this experiment to determine if they were correlated. “Statistical Analysis” software (SAS 9.4 Cary, NC) was used to perform the tests. A “p-value” is used to determine significance. If the p-value is less than 0.01, the comparison is highly significant; 0.05 is significant (95% confidence), and between 0.05 and 0.10 is significant with only 90% confidence.
Illinois Soil Nitrate Test (ISNT) (Figure 1):
Management showed a highly significant effect on ISNT in the high tunnel plots (p=0.008), while showing slightly significant effect on ISNT in field plots (p= 0.076). Organic ISNT was higher than conventional in both environments (high tunnel and field). Neither the high tunnel nor field plots showed fertility effect (Table 1).
Potential Mineralizable Nitrogen (PMN) (Figure 2):
Management had a significant effect on PMN in high tunnel plots (p=0.0081) where organic PMN was higher than conventional, while having no effect on field plots (p= 0.401). Fertility showed no significant effect in either environment (high tunnel and field) (Table 2).
ISNT relationship with PMN, OM, TC and TN: correlation p-values are listed in (Table 3 &4)
ISNT and PMN relationship: ISNT was found to be correlated to PMN in high tunnel plots (p=0.039), the Pearson correlation analysis (SAS, 9.4) determined an r2=0.358 (Figure 3). For field plots, there was a strong correlation between ISNT and PMN (p<0.0001), the Pearson regression analysis determined an r2= 0.88 (Figure 4). For tests of correlation, the higher the r2 the stronger the relationship between the two variables.
ISNT and OM relationship: Although the ISNT was slightly correlated to OM (p=0.07, r2= 0.358) in high tunnel plots, it was strongly correlated to OM in field plots (p=0.0052 r2= 0.56). In work published by (Khan, 2001) and (Klapwyk and Ketterings, 2005) the ISNT and OM were strongly correlated (p<0.001, r2= 0.95).
ISNT, total carbon (TC) and total nitrogen (TN) relationship: ISNT showed a strong correlation with TC (p=0.004, r2= 0.58) and TN (p=0.001, r2= 0.65) in high tunnel plots, but no correlation in field plots.
Educational & Outreach Activities
The results from this grant and other USDA projects are presented in my PhD dissertation, with a scheduled defense date in mid-January, 2016. We plan to submit at least two articles to refereed journals from this work. I also attended the American Society for Horticultural Science conference at Orlando, FL in the summer of 2014 and presented preliminary data on tomato yields and soil nutrient status in a poster session.
Management practices (organic or conventional fertilizers) had a highly significant effect on ISNT and mineral N in the high tunnel, and a modest effect on ISNT in the field. In all cases, the ISNT values were below the critical value of 230 ppm proposed by (Khan, 2001) which is associated with sites that are low and needed additional N. We also did not see an effect of compost addition between 2007 and 2010 in the organic plots, as the effect of fertility was not significant. The ISNT regression equation was able to predict PMN in the field plots (r2= 0.88), but the relationship with PMN in the high tunnel plots (r2= 0.35) is not strong enough to use ISNT as a practical means to predict the PMN. More field testing is needed to validate these equations. ISNT was found to be correlated with OM in both high tunnel (r2= 0.39) and field (r2= 0.56), which can be used as an alternative test.
The ISNT has gained interest as a predictor of N responsiveness and as a replacement of the pre-sideress nitrate test PSNT. The performance of the ISNT method has been studied by research groups in several states (Baker, 2006; Klapwyk, 2006; Laboski, 2006; Mulvaney, 2006; Williams, 2007). Khan, (2001) showed that ISNT results were highly correlated with amino sugar N, which is considered a stable but mineralizable fraction of soil N. Mulvaney, (2006) concluded that the ISNT was a powerful predictor of the error in the yield goal recommendations and was significantly correlated to crop N requirements. Other studies evaluated the applicability of the ISNT for other crops and or climatic conditions, found a poor correlation between ISNT and N response in wheat (Torrie, 2004). In Iowa, (Barker, 2006b) found no positive correlation between ISNT and corn N responses, relative yield, yield response to applied N, or economically optimum N rate across a range of soil and climatic conditions. Barker, (2006b) and (Klapwyk and Ketterings, 2006) reported the ISNT was unable to differentiate responsive from non-responsive corn sites in Iowa and New York, respectively. According to 15N Analysis Service (2002), use of the ISNT with other crops and or climatic conditions may require different critical values.
More studies need to be done to determine if the ISNT method can be used as a predictor for PMN in tomato production
An economic analysis was beyond the scope of the project as proposed. However, when we look at the costs of the various soil test performed relative to the usefulness to farmers in determining the possible nitrogen available to crops, we see that organic matter and soil carbon tests are both widely available, and economical for farmers to obtain from a commercial lab. The ISNT test is affordable ($10 per sample), but unless it tells us something new, or in addition to the organic matter test, we can’t recommend it at this time. Our results show that it correlates with potentially mineralizable nitrogen, but wasn’t necessarily a stronger relationship than organic carbon. The PMN (Potentially mineralizable nitrogen) test is not commercially available, and would be expensive for farmers to access, as they’d need to pay someone to perform the test as a special project or favor.
This experiment was a “proof of concept” study, and more field testing is needed before it is recommended to farmers.
Areas needing additional study
Results and discussion from this study show that the main focus in future work needs to be done on validation of ISNT as a predictor of nitrogen mineralization. This would be done by conducting more field experiments to test the regression equations generated and to correlate them with yield. Studies need to be repeated over years and on several soil types to get a better sense of variability and to determine how temperature and moisture impact the results and interpretation of ISNT.
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