- Author: Jordon Wade
- Contributor: Hannah Waterhouse
- Contributor: Martin Burger
In order to be accurate and effective, fertilizer recommendations must factor in a wide range of considerations, ranging from the site-specific to the climatic. To help guide these decisions, “the 4 R's” have been developed: Right rate, Right place, Right time, and Right form. These 4 R's can be utilized in tandem to maximize a given goal, whether that is maximum yield, maximum profitability, minimize adverse environmental effects, or perhaps a combination of factors. However, the specific recommendations will vary according to farm- or field-specific factors, such as climate, soil mineralogy, crop choice, or labor constraints. As such, it is difficult to make “best management” prescriptions across regions.
Several UC Davis researchers—Hannah Waterhouse, Martin Burger, and Will Horwath—recently investigated the 3 of the 4 R's of corn production over two years (2013-2014) on a farm near Stockton in the San Joaquin Valley. They were particularly interested in how nitrogen fertilizer rate, placement, and timing affected nitrous oxide (N2O) emissions. Additionally, they were comparing emissions and yields between drip and furrow-irrigated corn.
Right Rate: For both years of this study, fertilization rates were adjusted using the preplant (or residual) nitrogen levels, which were 65 lbs/ac in 2013 and 77 lbs/ac in 2014. These rates of residual nitrogen were then subtracted from the target fertilization rates to have an equal level of available N across years. To learn more about calculating residual nitrogen rates, visit our page on residual nitrogen budgeting. Overall, emissions increased with increasing rate, although there was a high degree of variability. Yield-scaled emissions, which allow for emissions to be examined in terms of agronomic efficiency, also increased as N rates increased. Using the corn stalk nitrate test in 2014, they found that there was no N deficiency, except a marginal deficiency in the 65 lbs/ac rate. At the highest rates (227 lbs/ac and 307 lbs/ac), the corn stalk nitrate test found hugely excessive levels of plant-available N.
Right Place: They also looked at the effect of applying fertilizer in a single band or a double band. They applied fertilizer at the same rate—202 lbs/ac in 2013 and 227 lbs/ac in 2014—on either the inside (1-band) or both sides (2-band) of the corn plant line. Comparing emissions from the single band vs. the double band, they saw twice as many emissions from the single band in 2013 and 3-4 times as much emissions in the single band in 2014, without seeing any differences in yield. There was also much higher residual nitrogen in the 1-band application, resulting in a higher fertilizer use efficiency in the 2-band treatment.
Right Time: For both years of the study, the majority of the fertilizer was applied as a sidedress during V2 stage of crop growth in 2013 (202 lbs/ac) and during V4/V6 in 2014 (227 lbs/ac). The use of the nitrification inhibitor AgrotainPlus helped to maintain the fertilizer in the less mobile ammonium form for longer, to better sync nitrogen supply with crop nitrogen demand. In the first year (2013), the application of fertilizer and nitrification inhibitor at V2 was a bit too early and did not reduce emissions. In 2014, the fertilizer and nitrification inhibitor were timed better to coincide with crop N demand and reduce emissions by 60%, although no yield difference was observed. This better syncing also resulted in an “excess” reading from the stalk nitrate test, suggesting that fertilization rates could likely be decreased in subsequent years.
These results were supported in another field trial of corn by the same group of researchers in Yolo County, where the AgrotainPlus also decreased emissions by approximately 50% in the sandier, coarser soils. In this study, AgrotainPlus also decreased easily-leached residual nitrate by 10 lbs/ac.
Irrigation Method: In 2013 and 2014, irrigation types were varied in the 202 lbs/ac and 227 lbs/ac treatments, respectively. Using subsurface drip to supply fertilizer and irrigation to the corn resulted in a 50-80% reduction in nitrous oxide emissions, relative to the furrow-irrigated field. The drip also had double the grain yield of furrow-irrigated corn in 2013, but no difference in total yield when growing for silage in 2014.
While the results of this study are subject to much of the same inherent variability associated with agricultural studies, it does support much of the current body of knowledge and show that California is not an exception. The central take-home messages from this research (that are well-supported by other studies) are:
- Testing for residual nitrate prior to planting helps to adjust fertilizer recommendations to minimize environmental effects, such as nitrous oxide emissions.
- Concentrating N fertilizer (especially ammonia/ammonium) into a single applied band will greatly increase emissions and decrease your fertilizer N use efficiency.
- Nitrification inhibitors can substantially decrease nitrous oxide emissions and increase your fertilizer N efficiency. Although they might not increase yields, they have the potential to increase N cycling within the system.
- Using subsurface drip irrigation can increase your yields (especially grain yields) while cutting your N2O emissions in half.
- Author: Yoni Cooperman
- Contributor: Jordon Wade
A variety of cover crops exist, way too many to be fully covered in this blog post. Generally speaking, cover crops tend to be grasses or legumes, and many growers utilize mixes to achieve targeted results. Legumes can be a source of N fertilization, though they can also contribute to N pollution if N levels exceed crop needs. Grasses have the potential to hold on to excess soil N and limit losses through nitrate leaching. Mixes of multiple cover crop types with different uses are used to maximize inputs of organic matter in hopes of building soil carbon.
While cover crops can have many potential benefits, like any other management decision tradeoffs are involved. While competition for soil water and nutrients can be used to control vigor, under certain conditions this can be harmful for vine development. Another possible downside to using cover crops, their potential to increase the production of greenhouse gas emissions, was the focus of our study conducted in a three year old Merlot vineyard in Lodi, CA. The vineyard soil is classified as a Devries sandy loam.
In our two year study, we compared rates of greenhouse gas (GHG) emissions from vineyard alleyway soil grown under three different cover crop mixes: a legume mix, a “soil builder” mix, and a ryegrass treatment all planted at 100 lbs/ac.
These three treatments were chosen to represent three reasons growers might utilize cover crops in a vineyard. The legume mix was chosen to be a “green manure” and increase soil nitrogen. The “soil builder” mix was meant to maximize plant biomass and increase soil organic matter. The ryegrass was chosen as a “catch crop” that can take up large amounts of soil N, limiting N losses through nitrate leaching.
After our two year monitoring period, we found that cover crops had little effect on soil N2O emissions, while they increased soil CO2 emissions. While CO2 emissions were higher when cover crops were used, there were no differences between the different cover crop types. These findings suggest that during drought years, growers are free to choose the cover crop mixes they think will best serve their needs, without being overly concerned about excess N2O emissions stimulated by cover cropping. However, the legume mix did result in higher levels of soil N and the ryegrass treatment did decrease leachable soil nitrate. It is unclear if the "soil builder" mix resulted in increased soil organic matter, although that is to be expected, considering it takes several years to substantially increase soil organic matter content.
For more information about utilizing cover crops visit the Solutions Center for Nutrient Management page on cover crops.
- Author: Sara Tiffany
- Author: Dr. Martin Burger
Soil nitrate sampling protocol
For maximum accuracy that can reliably predict nitrate availability in the soil, growers should sample according to the following protocol:
- For fields with 60-inch beds: soil cores should be taken at 3 lateral distances from drip tape, in at least 4 locations within a field.
- For fields with 80-inch beds: soil cores should be taken 2 lateral distances from drip tape, in at least 3 locations within a field.
click here to read full summary (or scroll down)
Research on agricultural greenhouse gas emissions in tomatoes
- The adoption of subsurface drip irrigation substantially reduces greenhouse gas emissions in tomato production (compared to furrow irrigation).
- Use of nitrification inhibitors lowers nitrous oxide emissions in tomato fields with subsurface drip irrigation.
click here to read full summary (or scroll down)
UC Davis researcher Dr. Martin Burger presented the results of a survey conducted by post-doctoral scholar Cristina Lazcano on pre-plant nitrate, phosphorus (Olson-P), and exchangeable potassium levels in 16 processing tomato fields in Yolo, San Joaquin and Fresno counties. The purpose of the study was to develop an economical sampling protocol that reliably predicts nitrate availability and allows growers to adjust fertilizer rates taking the residual soil nitrate into account.
While the conversion to subsurface drip irrigation has enabled growers to precisely deliver water and nutrients close to plant roots, there is still pressure for growers to increase nitrogen use efficiency, for example to reduce the risk of nitrate leaching. Previously, the spatial distribution of macronutrients in fields under drip irrigation was not well known. One concern has been that nitrate may accumulate at the periphery of the wetted soil volume, whereas the less mobile nutrients phosphorus and potassium may be depleted near the drip tape where roots can be expected to proliferate.
According to the survey encompassing more than 1000 soil analyses, pre-plant nitrate levels in the 16 fields varied widely, ranging from 45 – 438 lbs NO3- - N per acre in the top 20 inches of soil, with higher levels of nitrate found in fields under consecutive tomato cultivation. No depletion of Olsen-P or potassium in the root feeding areas close to the drip tape was detected. The majority of the fields showed phosphorus concentrations lower than 15 ppm, which based on earlier research is the threshold below which a yield response can be expected from a P addition. In contrast, potassium levels were higher than previously reported values, ranging from 293 ppm on average in Yolo County to 468 ppm in Fresno County.
The nitrate sampling protocol was based on a Minimax analysis by selecting the minimum number of samples within the field and locations within the beds (i.e. lateral distance from the drip tape). The combination of samples with the lowest relative error across all fields (< 5% from the field average) and the lowest number of samples taken was selected as the best sampling procedure to estimate average soil NO3-N. The analysis showed that soil cores should be taken at three (60-inch beds) or two (80-inch beds) lateral distances in at least four (60-inch beds) or three (80-inch beds) locations within a field.
Table 1. Pre-plant nitrate sampling protocol for 60-inch beds in Yolo (Y), San Joaquin (SJ), and Fresno (F) County SDI tomato fields.
Table 2. Pre-plant nitrate sampling protocol for 80-inch beds in Yolo (Y), San Joaquin (SJ), and Fresno (F) County SDI tomato fields.
***The full article about this study will appear in the Oct-Nov-Dec 2015 issue of California Agriculture.
An update on agricultural greenhouse gas emissions research included results of field studies testing a nitrification inhibitor for mitigation of nitrous oxide in subsurface drip irrigated tomato.
Nitrous oxide (N2O) is arguably the most important greenhouse gas produced in the agriculture sector, with its global warming potential 300 times that of Carbon Dioxide. N2O is produced by soil microbes during N transformations. N2O is a by-product of nitrification and denitrification.
Recent studies have shown that N2O produced during nitrification can be as important as that resulting from denitrification (Zhu et al., 2013). The highest rates of N2O emissions typically occur shortly after N fertilizer applications when soils are re-wet. The main regulatory factor is the availability of oxygen since microbes use nitrate (denitrification) and nitrite (nitrification) as electron acceptors of respiration when oxygen is in short supply. Soil processes that consume oxygen, such as the presence of a carbon source, and conditions that limit replenishment of oxygen levels in the soil, such as high soil water content, promote N2O production in soil. Compacted soils lead to rapid depletion of oxygen because of the reduced air spaces and greater tortuosity of pathways of oxygen diffusion.
Although the use of the nitrification inhibitor significantly lowered nitrous oxide emissions in SDI tomato in one of the two years of the study, the reduction in absolute values is rather small (64 lbs carbon dioxide per acre) to make a significant contribution to California's greenhouse gas inventory. With the adoption of subsurface drip irrigation, tomato growers have already lowered the impact of greenhouse gas emissions from tomato production substantially as furrow irrigation generated leads to greater nitrous oxide emissions than SDI.
Zhu, X., Burger, M., Doane, T.A., Horwath, W.R., 2013. Ammonia oxidation pathways and nitrifier denitrification are significant sources of N2O and NO under low oxygen availability. Proceedings of the National Academy of Sciences of the United States of America 110, 6328-6333.