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:

1. Testing for residual nitrate prior to planting helps to adjust fertilizer recommendations to minimize environmental effects, such as nitrous oxide emissions.
2. Concentrating N fertilizer (especially ammonia/ammonium) into a single applied band will greatly increase emissions and decrease your fertilizer N use efficiency.
3. 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.
4. Using subsurface drip irrigation can increase your yields (especially grain yields) while cutting your N₂O emissions in half.

For more on nitrogen budgeting and nitrous oxide emissions, visit our Focus Topic pages.

“Soil health”, as both a term and a concept, has been gaining traction in the last few years. The National Resource Conservation Service (NRCS) has defined soil health as “the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans”. Generally, soil health is considered to be the intersection of soil physical, chemical, and biological factors. Each soil will have differing levels of health within these three areas. Some of these constraints are inherent to a soil due to differences in the geologic material that it was formed from, but each individual soil can be optimized for agronomic performance by using management strategies that will improve the health of an individual soil.

In recent years, one area of soil health that has received a lot of attention is the biological component. Researchers are diving into the complexity of soil microbiology and looking for ways to translate these complex, micro-scale interactions into reliable, actionable recommendations for growers to use. These recommendations are usually going to be adjusted using results from commercial soil test labs. One biologically-based test that has been increasing in popularity in recent years is respiration, or the production of CO₂ by soil microbes, after an air-dried soil is rewetted. Measuring CO₂ production rates is a quick, inexpensive method to estimate soil microbial community size and activity level, as well as the availability of carbon to those soil microbes. For these reasons, respiration can serve as one useful tool in the toolbox for assessing soil health. However, translating these respiration measurements into a recommendation for growers has proved difficult.

One proposed use of respiration is to predict the inherent ability of a soil to supply nitrogen to a crop. The breakdown of soil organic matter into plant available forms of nitrogen, a process known as mineralization, is largely controlled by soil microbial activity, so a measure of microbial activity (e.g. respiration) has the potential to predict this process. If nitrogen mineralization can be predicted prior to planting, nitrogen fertilizer recommendations can be reduced, resulting in decreased fertilizer costs for growers and decreasing the potential for adverse environmental effects.

Funded by CDFA's Fertilizer Research and Education Program (FREP), UC Davis researchers Jordon Wade, Martin Burger, and Will Horwath sought to test the ability of respiration to predict nitrogen mineralization in California's diverse cropping systems. The study used soils from four distinct regions—Yolo County, San Joaquin County, Fresno/Kern Counties, and Monterey County—and two differing management strategies—fields receiving a winter cover crop and those not receiving a cover crop. A suite of common commercial test lab indicators were used to predict N mineralization.

Most soils showed the ability to contribute a substantial amount of plant-available nitrogen, with most of it coming within the first 30 days of the growing season (Table 1). Fields with cover crops had overall greater nitrogen supplying capacity than fields that did not receive cover crops.

The ability of our soil tests to predict a soil's N mineralization potential was very low. Respiration was able to predict nitrogen availability in cover cropped fields better than in non-cover cropped fields, although neither was a particularly strong relationship. Combining biological and chemical tests did improve the predictive ability, but the relationships were still weak. Additionally, they were inconsistent in their accuracy across growing regions. There is a potential to use N fertilizer rate trials to calibrate these tests for an individual field or set of fields, but large-scale recommendations at this time would likely be inaccurate.

While respiration may not be able to predict a specific outcome (such as N availability), it has been shown to be correlated with increased yields in corn (both grain and silage) and processing tomatoes in California agricultural systems. Together, these findings suggest that measuring respiration does have limited agronomic utility, but that its ability to predict specific outcomes is uncertain.

Cover crops have become a popular tool in vineyard floor management, and winegrowers utilize cover crops for a variety of reasons. Maintaining plant coverage of soil for as much of the season as possible helps limit soil erosion. Competition between cover crops and vines can be used to limit vine access to water and nutrients, helping control vine vigor. Incorporation of cover crops into the soil can increase soil organic matter and increase the amount of water soil can hold. Cover crops are also used to control weeds and can serve as habitat for beneficial predators.

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.

A growers' workshop on nutrient and irrigation management featured the Solution Center for Nutrient Management last week in San Benito County. The workshop was funded by the San Benito County Water District and brought together a wide variety of attendees including: growers, seed representatives, consultants, scientists, an operational engineer, an aide to Senator Canella and the President of the San Benito County Farm Bureau. Beginning with a presentation on the latest water quality and nitrogen reporting requirements from the Central Coast Regional Water Quality Board, the topics covered on nitrogen and water management generated lots of dialogue and participation throughout the event. Monterey County UCCE Farm Advisor Richard Smith gave a succinct presentation on nitrogen budgets, illustrating findings from several field trials throughout the central coast of California. The findings highlighted the need to take residual soil nitrogen into account when calculating how much fertilizer to apply, and matching this to crop nitrogen uptake as closely as possible. Smith also presented on several fertilizer technologies for improving nitrogen use efficiency such as slow release fertilizers and nitrification inhibitors. Another Monterey County Farm Advisor, Michael Cahn presented on irrigation management as it pertains to nutrient management. He pointed out that because nitrate will go wherever water goes, good irrigation management is good nutrient management. His presentation covered challenges to achieving high irrigation efficiency such as distribution uniformity in drip irrigation systems. Distribution uniformity is also very important in the distribution of fertilizer through fertigation in drip systems. In his presentation of CropManage, an online decision tool to assist growers with irrigation and nutrient management, he suggested that irrigation scheduling is really about when to irrigate and how much to irrigate when it's time. This online decision tool is aimed at helping growers decided when and how much to fertilize and irrigate taking into account local weather data, soil type, crop and a myriad of other factors. CropManage can also be a great way to keep accurate records of fertilizer application. The workshop concluded with a presentation on the Solution Center for Nutrient Management and its role in providing growers and CCAs with sound resources needed to address the current challenges in nutrient and irrigation management in California. A representative from the Fertilizer Research and Education Program pointed some key resources that FREP has developed such as the Crop Fertilization Guidelines.