Soil nitrate (NO₃ ⁻) tests are an integral part of nutrient management in annual crops. They help growers make field-specific nitrogen (N) fertilization decisions, use N more efficiently and, if necessary, comply with California's Irrigated Lands Regulatory Program, which requires an N management plan and an estimate of soil NO₃ ⁻ from most growers. As NO₃ ⁻ is easily leached into deeper soil layers and groundwater by rain and excess irrigation water, precipitation and irrigation schedules need to be taken into account when sampling soil and interpreting test results. We reviewed current knowledge on best practices for taking and using soil NO₃ ⁻ tests in California irrigated annual crops, including how sampling for soil NO₃ ⁻ differs from sampling for other nutrients, how tests performed at different times of the year are interpreted and some of the special challenges associated with NO₃ ⁻ testing in organic systems.

Adequate nutrients in forms available to plant roots are essential for sustainable crop production. Soil testing for phosphorus and potassium availability allows growers and crop advisers to determine whether a soil is likely to respond to fertilization. As yields have risen with improved management and production systems, crop nutrient demand and the removal of nutrients with harvested crops have increased. An in-depth discussion of soil tests for phosphorus and potassium and their use in California cropping systems is clearly needed. We review how these nutrients become available to plant roots, how samples are taken and test results interpreted, complementary ways to assess the adequacy of supplies and what research is needed to improve soil testing for phosphorus and potassium.

Predicting nitrogen mineralization has been the focus of many agricultural researchers for decades. This project was designed to investigate the soil properties that influence potential nitrogen mineralization in the northern Central Valley of California. A total of seventeen sites featuring annual cropping systems with a variety of common crops such as corn (Zea mays), watermelon (Citrullus lanatas), safflower (Carthamus tinctorius), sunflower (Helianthus annuus), sorghum (Sorghum bicolor), processing tomatoes (Solanum lycopersicum), and cotton (Gossypium hirsutum) were included, spanning from Colusa to Tulare Counties. We studied nitrogen mineralization using both field trials as well as laboratory incubations of undisturbed soil cores, in conjunction with continuous soil sampling of those sites every 5-6 weeks throughout the growing season to capture seasonal nitrogen turnover. Those soils were characterized and the results statistically analyzed, and a model was developed that determined particulate organic nitrogen and silt content as the best predictors of nitrogen mineralization in the undisturbed soil cores (adjusted-R2 = 0.65, P < 0.05). Over 70% of the variability in seasonal nitrogen mineralization at optimal soil moisture was described by only particulate organic nitrogen, while soil moisture was confirmed as a major controlling variable in determining nitrogen mineralization. Furthermore, even at the lowest soil moisture contents, nitrogen mineralization continued. Two separate smaller studies were also performed to determine crop residue effects on nitrogen availability. One included a field trial using seven different crop residues followed by a laboratory incubation of undisturbed soil cores, and the other included a laboratory incubation with two different residues at varying residue and soil moisture content. Seven different common crop residues (namely corn (Zea mays), watermelon (Citrullus lanatas), safflower (Carthamus tinctorius), sunflower (Helianthus annuus), sorghum (Sorghum bicolor), processing tomatoes (Solanum lycopersicum), and cotton (Gossypium hirsutum) were incorporated in the field trial. By the end of the incubation, cotton residues resulted in the greatest increase in soil mineral N, and sunflower residues resulted in the greatest nitrogen immobilization when compared to the control without residue. The difference between the two treatments was 13.7 mg N kg-1 OD soil, or approximately 28.4 kg N ha-1 in the top 15 cm of the soil. Finally, we performed two 10-week laboratory incubations investigating soil and residue moisture’s effect on nitrogen mineralization using processing tomato and broccoli residues. At the beginning of both incubations, residue moisture had a significant effect on nitrogen mineralization, however, by the end of both incubations, residue moisture was no longer significant, while soil moisture became increasingly important (P < 0.05). During the 10-week incubation, 21.6% and 50% of the total nitrogen in tomato and broccoli residues was mineralized, respectively. Differences are likely due to the residue’s carbon:nitrogen ratios and lignin content. The results of these studies will be used by growers in the region to improve upon their fertilizer management practices and make informed decisions based on cropping history and soil properties.

Over 400,000 acres of California farmland are certified organic and receive all fertility from organic inputs. In addition, due to new legislation requiring the diversion of organic waste from landfills into composting facilities, compost usage is likely to increase even on non-organically managed land. Organic inputs can potentially have many positive effects on soil health, such as increasing the soil organic carbon (C) content, nitrogen (N) cycling for plant growth, and improving the resilience of soil functions to stressful events. However, they may also have negative effects: they can be expensive, and inappropriate application may lead to poor crop growth or environmental pollution. To manage organic inputs in such a way as to maximize the benefits and reduce the risks, California growers need good region- and system-specific information on the effects of a range of organic inputs on the many facets of soil health. The goal of this study is to provide research-based information about the effect of several organic inputs in irrigated California cropping systems on different aspects of soil health, especially as they relate to soil N cycling, C accumulation, and biological function. A secondary focus is to identify important sources of variability both within those effects and within metrics themselves, as soils, amendments, and cropping systems are all intrinsically variable, and “one-size-fits-all” recommendations may not be appropriate. One goal of applying organic materials is short-term N fertility, but there is uncertainty about how much N mineralization can be expected. I used lab experiments to measure the range and variability of net N mineralization from different organic amendments and compost types in common use in California, and tested how this mineralization is affected by soil management history, and how it can be predicted by simple biochemical measurements. I found that materials had a wide range of N mineralization potentials, from immobilization to almost 100% availability, and that availability was broadly predicted by the amendment C to N ratio. Soil management history did not affect the N release rate. Another potential short-term effect of organic inputs is to buffer the soil community against shocks. My second experiment explored whether a recent compost addition could increase the resilience of microbial C and N cycling functions to a sudden chemical stress, the addition of elemental sulfur. I found that stress greatly reduced most C cycling indicators and inhibited nitrification, but tended to increase N mineralization, particularly as the soils adapted to stress. Legume residues added to the stressed soils stimulated catabolic processes (i.e. respiration and net N mineralization) to a much greater extent than anabolic processes (accumulation of biomass C), suggesting a reduced efficiency in the stress-adapted community. Compost addition did not affect how C mineralization or microbial C responded to stress. However, it consistently increased N mineralization, particularly when legume residues were added to the stressed soils, suggesting compost facilitated a community shift towards one with a relatively low need for N relative to C. In the third and fourth parts of this work, I explored the potential effects of long-term compost and cover crop incorporation on soil health by doing a full assessment of biological, physical, and chemical soil health indicators in a long-term field research experiment in which corn and processing tomato have been either conventionally or organically farmed for over 25 years. In this two-pronged approach, I firstly characterized the sensitivity of the different soil health indicators to management, as well as how they vary with growing season, crop type, and sampling date. In the second part, I assessed the management effects on three essential soil functions: C storage, N mineralization and crop yields. I assessed which indicators are most closely and consistently related to these functions across years and crop types. Finally, I used structural equation modeling to test the hypothesis that healthy soils lead to less stressed crops and thus to higher yields. This study found that organic inputs had strongly increased organic matter and biological activity at these sites. Indicators of C storage and especially biological activity tended to fluctuate between years and sampling dates, but were generally unaffected by crop phase. Different indicators were appropriate for predicting C storage and N mineralization, and I concluded that a holistic soil health assessment should include both indicator types. However, since yields were lower in the organic than conventional system for both crops, my hypothesis that healthier soils would produce healthier crops was not supported. The results of the path analysis suggested the presence of an unmeasured factor strongly related to both management and yields. In the years of this study this factor was likely disease pressures which built up over the years in the organic system. Together, the data from these experiments will be useful to growers, researchers, and policymakers wishing to assess the potential benefits, limitations and risks of different organic amendments over short and long time scales.

In one of the most agriculturally diverse production regions in the world, California researchers and producers face unique and complex challenges. With this study, we have elucidated several important research questions addressing those challenges, namely, how organic matter content affects nitrogen (N) mineralization across the tapestry of soil types under cultivation using an N-budget approach, the connection between expected yield and maximum N accumulated in aboveground biomass for two important cash crops, the diversity in chemical composition of the particulate organic matter fraction and how it pertains to organic matter and nutrient cycling, and how infrared chemical signatures can potentially be used as a surrogate for time-consuming laboratory analyses. We observed that high organic matter soils (> 30 g C kg-1 soil) mineralized an average of 2.2 kg N ha-1 day-1 over the growing season and that low organic matter soils mineralized 1.1 kg N ha-1 day-1. We found that for grain corn and hybrid sunflowers that crop yield and maximum N accumulated in the aboveground biomass were significantly correlated. We also found that particulate organic matter (POM) originating from high organic matter soils was more decomposed and had lower recalcitrance than the less decomposed and more recalcitrant low organic matter POM, using infrared spectra. We were also able to the estimate potential N mineralization from undisturbed soil cores with 90.1% accuracy in high organic matter soils (n=25) using solely infrared spectra and partial least squares regression. This information will allow those who work in agroecosystems across the state to make informed decisions regarding issues such as fertilizer management, further studies on organic matter influences on biochemical cycling, and the most efficient approach for estimating soil characteristics.

Many farms in northeast California are experimenting with organic production to take advantage of price premiums and niche markets. A common challenge in organic farming is finding dependable nitrogen sources to meet the needs of vegetable and grass crops, especially in fields with low soil nitrogen. This study assessed the use of cover crops and organic amendments for increasing soil nitrogen for potato production at the Intermountain Research and Extension Center in Tulelake. Researchers evaluated several cover crop species, three planting dates and multiple cover crop mixes. Amendments included composts, manures, bloodmeal and soymeal. The data collected in the study included total nitrogen from cover crops and amendments, plant-available nitrogen in the soil, potato petiole nitrate and crop yield and quality. Vetches and field peas, managed as green manure, were successful at satisfying potatoes' in-season nitrogen demand. These cover crops, grown alone or in mixes with non-legume species, produced potato crops whose yield and quality were similar to crops grown with conventional fertilizers. The cover crops' influence on potato pest pressure was neutral. Chicken manure was the most cost-effective amendment for satisfying potatoes' in-season nitrogen demand.