In many horticultural cropping systems, leaf sampling with comparison to established critical values (CV’s) represents the primary tool for fertilizer decision-making. In the majority of crop production systems, however, the use of CV’s is inadequate as a result of flaws in both practice and interpretation. Ideally, CV’s will have been established by carefully controlled experiments in which the relationship between yield and nutrient concentration is closely followed. In the majority of crops, with the exception of the major staples and broad acre field crop, trials of this kind are very limited and hence the integrity of the CV's is questionable. The utility of the CV as a management tools is further compromised by the very significant difficulty in obtaining representative tissue samples as a result of within plant, spatial and temporal variability. Further, CV’s established on the basis of the nutrient concentration at which a species’ attains near full yield (physiological CV), provide only limited information on the response of a population of plants under field conditions. The important difference between a ‘physiological’ CV and a ‘population’ CV, is rarely considered in field practice. Managing a field so that the mean plant tissue concentration is at or just above the CV will result in up to 50% of individuals falling below critical value, while managing to ensure that 95% of all individuals exceed the critical value thus ensuring maximal field productivity, will result in over-fertilization of the majority of the field. While attempts to reduce the cost of sampling, to refine the speed or accuracy of monitoring techniques or to create fertilization practices based upon models of crop demand or nutrient budget, can all help improve efficiency incrementally, these approaches alone cannot substantially contribute to enhanced fertilizer use efficiency unless they are combined with attempts to quantify and manage field variability.

Potassium (K) is one of essential elements for plant growth, especially orchard crops, and the third most important macronutrient after nitrogen and phosphorus. Despite the fact that K reserves are relatively abundant on earth, consisting of 2.1 – 2.3% of the earth crust, a large proportion of K is not available for plant uptake due to its uneven distribution. Potassium inputs come from different sources such as irrigation water or soil amendment applications. To supply K replenishment in agriculture, the application of K fertilizer is the most effective to increase K nutrient in soils. Thus, this study will focus on K behavior in the plant as well as dynamics in soil context. The K pools in soil are solution K, exchangeable K, non-exchangeable K, and structural K. Both solution K and exchangeable K portions are considered plant-available K, although they are usually low in total soil K. A number of factors govern the amount of K pools in soil, including soil texture, cation exchange capacity (CEC), pH, moisture, temperature and clay mineralogy. These factors not only influence the K efficiency of plant uptake, but also affect the K distribution of K availability by soil depths. In terms of precise estimation of available K for crop uptake, the current K fertilizer application recommendations are based on ammonium acetate extraction method, which only measures the solution K and exchangeable K. A novel sodium tetraphenylboron (NaTPB) method was introduced and discussed to help better estimate the plant-available K. In plants, K is the key component that maintains numerous metabolic functions at all levels. For instance, enzyme activation, protein synthesis, photosynthesis, carbon assimilation and transport, and water balance. Therefore, a lack of K nutrition leads to malfunctions of these processes and eventually reduces the crop yield and fruit quality. In annual crops, K deficiency is easy to observe. Generally, the most common symptoms of K deficiency firstly occur in the older leaves and include chlorosis and necrosis. While in orchard crops, low K condition in trees does not immediately become visible and result in low yield or reduced quality of harvest. Hence, a regular leaf K analysis is recommended for growers to monitor tree K levels on purpose. For orchard production, a proper K management is especially important to secure yields. However, different orchard crops require different amounts of K. For instance, in almonds 75 kg K was removed in fruit per metric ton of kernel yield. Usually, orchard crops are not sensitive to K fertilizer sources but the application rates. For example, in pistachios, Zeng & Brown (2001) proposed that a K application rate between 110 – 220 kg ha-1 was sufficient to sustain highly productive pistachio orchards with any K sources while excessive K fertilizer (> 220 kg ha-1) was applied, nut yield reduced in the subsequent year. During the course of weathering, the rate of K release and fixation is highly correlated to the clay mineralogy. Some secondary clay minerals, such as montmorillonite, are high K-fixed, which makes K become unavailable. In addition, the alternate wetting-drying cycle regimes greatly reduce K diffusive flux and K mobility in soil. Thus, proper K fertilizer management and application become crucial in agriculture. In this study, three K fertilizers are discussed – muriate of potash (MOP), sulfate of potash (SOP), and polyhalite (POLY). There is still little information available to describe the properties of K fertilizers and their efficacy for orchard production, especially almond production. To better understand K behavior in the soil system, the experiment was conducted to explore K dynamics using a soil-column experiment by comparing the behavior and distribution of KCl, microfine SOP and POLY in three different soils (Hillgate sandy loam and two different loams San Ysidro and Yolo) obtained from almond orchards in California’s Central Valley. Three replicate columns with each K fertilizer along with an unfertilized control were subject to four wetting events over a two-month period, and different K pools were measured at depth of 0-10, 10-20 and 20-30 cm. POLY resulted in a higher solution K concentration in all soils. After complete incubation, a large proportion of unaccounted K (e.g. 921 mg K column-1, which is equivalent to 42% of input K in the San Ysidro soil) was observed, perhaps due to movement of K into the non-exchangeable pool. The slow-release characteristic of POLY-K and the additional content of Ca and Mg increased total extractable base cation levels in soils, and increased leaching of base cations after four wetting compared to other two K fertilizers.

Returning almond hulls and shells to the orchard soil as organic matter amendments can provide numerous soil, water, and plant benefits. Billions of pounds of almond hulls and shells accumulate at California almond processors annually and need a convenient and sustainable outlet. As hulls and shells contain substantial potassium (K) concentrations, surface-applying these materials as organic matter amendments onto nearby almond orchard soil could reduce K fertilizer costs for growers by around 80% by retaining plant K in the orchard. Prior studies have found many crop residues including nutshells release K readily under water application, can improve soil-plant water dynamics, and support higher levels of soil microbiology. However, field trials characterizing the effects of nutshell amendments on K cycling, soil-water dynamics, and soil microbial responses are scarce. Research is needed to evaluate K release from nutshells and impacts on soil K availability and tree K status in commercial orchards. The effects of surface-applied nutshell amendments on soil-plant water dynamics and impacts on soil microbial community composition over time merit further research to assess potential benefits to soil and plant function. Three field trials were conducted to evaluate the effects of this practice on: hull/shell K solubilization, soil K availability, plant nutrient status, yield, soil-plant water dynamics, decomposition rates, and soil microbial functional community composition. Research questions were investigated in field trials established in 2020 in mature commercial California almond orchards. All field trials are randomized complete block designs with each treatment applied to the entire row (at least 40 trees per experimental unit). Each site was approached as a case study with distinct research questions and treatments tailored to grower priorities and current research gaps. This research was funded through a Western SARE grant. Results indicate surface-applied almond hull and shell organic matter amendments can increase K cycling and plant K status, reduce soil surface evaporation, maintain higher moisture in the upper soil layer, moderate plant water stress during dry periods, and increase microbial biomass of many beneficial functional groups in the soil and the amendment layer including arbuscular mycorrhizal fungi. Off ground harvest machinery can be used to maintain the microbially rich amendment organic layer on the soil surface in the tree row and release a more complete percentage of total amendment K. Findings can be used to support grower decision-making, practice implementation, and future research across different orchard contexts and management approaches.

This thesis explores the broad physiological responses of tomato (Solanum lycopersicum) in solution culture to various spatial potassium (K) distributions under heterogeneous rootzone salinity (NaCl). Chapter 1 is a collaborative review of heterogeneous soil salinity, introducing how management and environment influence salt distribution patterns, and reviews ensuing physiological responses. The review also summarizes the limited research on interactions between heterogeneous salinity and nutrient distribution, particularly split-root experiments, a line of inquiry which this research seeks to enrich. Chapter 2 outlines an original experiment where tomato plants were grown in solution culture with roots evenly divided between two compartments. Except for a salt-free control group (Treatment 0), the same overall amount of salt (NaCl) was either provided to the plant uniformly across the entire root zone (treatment 1) or provided to only one half of the root system (treatments 2, 3, and 4). Treatments 2, 3, and 4 feature an increase in the share of the K budget which is supplemented in the saline compartment compared to the non-saline compartment. Treatment 2 provides nutrients including K to one side and NaCl to the other. Treatments 3 and 4 increase K in the saline compartment to 40% and 80% of the K budget, respectively. The impacts on biomass accumulation, biomass partitioning, water uptake, sodium uptake, and potassium uptake were measured and analyzed for statistical significance. There was no difference in the total biomass, overall water uptake rate, or root distribution between root halves across uniform treatment groups (0 and 1) despite a major difference in overall solution NaCl concentration (0 mM and 20 mM average, respectively). In all treatments where supplemented sodium (40 mM) was confined to half of the root zone (2, 3, and 4), plant water uptake was restricted almost completely to the non-saline compartment demonstrating a remarkable plasticity of root response to local saline conditions. Whole plant water uptake rates were generally comparable irrespective of saline distribution. Saline compartments of treatments 2, 3, and 4 did not show sodium or potassium uptake, regardless of potassium richness. Across all treatments, there was a strong tendency for water, and potassium uptake, as well as root growth, to occur in the Na-free compartment. The only instance of plants utilizing solution K in the presence of NaCl occurred in treatment 1, where K was supplemented along with all other nutrients uniformly in an overall saline root environment. Interestingly, this treatment was also the only clear instance of sodium uptake to occur among any treatment groups.

The results of these experiments suggest a salt-avoidant response, whereby the presence of any salt-free and nutrient-rich root zone will result in preferential water uptake from that zone. Research (summarized in Chapter 1) also demonstrates that the provision of a full nutrient supply exclusively to the saline side of a split root system will result in considerable water uptake from the saline compartment and increase whole plant salt uptake. This research was conducted to determine which of the nutrients in the nutrient-rich zone was responsible for plant activity in the saline zone that may otherwise have been avoided. The research performed here demonstrates that the driving dynamic for this plant response is not K alone. The goals of minimizing the incidence of salinity stress and maximizing nutrient use efficiency are inextricable in the agronomic system. Understanding the relationship between nutrient use and salt localization is important if we are to optimize management systems under the heterogeneous ion distributions that are commonplace in irrigated agriculture.

This thesis explores the use of hull split regulated deficit irrigation in combination with early harvest of almonds (Prunus dulcis (Mill.) D. A. Webb) for improved irrigation efficiency and control of navel orangeworm (NOW) (Amyelois transitella (Walker)) and Hull Rot (HR) (Rhizopus stolonifer (Ehrenb:Fr.) Vuill.; Monilinia fructicola (G. Wint.) Honey). Chapter 1 contains a review of the literature regarding California almond production, almond phenology, almond water status monitoring, and NOW and HR phenology and control. Chapter 2 describes research carried out during the 2020 growing season to test the efficacy of early almond harvest for NOW and HR control and water use efficiency. Standard Harvest (SH) and Early Harvest (EH) treatments were applied to trees in two almond orchards in Denair and Woodland, California using a randomized controlled block design; EH treatments were harvested three to four weeks prior to SH. Midday Stem Water Potential (SWP) measurements were used to monitor tree water status and manipulate irrigation to prepare trees for EH treatments. Trees were maintained at -14 to -18 bars SWP for the two to three weeks leading up to both EH and SH. There were significantly lower HR strikes in EH treatment trees than in SH treatment trees. No significant difference was observed in NOW incidence between treatments, although neither site had a history of high NOW pressure. No significant kernel yield, marketable yield, or shaker efficacy differences were seen between the two treatments. Although 3.28 and 1.61 inches of water less was applied to EH trees than SH trees at the Denair and Woodland sites respectively, the trees from both treatment groups at both sites returned to approximately the same water status after Standard Harvest and mostly recovered from the induced moderate water stress by October 12. With further development of complementary harvest and processing strategies, early harvest of almonds shows promise as a strategy for pest and disease control and improved water use efficiency in almond orchards.

The use of organic matter amendments (OMAs) is an ancient practice for improving agricultural soils, but our understanding of the microbial mechanisms by which benefits to soil ecosystems occur remains incomplete. The 1.6 million acres of almond orchards in California need effective and sustainable management practices that promote soil as a living ecosystem. This thesis examines OMAs in the context of emerging microbiome science, our current understanding of relevant soil processes as affected by OMAs, and the challenges involved in modeling nutrient cycling through the microbiome. Results from two research trials conducted in 2019-2021 to evaluate three soil health practices in almond agroecosystems are presented. First, a field trial explored the effects of an almond hull and shell amendment and off-ground harvest on soil C and N and soil microbial biomass in almond orchards. A 210-day incubation trial further evaluated the same amendment on soils with differing management histories by utilizing soil that had previously received a green waste compost OMA for three years. Results highlight the suitability of the almond hull and shell amendment as a soil health treatment, finding that it increased microbial respiration and microbial biomass without significantly affecting nitrogen immobilization. Soil with a history of past OMA application displayed higher microbial respiration, dissolved organic carbon, and net N mineralization than soil without that history when receiving a new amendment of almond hulls and shells. Decomposition of amendment residue progressed at nearly identical rates in the field trial and the incubation trial and exhibited characteristics comparable to forest litter layers. These findings add to our understanding of the processes by which OMAs impact the soil microbiome and our ability to utilize these processes to achieve specific outcomes.

A large scale multi-year experiment has been set up to develop a phenology and yield based nutrient model for Almond, to develop fertilizer response curves to relate nutrient demand with fertilizer rate and nutrient use efficiency and to determine nutrient use efficiency of various commercially important N and K fertilizer sources. The goal was to develop an integrated nutrient best management practice for almond. The treatments consists of four rates of nitrogen 140kg/ha, 224kg/ha, 308kg/ha and 392kg/ha and two commercially important sources of nitrogen, UAN 32 and CAN 17. There are three treatments for potassium rates: 112kg/ha, 224kg/ha and 336kg/ha and three sources of potassium: SOP, SOP+KTS and KCl. Leaf and nut samples were collected from the 768 individual trees in April, May, June, July, August and October and analyzed for N, P, K, Ca, S, Mg, B, Zn, Cu, Mn and Fe. A clear effect of N rate on the pattern and total quantity of annual nitrogen accumulation was observed. Changes in the pattern of total fruit N accumulation, leaf and fruit N concentrations suggest that fruit growth is the primary determinant of tree N accumulation and that resorption of N from fruit into perennial tree structures occurs as fruit matures. Similar patterns of resorption were observed for P but not for the other elements measured.

Regenerative agricultural practices strive to produce food that have lower economic, environmental, and social impacts defined by the processes used (i.e. using cover crops, reducing tillage) or by proposed outcomes (i.e. improving soil health, improving biodiversity). In almond orchards the regeneration of ecosystem health and productivity can be achieved through practices including recycling of almond hull and shell resources as soil amendments, reducing dust production during harvest, and minimizing synthetic fertilizer inputs. The current and set standards of conventional almond production poses a unique challenge when proposing regenerative agriculture goals. To facilitate almond harvest and simplify irrigation and fertigation practices, almond orchards are typically managed with the orchard floor bare, and trees are grown in rows with irrigation and fertilization localized exclusively to the 3.6 meter (12-ft) wide soil strip below the tree canopy. The alleyways are often left unirrigated and bare to facilitate almond harvest. These conventional practices reduce soil health, whereas with the advent of off-ground harvesting, opportunities exist to incorporate cover crops (CC) and the use of organic matter amendments (OMA) including almond hulls and shells (AHS) to maintain a more permanent mulch layer to protect the soil. This thesis examined the changes in soil physical, chemical, and biological properties from the use of AHS and a CC mix of 60% oats, 35% spring peas, and 5% yellow mustard in a randomized complete block design field trial. Nutrient cycling and OMA decomposition dynamics was analyzed one and two years after practice adoption. Effects on soil aggregate stability, particulate organic matter (POM), mineral associated organic matter (MAOM), microbial biomass, and soil moisture in amended trees harvested using on-ground and off-ground harvest machines were determined. Results showed increased C and N in microaggregate and macroaggregate soil organic matter fractions, increased saprophytes and arbuscular mycorrhizal fungi, and improved soil moisture in amended soils. Exchangeable potassium (XK) and soil organic matter (SOM) increased in the amended on and off-ground harvest treatments and was highly correlated with microbial communities measured. We found that after 3 years of AHS OMA and catch-frame harvest, MAOM and soil aggregate stability increased due to this undisturbed forest litter layer which may allow more microbial processes to contribute to more MAOM over time. Cover crop biomass was highest in oats but peas contributed the highest soil N. 192 days after AHS application, an average of 38% of the AHS material decomposed and 90 days after the CC were seeded, an average of 69% of the CC material decomposed.