Positive ions that are available in soils absorb on grain surfaces. The total sum of cations that can be absorbed bij a soil/sediment at a certain PH is defined by the cation-exchange capacity (CEC, in meq g-1: mol equivalents per gram). The uptake of cations is an important parameter in agriculture and the larger the CEC, the more cations can be absorbed to the soil. The CEC depends highly on the pH of soil and sediments, where the CEC decreases with decreasing PH (increasing acidity). The exchange of ions on sediments occurs commonly fast on geological time scales, but the kinetics of adsorption in natural environments is still poorly understood. The strength of the bonding between the cations and the sediments varies from weak Van der Waals bondings (physical adsorption) to strong chemical bonds. The CEC is widely used for agricultural assessment because it is a measure of general soil fertility as well as an indicator of structural stability because CED is capabel of enhancing development of shrinkage cracks. The list below shows the CEC for different types of minerals. The data indicate that the CEC can vary over 2 orders of magnitude for various types of , minerals and can vary one order of magnitude within one soil type. Cation exchange capacity for different types of sediment (Eisma, 1992; Locher and de Bakker, 1990):

Cation exchange capacity.

Nitrogen is the most limiting nutrient for crop production in many of the world's agricultural areas and its efficient use is important for the economic sustainability of cropping systems Furthermore, the dynamic nature of N and its propensity for loss from soil‐plant systems creates a unique and challenging environment for its efficient management Crop response to applied N and use efficiency are important criteria for evaluating crop N requirements for maximum economic yield Recovery of N in crop plants is usually less than 50% worldwide Low recovery of N in annual crop is associated with its loss by volatilization, leaching, surface runoff, denitrification, and plant canopy Low recovery of N is not only responsible for higher cost of crop production, but also for environmental pollution Hence, improving N use efficiency (NUE) is desirable to improve crop yields, reducing cost of production, and maintaining environmental quality To improve N efficiency in agriculture, integrated N management strategies that take into consideration improved fertilizer along with soil and crop management practices are necessary Including livestock production with cropping offers one of the best opportunities to improve NUE Synchrony of N supply with crop demand is essential in order to ensure adequate quantity of uptake and utilization and optimum yield This paper discusses N dynamics in soil‐plant systems, and outlines management options for enhancing N use by annual crops

Enhancing nitrogen use efficiency in crop plants

Physiological perspectives of changes over time in maize yield dependency on nitrogen uptake and associated nitrogen efficiencies: A review

[1] One of the main causes of the low efficiency in nitrogen (N) use by crops is the volatilization of ammonia (NH3) from fertilizers. Information taken from 1667 NH3 volatilization measurements documented in 148 research papers was summarized to assess the influence on NH3 volatilization of crop type, fertilizer type, and rate and mode of application and temperature, as well as soil organic carbon, texture, pH, CEC, measurement technique, and measurement location. The data set was summarized in three ways: (1) by calculating means for each of the factors mentioned, in which findings from each research paper were weighted equally; (2) by calculating weighted median values corrected for unbalanced features of the collected data; and (3) by developing a summary model using linear regression based on weighted median values for NH3 volatilization and by calculating global NH3 volatilization losses from fertilizer application using 0.5° resolution data on land use and soils. The calculated median NH3 loss from global application of synthetic N fertilizers (78 million tons N per year) and animal manure (33 million tons N per year) amount to 14% (10–19%) and 23% (19–29%), respectively. In developing countries, because of high temperatures and the widespread use of urea, ammonium sulfate, and ammonium bicarbonate, estimated NH3 volatilization loss from synthetic fertilizers amounts to 18%, and in industrialized countries it amounts to 7%. The estimated NH3 loss from animal manure is 21% in industrialized and 26% in developing countries.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000GB001389

Estimation of global NH3 volatilization loss from synthetic fertilizers and animal manure applied to arable lands and grasslands

Agricultural robots for field operations: Concepts and components

3 Soil properties and weather conditions are known to affect soil nitrogen (N) availability and 4 plant N uptake. However, studies examining N response as affected by soil and weather 5 sometimes give conflicting results. Meta-analysis is a statistical method for estimating treatment 6 effects in a series of experiments to explain the sources of heterogeneity. In this study, the 7 technique was used to examine the influence of soil and weather parameters on N responses of 8 corn (Zea mays L.) across 51 studies involving the same N rate treatments which were carried out 9 in a diversity of North American locations between 2006 and 2009. Results showed that corn 10 response to added N was significantly greater in fine-textured soils than in medium-textured 11 soils. Abundant and well-distributed rainfall and, to a lesser extent, accumulated corn heat units 12 enhanced N response. Corn yields increased by a factor of 1.6 (over the unfertilized control) in 13 medium-textured soils and 2.7 in fine-textured soils at high N rates. Subgroup analyses were 14 performed on the fine-textured soil class based on weather parameters. Rainfall patterns had an 15 important effect on N response in this soil texture class, with yields being increased 4.5-fold by 16 in-season N fertilization under conditions of “abundant and well-distributed rainfall.” These 17 findings could be useful for developing N fertilization algorithms that would allow for N 18 application at optimal rates taking into account rainfall pattern and soil texture, which would lead 19 to improved crop profitability and reduced environmental impacts. 20

Corn response to nitrogen is influenced by soil texture and weather

Low use efficiency has frequently been observed when N fertilizers, particularly those containing urea, are applied directly on the surface residue in no-till corn (Zea mays L.) production systems. A portion of this low use efficiency has been attributed to volatilization of NH₃. Field experiments were conducted at two locations in Indiana to make direct measurements of NH₃ volatilization from granular urea, cogranulated urea-urea phosphate (UUP), urea-ammonium nitrate solutions (UAN) and prilled ammonium nitrate (AN). The fertilizers were applied directly on corn residue at a rate of 168 kg N ha⁻¹. On a Lyles sandy loam soil (Typic Haplaquolls) 50.9 kg of the 168 kg N ha⁻¹ applied as granular urea was lost in 120 h, with essentially all that loss occurring in the first 50 h after application. Peak loss rates of 3.0 kg N ha⁻¹h⁻¹ occurred 23 h after application. No significant volatilization loss occurred with AN, while 26.0 kg N were lost through volatilization with UUP and 14.6 kg N lost from UAN. Peak loss rates at 23 h after application were 1.83 and 0.37 kg N ha⁻¹h⁻¹ for UUP and UAN, respectively. A 25-mm rain shower effectively stopped volatilization approximately 50 h after application. On a Bedford silt loam soil (Typic Fragiudalfs) volatilization losses were considerably lower than those found on the lower cation exchange capacity Lyles soil. Though overall losses were lower, relative losses from the different N fertilizers were similar to those found on the Lyles soil.

Ammonia Volatilization from Nitrogen Fertilizers Surface Applied to No-till Corn

Recent attention has focused on management of soil acidity in no-till (NT) soils due to the limited movement of surface-applied lime in these systems. Interactions of exchangeable Al and organic matter have been recognized for many years, but limited data exist investigating how these interactions should affect management decisions for NT soils. This study was conducted to identify effects of rotation and tillage on soil pH and soil organic carbon (OC) content and to determine the influence of soil pH and OC on KCl and CuCl 2 extractable-Al (Al KCl and Al CuCl2 , respectively). Soil samples were collected to a depth of 15 cm, in 2.5-cm increments, from a long-term rotation and tillage study near Manhattan, KS. Soil pH and OC concentrations were influenced by rotation and tillage, especially in the surface 2.5 cm. Organic C concentrations were on average 2.3 g kg -1 greater with NT than with conventional tillage in the surface 15 cm of soil. Aluminum extracted with KCl and Al CuCl2 increased exponentially with decreasing soil pH. Copper chloride extractable-Al values were on average 8% greater than Al KCl values. When using a regression model to predict the difference between Al CuCl2 and Al KCl , inclusion of OC explained only 4% more variability compared with inclusion of only soil pH in the model. A change in OC concentrations of 2.3 g kg -1 as observed in this study, after reducing tillage would likely not alleviate Al toxicity if pH became very acidic (pH < 5).

Changes in Soil pH, Organic Carbon, and Extractable Aluminum from Crop Rotation and Tillage

Protecting water quality and maintaining profitable corn (Zea mays L.) production requires diagnostics that can distinguish between N deficiency, sufficiency, and excess. This study evaluates initial recommendations on blacklayer basal-stalk NO 3 -N ranges and critical concentrations for diagnosing N status, and it compares the performance of this test with grain analysis. Observations (428) were collected from 13 N-response experiments. Linear response and plateau (LRP) and binary logistic regression (BLR) were used to characterize the relationships between yield and tissue-test values. With the LRP, stalk NO 3 -N and grain N concentrations separating deficient from sufficient observations were 0.42 and 13.1 g kg 1 , respectively, and the success rates of the two tests were comparable (77 and 75%, respectfully). The BLR also identified critical concentrations, but the values increased with decreasing yields, a desirable decision-rule attribute given that extreme deficiency can result in higher-than-expected tissue concentrations. The success rates of multiple BLR functions using yield and stalk or grain analysis as factors were again comparable (88 and 87%, respectively), but they were significantly greater than with the LRP analysis. Stalk analysis was superior to grain analysis for distinguishing sufficiency from excess. A constant stalk NO 3 -N concentration (1.67 g kg 1 ) separated sufficient from excessive cases, and fertilizer efficiency approached zero at 2.9 g kg 1 . Premature sampling resulted in stalk NO 3 -N levels that were 40 to 600% greater than levels observed after blacklayer formation, with the greatest error occurring when N fertility was low. When not testing for N excess, the advantages of grain analysis are the ease of sampling during harvest and the reduced risk of error associated with premature sampling.

Diagnostic Efficiency of the Blacklayer Stalk Nitrate and Grain Nitrogen Tests for Corn

Increasing acreage of no-till (NT) cropping systems with surface applications of N fertilizer brings forth the important issue of management of acidic soils in these systems. Our objectives were to determine the vertical movement of surface-applied lime, and to determine if frequency or type of lime applied affects the rate of movement; to evaluate the effect of surface application of lime on soil chemical properties; and to determine the correct application rate of lime for acidic NT soils. Three NT field sites were identified that had below-optimal soil pH (<6.0) in the surface 15 cm. Various lime treatments were established in 2000 or 2002, consisting of differing rates [as kg effective calcium carbonate (ECC) ha 1] of either limestone (commercially available) or pelletized limestone, plus an unlimed control. In the spring of 2005, soil samples were taken to a depth of 30 cm; the surface 15 cm was separated into 2.5-cm increments and the lower 15 cm was separated into 7.5-cm increments. Evidence of lime movement was limited to 7.5 cm or less at all three NT sites, as indicated by significant increases in pH compared with the control. Type of liming material or frequency of limestone application seemed to have no effect on any variables measured. Significant yield increases were not observed for crops as a result of limestone applications. Limestone recommendations for NT production systems need to be based on correcting pH in the surface 7.5 cm for production systems receiving 800 to 1000 mm of annual precipitation.

David B. Mengel

Papers

Corn response to nitrogen is influenced by soil texture and weather

Ammonia Volatilization from Nitrogen Fertilizers Surface Applied to No-till Corn

Changes in Soil pH, Organic Carbon, and Extractable Aluminum from Crop Rotation and Tillage

Diagnostic Efficiency of the Blacklayer Stalk Nitrate and Grain Nitrogen Tests for Corn

Management of Soil Acidity in No-Till Production Systems through Surface Application of Lime