Open Access. Powered by Scholars. Published by Universities.®

Digital Commons Network

Open Access. Powered by Scholars. Published by Universities.®

Articles 1 - 30 of 98

Full-Text Articles in Entire DC Network

Evaluation Of Cover Crop Mixtures For Weed Management And Soil Fertility Improvement In Organic Agriculture, Christos Vasilikiotis Jan 2018

Evaluation Of Cover Crop Mixtures For Weed Management And Soil Fertility Improvement In Organic Agriculture, Christos Vasilikiotis

Creative Components

Cover crops are promoted as a tool to preserve agrobiodiversity, to enhance soil chemical and physical properties, and to help suppress weeds. This study was conducted at two sites in northern Greece with similar climates, but different soil types, over two growing seasons in 2014-15 and 2017-18. Two multi-species cover crop mixtures with seven and eight types of legumes and grasses, respectively, were monitored to determine their weedsuppressing and soil fertility enhancement attributes compared to a vetch/oat (Vicia sativa L./Avena sativa L.) mixture, and single-species crops of triticale (×Triticosecale Wittm.), rye (Secale cereale L.) and ryegrass (Lolium multiflorum ...


Improved Production Systems For Common Bean On Ferralsol Soil In South-Central Uganda, Lance H. Goettsch, Andrew W. Lenssen, Russell S. Yost, Ebby S. Luvaga, Onesmus Semalulu, Moses Tenywa, Richard Miiro, Robert E. Mazur Jun 2017

Improved Production Systems For Common Bean On Ferralsol Soil In South-Central Uganda, Lance H. Goettsch, Andrew W. Lenssen, Russell S. Yost, Ebby S. Luvaga, Onesmus Semalulu, Moses Tenywa, Richard Miiro, Robert E. Mazur

Agronomy Publications

Common bean (Phaseolus vulgaris L.) is the most important source of dietary protein in Uganda but current grain yields are low. Production is particularly low on the degraded Ferralsol soils that dominate the landscape because these soils are generally weathered, acidic and infertile. A study was done on a Ferralsol soil in Masaka District, Uganda to compare productivity and economic return to labor and management (ERLM) for four bean cultivars grown under three management systems. The experimental design was a randomized complete block in a split-plot arrangement. Management system was the whole-plot factor and included the Conventional Farmer (CFS), Improved ...


Improved Production Systems For Common Bean On Phaeozem Soil In South-Central Uganda, Lance H. Goettsch, Andrew W. Lenssen, Russell S. Yost, Ebby S. Luvaga, Onesmus Semalulu, Moses Tenywa, Robert E. Mazur Nov 2016

Improved Production Systems For Common Bean On Phaeozem Soil In South-Central Uganda, Lance H. Goettsch, Andrew W. Lenssen, Russell S. Yost, Ebby S. Luvaga, Onesmus Semalulu, Moses Tenywa, Robert E. Mazur

Agronomy Publications

Common bean (Phaseolus vulgaris L.) is the most important grain legume in Uganda. Beans managed under conventional systems range in yield from 500 to 800 kg ha-1, with a yield gap of about 75%. The objective of this study was to compare the productivity and net profitability of four bean cultivars grown under three management systems on Phaeozem soil (Mollisol) in Masaka District, Uganda. The experimental design was a randomized incomplete block in a split-plot arrangement. Management system was the whole-plot factor and included the Conventional Farmer (CFS), Improved Farmer (IFS), and High Input systems (HIS). Management systems differed for ...


Improved Production Systems For Common Bean In South-Central Uganda: I. Liddugavu Soil, Ii. Limyufumyufu Soil, Lance Henry Goettsch Jan 2016

Improved Production Systems For Common Bean In South-Central Uganda: I. Liddugavu Soil, Ii. Limyufumyufu Soil, Lance Henry Goettsch

Graduate Theses and Dissertations

Common bean (Phaseolus vulgaris L.) is the most important source of dietary protein in Uganda but current grain yields are extremely low. Beans are produced on a variety of soils in south-central Uganda but the two most important soils for bean production are the highly weathered Limyufumyufu (Ferralsol) and the relatively fertile Liddugavu (Phaeozem) soils. These two soils vary in level of pH and fertility and therefore must be managed appropriately. Beans managed under conventional systems have a yield gap of about 75% due to poor agronomic practices, soil infertility, lack of seed from improved cultivars, moisture stress, weed competition ...


On-Farm Evaluation Of Fluid Starter Potassium Fertilization For Corn Using Precision Agriculture Technologies, Nicolas Bergmann Jan 2009

On-Farm Evaluation Of Fluid Starter Potassium Fertilization For Corn Using Precision Agriculture Technologies, Nicolas Bergmann

Graduate Theses and Dissertations

Use of fluid N–P or N–P–K starter mixtures in most research studies precludes firm conclusions about specific starter K effects on early crop growth and grain yield. This study evaluated corn (Zea mays L.) early growth, early plant K concentration and uptake, and grain yield response to fluid starter K fertilizer. Eight trials were conducted during 2007 and 2008 in Iowa farmers’ fields using a strip trial methodology, global positioning systems, yield monitors, and geographical information systems. Six sites were managed with chisel–plow tillage and two with no–tillage. Soil–test K (STK) was measured using ...


Use Of Precision Agriculture Technologies In Studying The Relationships Among Soil Ph, Calcium Carbonate Equivalent, Soybean Cyst Nematode Population Density, And Soybean Yield , Natalia Rogovska Jan 2006

Use Of Precision Agriculture Technologies In Studying The Relationships Among Soil Ph, Calcium Carbonate Equivalent, Soybean Cyst Nematode Population Density, And Soybean Yield , Natalia Rogovska

Retrospective Theses and Dissertations

Iron deficiency chlorosis (IDC) and soybean cyst nematode (SCN; Heterodera glycines) infestation are major factors that contribute to soybean (Glycine max Merr.) yield reduction in the Midwest. The IDC is often associated with soybean grown on high pH, calcareous soils. In addition, it was documented that SCN population density is higher in high pH soils. The objectives of this paper were to assess the proportion of within-field soybean yield and SCN variability that could be explained by soil pH, calcium carbonate equivalent (CCE), and a previously defined alkalinity stress index (ASI). Aerial images from 21 fields planted to SCN-resistant or ...


On-Farm Evaluations Of Nitrogen Management For Corn Production With Precision Farming Technologies , Bradley Wilbert Van De Woestyne Jan 2005

On-Farm Evaluations Of Nitrogen Management For Corn Production With Precision Farming Technologies , Bradley Wilbert Van De Woestyne

Retrospective Theses and Dissertations

Guidelines, or recommendations, for nitrogen (N) management in the Corn Belt are important to corn producers because fertilizer N must be applied for profitable corn production, because profit margins are narrow, and because N fertilization has been linked to local and regional water quality problems. Producers rely on these recommendation because it has been impractical for individuals to identify optimal rates, times, and methods of N fertilization for corn. Precision farming technologies, however, enable organized networks of crop producers to evaluate management practices used on their farms and within regions. These technologies and on-farm trials were used to assess optimal ...


Calculating Ex Post Economic Optimum Rates Of Nitrogen Fertilization For Corn , Petro M. Kyveryga Jan 2005

Calculating Ex Post Economic Optimum Rates Of Nitrogen Fertilization For Corn , Petro M. Kyveryga

Retrospective Theses and Dissertations

Rates of nitrogen (N) fertilization usually are selected with the intent of maximizing profits for producers. The most commonly used method of estimating N fertilizer needs, however, does not directly relate rates to profits. This problem was addressed by developing methodology for calculating economic optimum rates (EORs) of N fertilization from large amounts of data collected in trials using precision farming technologies. Distinctions were made between ex post and ex ante EORs and between EORs for individual trials and for samples of trials. Emphasis was placed on collecting and identifying appropriate samples of yield responses to N. Because ex post ...


Soil And Runoff Phosphorus As Affected By Fertilizer And Manure Application , Brett Lee Allen Jan 2004

Soil And Runoff Phosphorus As Affected By Fertilizer And Manure Application , Brett Lee Allen

Retrospective Theses and Dissertations

Fertilizer or manure applications are increasing soil P in agricultural fields. This study assessed the impact of P application on soil P fractions and P loss with surface runoff based on P rate field trials and both indoor and field rainfall simulations. Bray P, Mehlich-3 (M3) P, Olsen P, total P (TPS) and soil P saturation (Psat) estimated by M3 or oxalate extractable P, Fe, and Al molar ratios increased with cumulative P application (0 to 1098 kg P ha-1 after 4 to 23 yr) at plots from 11 trials. Soil test P, TPS, and Psat were linearly correlated. Routine ...


Liquid Swine Manure As A Phosphorus Source For Corn-Soybean Rotation , Mónica M. Barbazán Jan 2004

Liquid Swine Manure As A Phosphorus Source For Corn-Soybean Rotation , Mónica M. Barbazán

Retrospective Theses and Dissertations

Improving management guidelines may result in more efficient use of liquid swine manure and in reduced environmental risks. Two different studies were conducted on farmers' fields using liquid swine manure on corn and soybean crops. One study used conventional research plot methodology with objectives to evaluate (a) manure P effects on early plant growth, early P plant uptake, grain yield, and P removal with grain; (b) crop response to fertilizer P applied in addition to manure P applied once for two crops; and (c) evaluate liquid swine manure P application on soil P measured with three routine soil-P tests (Bray-1 ...


The Fate Of Phosphorus And Molybdenum After Biosolids, Swine Manure And Triple Superphosphate Were Applied To A Soil , Jorge David Hernandez Jan 2003

The Fate Of Phosphorus And Molybdenum After Biosolids, Swine Manure And Triple Superphosphate Were Applied To A Soil , Jorge David Hernandez

Retrospective Theses and Dissertations

The possible effects of the application of swine manure and biosolids to high phosphorus (P) test soils has been widely debated due to the potential increase in P pollution. The application of these materials to soils might result in changes in water, soil, and crop quality. Increasing P in an already high soil P test (STP) soil may result in changing P dynamics in agro-ecosystems and enhancing P movement away from sorption sites. The purpose of this study was to determine the effect of swine manure (SM), biosolid (BS) and triple superphosphate (TSP) applications on the soil P content in ...


Efficacy Of Multivariate Analysis And Zone Soil Sampling To Study Relationships Between Site Variables Affecting Crop Yield And Yield Response To Phosphorus And Potassium Fertilization , Jorge Sawchik Jan 2003

Efficacy Of Multivariate Analysis And Zone Soil Sampling To Study Relationships Between Site Variables Affecting Crop Yield And Yield Response To Phosphorus And Potassium Fertilization , Jorge Sawchik

Retrospective Theses and Dissertations

Crop yield and yield response to fertilization is frequently heterogeneous within-fields. A better understanding of this variability and the development of cost-effective soil sampling techniques to better represent plant nutrient levels are needed to improve nutrient management practices. Two studies were conducted in Iowa to address this general objective.;One study focused on correlations between several site variables and their relationship with soybean [Glycine max (L.) Merr.] yield using two multivariate techniques. Soil and crop variables were measured at five fields using a dense, 0.2-ha grid-point sampling approach. Factor analysis grouped correlated variables into three common factors across fields ...


Uniform And Variable-Rate Fertilizer And Manure Phosphorus For The Corn-Soybean Rotation , David John Wittry Jan 2002

Uniform And Variable-Rate Fertilizer And Manure Phosphorus For The Corn-Soybean Rotation , David John Wittry

Retrospective Theses and Dissertations

Precision agriculture has evolved from a concept into an accepted management practice. The challenge now is how to best utilize these technologies for the benefit of agriculture. Nutrient management could be improved and spatial variability reduced by variable-rate (VR) application. The objectives of this dissertation were to assess the value of VR P fertilization and P-based liquid swine manure application for corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production compared with the traditional uniform-rate (UR) application. On-farm research methods based on replicated, long narrow strips were adapted to precision technologies. These technologies included grain yield monitors, differential ...


Assessing Soil Phosphorus Status Under Different Agronomic Land Use , Carlos Henriquez Jan 2002

Assessing Soil Phosphorus Status Under Different Agronomic Land Use , Carlos Henriquez

Retrospective Theses and Dissertations

The study of the availability of soil phosphorus (P) to crops has been an important issue for years in different agroecosystems around the world. Because of the complex P cycling in soils this has been studied from different points of view. The study of soil P forms has been seen as a possible way to explain many processes and changes occurring in plant-soil interactions. The purpose of this research was to characterize soil P forms under different land uses and evaluate the relationship between different land use areas and the spatial distribution of soil P forms. The study was carried ...


Remote Sensing Of Nitrogen Deficiencies In Cornfields , Jun Zhang Jan 2002

Remote Sensing Of Nitrogen Deficiencies In Cornfields , Jun Zhang

Retrospective Theses and Dissertations

Remote sensing offers unique ability to characterize spatial patterns in nitrogen (N) deficiency symptoms within cornfields. Studies reported in this dissertation were conducted to learn how this unique ability could be used along with new precision farming technologies to address the practical problem of improving estimates of N fertilizer needs in production agriculture. A key focus was on development of methodology for diagnosing deficiencies of N in fields that are assumed to include important spatial variability in factors that influence both the supply and demand for N.;Studies were conducted in fields of the size normally used in production agriculture ...


Impact Of Variable-Rate And Starter Fertilizer Application Methods On The Crop Response To Phosphorus , Manuel Bermudez Jan 2002

Impact Of Variable-Rate And Starter Fertilizer Application Methods On The Crop Response To Phosphorus , Manuel Bermudez

Retrospective Theses and Dissertations

Crop production removes P and other essential nutrients from the soil that need to be replaced by fertilizer application in order to maintain higher yields. The overall objective of this research was to assess the impact of P fertilization methods on crop and soil-test P responses to P. Two distinct studies were conducted to achieve this general objective.;A starter fertilization study involved strip trials on seven cornfields. Treatments applied were no-starter and liquid starter with or without spring tillage. Results from this study showed that grain yield, early plant growth, and early N and P uptake often were greater ...


Dividing Cornfields Into Soil Management Units For Nitrogen Fertilization , Jason W. Ellsworth Jan 2001

Dividing Cornfields Into Soil Management Units For Nitrogen Fertilization , Jason W. Ellsworth

Retrospective Theses and Dissertations

The advent of applicators having the capacity to adjust rates of fertilization as they move across fields has created a need to subdivide fields into nitrogen (N) management units, or areas of soil that should receive a common rate of N. This dissertation explores methods for using yield monitors on combines, The Global Positioning System (GPS), remote sensing, and geographic information systems (GIS) to subdivide fields into N management units. Fertilizer treatments were applied in replicated strips 4.5--6 m wide and >500 m long across fields having several soil types. Combines with yield monitors harvested each strip as a ...


Early Season Losses Of Fertilizer Nitrogen During Corn Production , Kipling Shane Balkcom Jan 2000

Early Season Losses Of Fertilizer Nitrogen During Corn Production , Kipling Shane Balkcom

Retrospective Theses and Dissertations

Fertilizer nitrogen (N) usually is applied to Iowa cornfields before planting in late April or early May. Although it is well established that some of this N can escape from fields before the corn (Zea mays L.) begins rapid growth in June, the practical importance of these early season losses of N has not been established. This dissertation focused on learning more about the importance of early season N losses during corn production in Iowa. Three studies were conducted. The first involved a survey-type approach using the late-spring test for soil nitrate and the end-of-season test for cornstalk nitrate in ...


Fertilizer Management In No-Tillage Corn And Soybean , Samuel Saaka Jeduah Buah Jan 1999

Fertilizer Management In No-Tillage Corn And Soybean , Samuel Saaka Jeduah Buah

Retrospective Theses and Dissertations

Nutrient management practices in no-tillage systems may need adjustment to maximize profits because the distribution of nutrients and plant roots are affected by the lack of tillage. This 3-yr study evaluated (i) no-tillage corn response to surface broadcast and subsurface placement of fertilizer N, P, or K, (ii) no-tillage soybean response to broadcast and banded P or K as well as the direct and residual responses to P and K, and (iii) no-tillage corn hybrids response to complete starter fertilizer;The results suggest that at high soil test levels, response to fertilizers would be difficult to achieve with consistency regardless ...


Modeling Crop Response To Nitrogen , Nanchang Yang Jan 1999

Modeling Crop Response To Nitrogen , Nanchang Yang

Retrospective Theses and Dissertations

Curves used to describe observed relationships between incremental differences in rates of fertilization and yields of corn grain are commonly called crop response models. Such models are widely used to identify optimal rates of nitrogen (N) fertilization for corps. A major problem is that relationships between rates of N fertilization and yields vary greatly with many different factors that vary from site to site. Marked variability in these site factors makes it difficult to use observations made in the past to predict rates needed in the future;Data from 70 different response trials were used to explore the possible benefits ...


Soil Testing And Plant Analysis To Optimize Nitrogen Management In Manured Cornfields , David Jay Hansen Jan 1999

Soil Testing And Plant Analysis To Optimize Nitrogen Management In Manured Cornfields , David Jay Hansen

Retrospective Theses and Dissertations

Land application of animal manure is widely accepted as a method of recycling plant available nitrogen (N) within agricultural systems, but there is great need to improve the efficiency of this recycling. Soil testing and plant analysis are widely accepted as important tools for managing plant nutrients, but guidelines for manure management seldom employ these tools. The objective of this study was to develop guidelines for use of soil testing and plant analysis to optimize N management on manured cornfields. Data were collected in 205 on-farm trials where commercially prepared fertilizer N was applied at various rates to replicated and ...


Effect Of Soil Ph On Phytoavailability Of Nitrogen In Biotechnology By-Products And Animal Manures , Fahad Mohamad Alromian Jan 1999

Effect Of Soil Ph On Phytoavailability Of Nitrogen In Biotechnology By-Products And Animal Manures , Fahad Mohamad Alromian

Retrospective Theses and Dissertations

Biotechnology by-products and animal manures have received considerable attention in recent years because of the problems associated with their disposal and because of their potential use as soil amendments and as sources of nutrients for crops. A laboratory experiment was conducted to assess the potential N mineralization in five biotechnology by-products and three animal manures added to an agricultural soil with pH values of 5, 6, and 7 and incubated at 20°C or 30°C. Two greenhouse experiments were conducted by using the same soils to evaluate those materials as sources of N for plants; corn (Zea mays L ...


Foliar Fertilization Of Soybeans At Early Vegetative Stages , Mazhar Ul Haq Jan 1998

Foliar Fertilization Of Soybeans At Early Vegetative Stages , Mazhar Ul Haq

Retrospective Theses and Dissertations

This study evaluated soybean (Glycine max (L.) Merr.) response to foliar fertilization at early vegetative stages in Iowa soils that tested mostly high in P and K. Several studies evaluated fertilization effects on grain yield; N, P, K, composition of vegetative tissue; and photosynthesis. A 3-8-15 (N-P-K) mixture was evaluated at 21 trials in 1994. Treatments were a control, single applications of 19, 28, or 38 L ha-1 at the V5 stage, and 38 or 56 L ha-1 split one half at the V5 stage and one-half 8 to 9 days later. Three fertilizers were evaluated at 27 trials in ...


Phosphorus And Potassium Management For Corn And Soybeans In Conservation Tillage , Rogério Borges Jan 1998

Phosphorus And Potassium Management For Corn And Soybeans In Conservation Tillage , Rogério Borges

Retrospective Theses and Dissertations

This study evaluated the grain yield, P or K uptake, and early dry weight responses of no-tilled and ridge tilled soybeans [Glicine max (L.) Merr.] and ridge tilled corn (Zea mays L.) to broadcast, deep-band, and planter-band P and K fertilizer placements under Iowa soil and weather conditions. Long-term P and K trials were established in 1994 at five research centers and were evaluated for four years. Forty short-term P-K trials were established in farmers' fields from 1995 to 1997; Treatments were various P (0 to 56 kg P ha-1) and K (0 to 132 kg K ha -1) rates ...


Biotechnology By-Products And Animal Manures As Sources Of Phosphorus For Plants , Edy Sigit Sutarta Jan 1998

Biotechnology By-Products And Animal Manures As Sources Of Phosphorus For Plants , Edy Sigit Sutarta

Retrospective Theses and Dissertations

Biotechnology by-products and animal manures have received considerable attention in recent years because of the problems associated with their disposal. All these materials contain significant concentrations of macro- and micronutrients, especially N, P, and S. Two greenhouse experiments were conducted by using three Iowa surface soils (Gosport, Grundy, and Clinton soils) to assess the potential of biotechnology by-products and animal manures as sources of P for corn (Zea mays L.) and ryegrass (Lolium multiflorum L.). The biotechnology by-products tested were agricultural waste activated sludge (Ag.WAS), sterilized microbial biomass (SMB), corn stalk ash (CSA), base water (BW), and steamed bone ...


An Assessment Of Diagnostic Tools For Improved Soil And Crop Management , Moses Mang'eni Siambi Jan 1997

An Assessment Of Diagnostic Tools For Improved Soil And Crop Management , Moses Mang'eni Siambi

Retrospective Theses and Dissertations

Evaluation of soil quality requires the development of a minimum dataset that can be used at different levels and scales. Soil organic matter is a major reservoir of plant nutrients, and hence the dynamics associated with it due to management practices are important. Particulate organic matter (POM) has been suggested as one of the parameters that may be sensitive to management practices and hence can be used to monitor the changes in soil organic matter. The objective of the first study was to investigate the temporal and spatial changes in particulate organic matter when a field that had been under ...


Productivity And Profitability Of Strip Intercropping Systems As Implemented By Agronomists And Farmers , Derrick Noble Exner Jan 1997

Productivity And Profitability Of Strip Intercropping Systems As Implemented By Agronomists And Farmers , Derrick Noble Exner

Retrospective Theses and Dissertations

On-station experiments investigated whether three-crop strips produced net gains in yield and profit and examined different crop rotations in strips for profitability, border interactions, and fertilizer nitrogen requirements. The sites were the Northeast Research Station (NERC), at Nashua, Iowa, and Living History Farms (LHF), near Des Moines. A corn-soybean rotation was compared to three-year crop rotations that included a sole-seeded forage legume or a forage legume established with a small grains nurse crop;For three years, six cooperating farmers recorded yields and kept Iowa State University Crop Enterprise Records (CER) for strip intercropping and for single-crop blocks. Four compared three-crop ...


Sources Of Variability In Nitrogen Availability In Iowa Cornfields , Carlos Honorio Perdomo Jan 1995

Sources Of Variability In Nitrogen Availability In Iowa Cornfields , Carlos Honorio Perdomo

Retrospective Theses and Dissertations

Studies were conducted to learn more about the spatial structure of soil NO[subscript]3[superscript]- concentrations in cornfields in late spring and to identify some major causes of variability in these concentrations. This information is specifically needed to aid in design of efficient sampling strategies for the late-spring test for nitrogen (N) availability, which is a new management tool that enables site-specific evaluation and adjustment of fertilization practices. One study assessed the importance of windrows of plant residue left by combines as a source of variability in soil nitrate (NO[subscript]3[superscript]-) concentrations for the next crop. Observations ...


Corn Responses To Nitrogen Forms, Ammonium/Nitrate Ratios And Potassium In Iowa And Kenya , Nesbert Mangale Jan 1995

Corn Responses To Nitrogen Forms, Ammonium/Nitrate Ratios And Potassium In Iowa And Kenya , Nesbert Mangale

Retrospective Theses and Dissertations

Nitrogen in soils occurs in both the organic and inorganic forms; but a crop absorbs and utilizes mainly the inorganic forms. Of these inorganic N forms only the nitrate (NO[subscript]3) and ammonium (NH[subscript]4) forms are absorbed. In humid and sub-humid regions, NO[subscript]3-N is lost from crop rooting zones by leaching and/or denitrification;Preserving fertilizer N in the NH[subscript]4 form may significantly decrease leaching and denitrification losses of applied N because NH[subscript]4-N is not easily leached and is not denitrified. However, ammonium nutrition may be harmful to a crop, especially ...


Release And Phytoavailability Of Phosphorus In Soils Treated With Phosphate Rocks And Organic Acids , Kokoasse Kpomblekou-Ademawou Jan 1993

Release And Phytoavailability Of Phosphorus In Soils Treated With Phosphate Rocks And Organic Acids , Kokoasse Kpomblekou-Ademawou

Retrospective Theses and Dissertations

This work involved studies of 12 phosphate rocks (PRs), four each of low, medium, and high reactivity, obtained from various deposits (Kodjari, Tahoua, North Carolina, Gafsa, Khouribga, Tilemsi Valley, Central Florida, Sechura, Minjingu, North Florida, Hahotoe, and Parc W). The reactivity of the rocks was estimated by four methods (neutral ammonium citrate first and second extractions, ammonium citrate pH 3, 2% formic acid, and 2% citric acid). The unit cell a dimensions and the empirical formulae of the rocks were calculated from x-ray diffraction data and statistical models of francolites; the results agreed closely with those published. The concentrations of ...