Introduction
Foreword
“The Nation that destroys its soil destroys itself.”
–Franklin D. Roosevelt
This laboratory manual has been compiled and updated several times over the years. Some procedures have been modified to fit the equipment in the 354 laboratory. The intent of this manual is to use for educational purposes and not to duplicate procedures used in a routine soil testing laboratory. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the authors.
About the authors: Dr. Loynachan is recently retired (July 2016) after teaching the lecture portion of Agronomy 354 for 38 years; he taught 354 laboratories early in his career. Dr. Troeh is professor emeritus of the ISU Agronomy Department and was involved with the lecture and laboratory portions of Agronomy 354 for many years.
Last updated: July 2024
Introduction
This set of laboratory exercises is designed to provide students an opportunity to experiment with properties related to soils and plant growth. Each exercise was selected and designed to illustrate some factors related to the growth of plants in soil. The significance of the property being studied and the general theory of the experiment are outlined in a discussion at the beginning of each exercise. This material should be studied as background before the exercise is attempted.
The procedural part of each exercise is divided into a series of numbered steps to be easily followed whether or not the student has experience in analytical chemistry. Many steps have a procedural item followed by an explanatory paragraph. This paragraph is used in various ways, including explaining the reagents, what is happening to the sample being tested, what precautions are needed, or what alternatives are available in the procedure. These paragraphs are intended to help the student understand what is being done rather than simply pass through a series of procedures.
Many of the exercises are interrelated. It is desirable that the student works with a single soil sample for all of the exercises and observes how well the properties relate to one another. In a few instances, however, where larger quantities of soil are required or special soil is needed, common soil will be provided. Some indication of the degree of repeatability being achieved also is important. This is obtained by performing all analyses in duplicate.
Several of these exercises are the type of measurements that are made by soil testing laboratories as a basis for fertilizer recommendations. The interpretation of this type of laboratory data into fertilizer recommendations and the decisions that must be made before fertilizer is applied are also important parts of a soil and plant growth course. More will be discussed about correlation and calibration data in the lecture.
Some procedures in this manual have been modified to conserve chemicals and/or time. Standard references to methods of soil analyses should be reviewed when analyzing soils for research purposes.
We are deeply grateful to the many students, teaching assistants, and fellow staff members who contributed to the development of this manual. Also, the authors wish to specifically recognize Dr. Stanley J. Henning, Dr. Michael L. Thompson, Dr. Teresita Chua, Dr. Renuka Mathur, Dr. Amber Anderson-Mba, Patrick Mireku, Dr. Cole Dutter, Dr. Mostafa Ibrahim, and Dr. Marshall McDaniel.
Background of Soil Testing
About 1834, Jean Baptiste Boussingault began a series of field experiments on his farm at Bechelbronn in Alsace, France, and, as a result, became recognized as the father of the field-plot method of experimentation. In 1843, the oldest continuous agricultural experiment station was founded at Rothamsted, England by J.B. Lawes and J.H. Gilbert. In 1875, the first agricultural experiment station in the United States was established in Middletown, Connecticut. Shortly thereafter, many state experiment stations were founded. The Iowa Experiment Station was founded in 1888.
The early chemists did complete chemical analyses of soils. The theory was that if a soil had large total amounts of a nutrient then the plant could readily absorb the nutrient. This idea was in vogue at the time of Justis von Liebig (1830) who made one of the first chemical fertilizers and also who developed the Law of the Minimum. As more information was obtained about plants and soils, it became apparent that only a small portion of the total amount of any element in the soil was used by plants. Hence, attempts were made to simulate the action of the plant root by treating the soil with weak acids to extract the specific nutrient. Danberry used carbonic acid (H2CO3, which is formed by plant roots during respiration) to study fertility in 1845.
During the 1920s and early 1930s, Bray at Illinois, Hester at Virginia, Morgan at Connecticut, Spurway at Michigan, and Truog at Wisconsin developed laboratory and greenhouse methods for evaluating the fertility status of a given soil. While the U.S. workers concentrated mainly on chemical techniques, the German workers, Mitscherlich and Baule, developed mathematical approaches for predicting crop responses. In the 1950s and 1960s, the Iowa and North Carolina State Experiment Stations led in the development of factorial experiments and multiple regression statistical analyses to predict crop responses to added fertilizer.
Today, one goal in soil testing research is to find an extracting solution that will relate the amount of nutrients extracted from a soil to the quantities of nutrients taken up by plants during a growing season. These index numbers from the laboratory must be calibrated during field trials with plant responses, and then economic responses must be considered. Thus, the aim of soil testing is to obtain a reliable estimate of the amount of fertilizer needed on a certain soil on a given crop to achieve a given yield that will result in maximum profit per acre for the grower while protecting the environment.
Steps Needed to Establish a Soil Testing Laboratory
- Recognition of a nutrient deficiency has been the starting point for most research because it is very difficult to obtain yield responses to application of nutrients that are present in adequate amounts; that is, analysis of plants from unfertilized soil will be nearly the same as plants from fertilized soil, unless a definite deficiency exists.
- Addition of fertilizer to the soil should result in a yield response to the nutrient additions.
- A standard method of measuring the nutrient in the soil is needed. Experiments with chemical extracting solutions that will measure the specific nutrient of interest in the soil are studied and adapted to the particular soils of a state or region, because different soils will give varying results with a given technique. The chemical test needs to provide a measurement that can serve as an index related to the ‘nutrient-supplying power’ of the soil. This index is usually a fraction of nutrients that will become available during the growing season.
- A soil chemical test is selected, and this test is then correlated with the crop yield response to increasing fertilizer levels on a given soil. This is done for individual soils because each soil will have different ‘nutrient-supplying power.’ Several different response models have been used in the past, but today the linear-plateau model is one commonly used.
- Establishment of the most economically optimum fertilizer rates to increase yields, but not in excess of crop needs, is essential for growers to maintain a sound fertilization program.
- A recommendation of the amount of fertilizer to apply to obtain the maximum profit per acre in an environmentally sound manner is the hardest part of soil testing. The interpretation of soil information, and crop responses, is based on many site- and management-specific properties like the characteristics of surface and subsurface soil, drainage, erosion, presence of calcium carbonate, crop variety differences, stand level, pest management, etc.
