Northwest Ag: Interpreting Soil & Plant Tissue Tests

INTRODUCTION

This publication is a non-technical attempt at discussion of those controllable factors which are most commonly considered when evaluating a soil or plant tissue test report for the purpose of making specific fertility recommendations and an attempt to increase the application value of our test reports by increased understanding of our reported test results.

We are not attempting to consider all factors, and a report recipient should remember that in diagnosing the needs of plants, all factors, both controlled and uncontrolled, involved with crop yield need to be considered. A proper interpretation of the assembled facts must then be made and a fertility program decided on.

Selection of the kinds and amounts of soil fertility treatments depend, broadly upon (1) the crop yield goal and its nutrient requirements, (2) the ability of a given soil to supply these nutrients, {3) the climatic factors affecting fertility response, {4) the management factor, and (5) the presence or absence of inhibitory chemicals or pathogens. The economic maximizing of factor 2 within the limits of the other factors is the goal of any fertilizer application recommendation.

Factor 2 has, historically, been the factor farmers could most readily change. Early efforts were largely the result of chance observation or trial-and-error. As the body of scientific knowledge increased, a systematic scientific approach to the problems of soil fertility evolved. An outgrowth of this is chemical soil and plant tissue testing to ascertain the fertility status of a soil or plant and predict crop response or need for the addition of fertilizer.

The technical complexities and details of the chemical test methods are of limited interest here. They are discussed only where needed for clarification or background information. We are striving solely describe the meaning of test results and facilitate conversion of the chemical soil and plant tissue test figures into recommended amount of fertility treatment.

THE AUTHOR WOULD LIKE TO EMPHASIZE THAT THE SUGGESTIONS, RECOMMENDATIONS, TABLES, AND CHARTS PRESENTED HERE SHOULD BE USED ONLY AS A GUIDE IN MAKING FERTILIZER RECOMMENDATIONS.

PREVIOUS FERTILIZER PRACTICES WHICH HAVE PROVEN SATISFACTORY IN AN AREA SHOULD BE GIVEN MAJOR CONSIDERATION IN MAKING THE FINAL DECISION ON FERTILIZER TO BE APPLIED TO ANY GIVEN CROP.

IT IS RECOMMENDED THAT THE GUIDELINES PRESENTED IN THIS MANUAL BE USED TO MODIFY PRESENT PRACTICES OR USED WHEN NO PREVIOUS EXPERIENCE IS AVAILABLE.

The author wishes to acknowledge the liberal use of the published research data and the fertilizer guides of Washington State University, University of Idaho, and Oregon State University. This data along with my own experience and the experiences reported back to me by clients determined the final conclusions in making these recommendations.

I also wish to acknowledge the many helpful suggestions from the people in the fertilizer and chemical industry and the people in the food processing industry whose experiences have formed guidelines for fertilizer recommendations to growers.