PDF《Iowa》

40 Figure 6. Relationship between soybean relative grain yield and the P concentration of plants at the V5-V6 growth stage or uppermost mature leaves at the R2-R3 stage. QP, quadratic plateau;

LP, linear plateau;

critical concentrations are shown.

41 1 CHAPTER 1. GENERAL INTRODUCTION Phosphorus and potassium are two nutrients that frequently limit crop production in the U.S. and other regions of the world. These nutrients are important for crop growth and development. Phosphorus has important roles in nucleotide molecule composition, plant metabolism, and energy transfer within the plant. Potassium has a major role in membrane function, water transport, and cellulose synthesis. Yield potential can be expressed with proper assessment of the nutrient needs and appropriate fertilization. Nutrient status is typically estimated with soil test results. Soil sampling in the fall or spring allows farmers to assess the nutrient status of their fields and decide fertilizer application rates. Tissue sampling may provide an in-season complement to soil sampling and testing by a more direct method of assessing nutrient status. Successful use of tissue testing to guide fertilization requires further correlative research to interpret test results. Previous research has shown that nutrient concentrations in plant tissue vary greatly, not only with soil fertility but also with the plant growth stage, crop species or variety, the plant part sampled, and various environmental conditions. Therefore, tissue test results must clearly refer to specific plant parts sampled and growth stage, although to be useful in production agriculture, tissue testing needs to be useful across a wide range of cultivars and growing conditions. The most widely used method of implementing tissue testing in production agriculture is based on the sufficiency level concept. The basis for this concept is that there is a critical nutrient concentration above which there is luxury accumulation (the concentration increases but does not translate into increased yield) and below which there is deficiency. The concept

2 has been further developed and explained by others to address critical concentrations and ways for establishing sufficiency ranges. Research on critical tissue P and K concentrations for corn and soybean for Iowa and neighboring states that included several experiments across several sites or years are scarce, and most studies were conducted before the 1980s. These experiments have provided inconsistent results about the value of tissue testing for phosphorus and potassium in corn and soybean. Therefore, interpretations for tissue test results do not exist in Iowa, and in most states of the Corn Belt and few universities recommend tissue testing as a diagnostic tool to decide fertilization. Interpretations do exists for other regions, but these may not apply to Iowa conditions, genetics of cultivars grown, and both soil and climatic conditions. Therefore, the objective of this research was to determine critical P and K concentrations for corn and soybean for early vegetative and early reproductive growth stages. THESIS ORGANIZATION This thesis is organized as a paper titled Plant tissue analysis to assess phosphorus and potassium nutritional status of corn and soybean in Iowa for submission to the Soil Science Society of America Journal. It is organized into the following sections: Abstract, Introduction, Materials and Methods, Results and Discussion, References, Tables, and Figures. The paper is preceded by a General Introduction, and followed by General Conclusions.