Fertilizer Application Methods
Broadcasting involves spreading the material evenly over the entire area to be treated and is usually convenient but not very efficient in terms of the amount of nutrient that must be applied. A form of broadcasting is fertigation or application of nutrients dissolved in irrigation water. Localized placement methods include banded starter applications of solid and liquid fertilizers, dribble-on applications, point injection, drip fertigation, and perforation methods (the latter used mainly for individual trees). Foliar sprays are very effective for micronutrients in many situations, but have a limited capacity to supply macronutrients because of salt hazards.
Fertilizers may be applied by many different methods, depending on the situation. Methods (a) to (c) represent broadcast fertilizer, with or without incorporation. Methods (d) to (h) are variations of localized placement. Method (i) is foliar application and has special advantages, but also limitations. Commonly, two or three of these methods may be used in sequence. For example, a field may be prepared with (c) before planting; (d) may be used during the planting operation; (g) may be used as a side-dressing early in the growing season; and, finally, (i) may used to correct a micronutrient deficiency that shows up in the middle of the season.
Timing of Fertilizer Application
Timing of fertilizer applications in the field is governed by four basic considerations: 1) making a sufficient amount of the nutrient available when the plant needs it; 2) avoiding excess availability, especially of N, before or after the principal periods of plant uptake (especially during environmentally sensitive periods of groundwater recharge); 3) making nutrients available when they will strengthen, not weaken, long-season and perennial plants; and 4) feasibility of conducting field operations.
Diagnostic Tools and Methods
The field diagnostician must use all available information, including field observations, plant tissue analyses, and soil tests, to make sound recommendations and solve the puzzling problems that commonly arise in complex plant-soil systems. Organized, recorded observations should be made on soil variation, and plant symptoms (both above and below ground), including spatial and temporal changes. Deficiencies may produce recognizable symptoms. Management histories should be obtained, and a sketch map drawn.
Plant Analysis and Tissue Testing
If samples of a specific plant part (e.g. petioles, newly opened leaves, etc.) are taken at a specific time (e.g. a first flowering), and the tissue analyzed for nutrient element content, the results can be compared to values known to be in the sufficiency range or the critical range for the particular plant species. The balance among nutrients must also be considered in interpreting the tissue data.
The relationship between plant growth or yield and the concentration of an essential element in the plant tissue. For most nutrients there is a relatively wide range of values associated with normal, healthy plants (the sufficiency range). Beyond this range, plant growth suffers from either too little or too much of the nutrient. The critical range (CR) is commonly used for the diagnosis of nutrient deficiency. Nutrient concentrations below the CR are likely to reduce plant growth even if no deficiency symptoms are visible. This moderate level of deficiency is sometimes called hidden hunger. The odd hook at the lower left of the curve is the result of the so-called dilution effect that is often observed when extremely stunted, deficient plants are given a small dose of the limiting nutrient. The growth response may be so great that even though somewhat more of the element is taken up, it is diluted in a much greater plant mass.