Site Specific Soil Fertility Basics

The concept of site specific application of fertilizer is not new. Historically, fields were smaller than they are today and small areas within fields were frequently fertilized differently than the major portion in order to address special requirements for either nutrients used or rate of application. Today, computers and guidance systems have largely replaced techniques like counting rows or looking for atypical areas within a field. In addition, new technologies such as soil EC meter and the yield monitors have increased the awareness of variability within fields.

In general, there are two sampling strategies (grid, zone) that can be used to direct site-specific fertilizer management and lime application. Grid sampling uses a systematic approach that divides the field into squares or rectangles of equal size (usually referred to as "grid cells"). Soil samples are collected from within each of these "cells." The location of each "grid cell" is usually geo-referenced using global positioning system technology. This method is used when variability of soil pH and immobile nutrients within fields cannot be easily identified.

However, soils in our area do not change in squares. Zone sampling uses a more subjective and intuitive approach to divide any field into smaller units. Soil samples collected at random from within each zone are bulked together and analyzed to provide an average sample value for each unit. This approach assumes that variability of soils within a field can be easily identified. The Veris Electrical Conductivity (EC) meter, aerial or satellite photography, and multi-year yield maps can be used to divide production fields into management zones. Information from a yield monitor is essential in identifying zones that should be sampled separately. As with the grid system, sampling points can be geo-referenced.

The "grid" sampling strategy can be used for the following conditions: 1) a measure of non-mobile nutrients is the primary concern; with no movement, distribution will be affected less by topography and other fixed properties; 2) management practices used in the past will override natural variability; and 3) there is a history of manure use.

The criteria that would favor the use of zone sampling are: 1) cost of sampling and analysis is a major concern; zones may be larger than grid cells thereby lowering sampling costs; 2) a measure of mobile nutrients is the primary concern; and 3) there is no history of manure application.