Sampling - Field Sampling: Principles and Practices in Environmental Analysis - page 128

occur in an episodic fashion. The most common example is the episodic contamination of
water by agricultural chemicals, which occurs most frequently just before and after
planting. If a water sample is taken before or after the planting no unusual amount of
contamination may be found. It was not present yet or may later have been diluted to a
natural or undetectable level. Either sampling at regular intervals or some method of
taking a continuous sample, such as time-weighted average sampling based on solid-
phase extraction, thus may be appropriate.
A specific example would be the contamination of groundwater by nitrate from
agricultural fertilizer. The time to look for this is when fertilizer application is high and
plant use is low. Grabbing a sample at some other time will not illuminate the problem
[17, 18].
5.12.2. Agricultural Sampling
Farmland represents the largest area sampled and analyzed per year. Typically, only
surface samples 0 to 15 or 0 to 30 cm deep are obtained, and few if any deep samples are
taken. The typical soil analysis includes pH, cation exchange capacity, organic matter
content, and the amounts of calcium, magnesium, phosphorus, potassium, micronutrients,
and sometimes nitrogen. Such analyses are inexpensive and rapidly done.
There are three types of agricultural fields, each requiring different sampling strategies
(e.g., those planted to row crops, to grass, and to fields used for pasture). The most
commonly sampled are fields with row crops (i.e., any field containing crops planted in
discernible rows). Figure 5.10 shows a field planted to a row crop (corn) being sampled.
In this type of field, fertilizers and other agricultural chemicals may be broadcast, or
sprayed in such a way that the materials is spread evenly over