It is always best to consult the guidelines for sampling specific crops but many times a county agent or specialist will be in the field with a client and can't readily access this information. For those situations, the fo llowing general sampling guidelines will be helpful.
The concentration of the various nutrients required for optimum plant growth changes throughout the life of a plant. The sufficiency ranges supplied by most plant testing labs assume that the plant material submitted for analysis is sampled at a speci fic stage of growth. For the sufficiency ranges to be useful, the proper plant parts must also be sampled. The general rule-of-thumb is to sample the youngest mature leaves (also called the youngest fully expanded leaves) when the plant is near maturity but before senescence. This would be, for example, at initial pod filling for soybeans or near the fruit set stage for watermelons. If the petioles are unusually large (larger than a wire), they should be excluded from the leaf sample. Corn is a notable exception to this general rule. Ear leaves should be sampled at silking.
About half a small lunch bag volume of sample is required by the Ag Service Lab for most leaf analysis requests. Paper bags are preferred over plastic bags to prevent moisture condensation within the bag and subsequent problems with fungal growth.
When a nutrient problem is suspected with plants and they cannot be sampled at the proper time, a sampling from a "good" area of the field vs. a "bad" area of the field is useful for making comparisons regarding possible deficient nutrients.