Soil Testing, Plant Tissue Analysis, and Interpretation
Core concepts
Know the soil-testing workflow: representative sampling (proper depth, cores per composite, avoiding atypical spots), lab extraction methods (e.g., Bray-1 and Olsen for P, ammonium acetate for K and CEC), and how results map to soil test categories (very low to very high) that drive recommendations. Understand that different extractants give different numbers and are not interchangeable. For tissue analysis, know sampling timing (e.g., whole plant vs most-recently-matured leaf), critical values, sufficiency ranges, and the value of paired soil-plus-tissue data. Recognize that tissue tests diagnose current status while soil tests predict response potential.
Traps
Questions test whether you interpret a number against the correct extractant's scale, or whether you confuse deficiency with toxicity. Watch for the trap of recommending fertilizer purely on a single tissue sample without context, and for sampling errors (wrong plant part, wrong growth stage, contaminated samples). Also be alert to pH and CEC interpretation errors.
Rule of thumb and worked example
Sample to the crop's active root zone and composite 15–20 cores. Worked example: a corn tissue sample from the ear-leaf at silking reads 2.4% N against a sufficiency range of ~2.8–3.5%. That suggests a mid-season N shortfall; pair it with soil nitrate and recent rainfall before sidedress decisions, because a single low reading can reflect sampling error or transient stress rather than a true field deficiency.