How might you go about determining or ruling out soil-related causes for a symptomatic plant?

What is the plant supposed to look like at this stage?

Is it stunted or abnormally colored? Purple leaves in some varieties, particularly horticultural crops, may be normal. However, purple colors in a plant that is not supposed to have that coloration may indicate a phosphorus deficiency.

Pattern?

Is there a pattern in where symptomatic plants are occurring in the field (low spots, edge of field, steep areas)? Area with recent construction or newly replaced topsoil? How about within the plant (new growth vs old growth)?

Conditions?

What have conditions been like in the area recently? Have they been abnormally cold, wet, hot, dry? How about last season (potential herbicide carryover if dry)? Cool springs may contribute to more nutrient deficiency symptoms, as root growth and microbial activity may both be slower than normal.

Field operations?

What has happened to the site recently? Any major disturbance (topsoil removal, compaction, installation that displaced or inverted significant soil)? Application of nutrients or chemical materials?

Testing?

Has any testing been done that may help support a potential nutrient deficiency diagnosis? Soil pH is one that may be helpful if only limited testing has been done, as high pH soils may cause nutrient availability issues with metals like iron. At low pH values, there may be different nutrient deficiencies, or even toxicity of something like aluminum in highly weathered conditions.

Now what?

In order to support your diagnosis, you may want to gather additional evidence. For example, a soil test if you suspect a nutrient deficiency. Some issues may not have a feasible treatment at the point of diagnosis, like an herbicide carryover due to a dry year, but could be considered in future management of the area if another dry year is expected.

In annual crop management, seeing significant deficiency symptoms throughout the season mean that decreased yield is already expected and it may be too late to adjust conditions for the current crop, so adjustments or applications would likely be for the next season or crop. In perennial crops, other strategies might be more desirable, like foliar application, injections, or more intensive methods. Additionally, considering the soil and potential problems before they are planted means that some issues, like iron deficiency of a full-sized pin oak due to high pH soils, could be avoided. As in many things-an ounce of prevention is worth a pound of the cure.