How does plant testing fit in with soil testing?
Soil testing can be combined with plant testing to get a fuller picture of the nutrients available to the plant. Additional nutrients can then be provided through the irrigation system or as a foliar application. Plant testing can also be used as a way to calibrate the soil test under local conditions. Even if nutrients are ‘theoretically’ available in the soil according to a soil test interpretation, plants are not always able to take the nutrients up in the amounts needed for various reasons. Some of these reasons may be:
Poor root system
Extended periods of cloudiness
Excessively dry soil
Soil temperature (nutrient release is best at moderate soil temps
Interference from excessive amounts of another nutrient
Plant testing for nutrients is most often in the form of petiole (plant sap) or tissue (entire leaf) testing. The results of either of these tests only make sense when the results are compared to an ‘ideal leaf’ of a certain maturity. This is referred to as ‘the standard’. For this comparison most labs use a recognized ‘standard’ of the youngest fully mature (or fully expanded) leaf. Plant test nutrient levels are typically reported as a percentage of the standard.
Petiole tests are done on the sap flowing into a leaf. This gives the grower information about what is happening over the next few days after the test as the sap flows into leaf. Tissue testing is done on the entire leaf. This gives information on what has already occurred up that point in the growing season. Both of these approaches can provide valuable information to the grower. Some growers of high value crops will test their plants weekly so that they can make adjustments to their fertility inputs as the season goes along. This is usually done in situations where the irrigation system is drip or sprinkler; with these systems, adding significant amounts of nutrient inputs throughout the season is commonly done.