AG Histograms - where do they come from?
This is a frequently asked question and it came up again in our recent customer survey. The graphical presentation of soil and plant test results as histograms is intended to give some guidance as to what the test results are indicating. In order to do this, the lab must have some reference values to create the low, medium or high delineation. The concept of “agronomic optimums” is the main basis for setting the medium levels at Hill Laboratories, but where appropriate, account is also taken of levels typically found.
Where possible, Hill Laboratories uses New Zealand published references, but sometimes needs to adopt values from overseas if there is little NZ research for a particular crop. These references are identified in our Crop Guides. For soil tests, results falling within the “medium range” will usually mean that that nutrient is not likely to be limiting for the specified crop grown.
For some tests however, the published reference ranges may not be applicable to crop varieties grown in New Zealand. For instance, the original leaf test ranges for grapes were based on California data for Thompson Seedless table grape. Some years ago, Hill Laboratories carried out a data-mining exercise and developed varietal medium ranges for each of the main grape types grown here. A similar exercise was done for avocado soil and leaf samples in conjunction with the avocado Industry. For the new kiwifruit varieties, leaf test ranges have been set using a combination of similar variety published values and accumulated median-result data modifiers.
For a more detailed explanation of the histogram setting practice adopted by Hill Laboratories please see our Technical Note – Interpretation Criteria for Histogram Reports.
Fig 1. Example histogram report for G3 Kiwifruit variety leaf test at spring growth stage.