Interpretation of Soil Test Results (Horticultural Crops)

At the far left of the INTERPRETATION section are definitions for the “Soil Texture Code”. The Code lists the soil types contained in the broad classifications of “Coarse”, “Medium”, and “Fine”. You will refer to the Soil Texture Code when the texture of your soil is classified in the SOIL TEST RESULTS section.

The next three columns in the INTERPRETATION section graphically illustrate the relative levels of organic matter, soluble salts, and acidity in your soil, along with the buffer index. The graphs use stacked letters to form bar charts and they refer to the categories just below them in the SOIL TEST RESULTS section. In the Grapes Report “Organic Matter” has three stacked O’s, which tells you that the amount of organic matter is on the border between low and medium in this soil. “Soluble Salts” were not measured, but there is a stack of two H’s above “pH”, telling you that the soil is moderately acid. A category related to pH is the “Buffer Index”, which has four stacked B’s and tells you that the Buffer Index is just below the intermediate level or slightly acid.

In the Vegetables Report “Organic Matter” has three stacked O’s, which tells you that the amount of organic matter is also on the border between low and medium in this soil. “Soluble Salts” were not measured, but there is a stack of seven H’s above “pH”, telling you that the soil is slightly alkaline. The “Buffer Index” was not measured for this sample, because it is determined only if the pH of a mineral soil is less than 6.0.

The rest of the INTERPRETATION section is not separated into columns, but it follows a similar graphical format that illustrates the relative levels (from “very low” to “very high”) of plant nutrients that were tested for in these soils. Once again, they refer to the categories just below them in the SOIL TEST RESULTS section. In the Grapes Report, there is a stack of ten P’s above phosphorus (Bray 1 Phosphorus), a stack of seven K’s above potassium, a stack of three Z’s above zinc, a stack of ten C’s above calcium, and a stack of three M’s above magnesium, indicating that the soil tested in the very high range for phosphorus, at the high end of the medium range for potassium, in the low range for zinc, in the high range for calcium, and near the medium level for magnesium.

In the Vegetables Report, phosphorus and potassium were the only nutrients tested. Two different tests were done for phosphorus, Olsen and Bray 1, but the Olsen Phosphorus test is the relevant one for this soil. The reason for two tests and selection of the Olsen P measurement will be explained in the next section – SOIL TEST RESULTS. The stacks of four P’s (above the Olsen Phosphorus box) and five K’s tell you that both phosphorus and potassium were low to medium in this soil. Interpretation of the relative phosphorus level is not straighforward, because there are three crops on this report and the stack of four P’s refers to sweet corn. The relative phosphorus ranges for cabbage and tomatoes are different than the range for sweet corn, so the phosphorus level is actually in the very low range for cabbage and tomatoes. This difference between the crops is reflected in the P fertilizer recommendations on the report.

Soil texture, organic matter, pH, buffer index, P, and K are part of the Regular Soil Test Series and are the usual categories tested. The nitrate (NO3-N) test can be used in western Minnesota to determine nitrogen fertilizer recommendations for potatoes and sweet corn. Soluble salts and the rest of the plant nutrients listed on the report are most usefully tested for under specific circumstances where a problem is suspected or likely to occur. These circumstances include certain soil types and crops which are more prone than others to develop deficiencies of specific nutrients. See the Soluble Salts,Secondary Macronutrients, and Micronutrients sections in the University of Minnesota Extension bulletin Nutrient Management for Commercial Fruit & Vegetable Crops in Minnesota for more information on crops and soils where soil tests in addition to the Regular Series are recommended. Another useful nutrient management tool, especially for perennial fruit crops, is plant tissue analysis. Soil testing and plant analysis complement each other and are most effectively used in combination Plant Analysis. For vegetable crops, in-season monitoring of petiole/midrib nitrate-nitrogen can also be useful Tissue Nitrate Analysis.