Soil Test Levels and Crop Response to Fertilizers

Soil scientists and consultants often get confronted with these questions: “My soil analysis showed a high level of nutrient ‘X.’ Is all this ‘X’ actually available for uptake by plants? Might I see a response to applications of ‘X’?”

The answer to the first of these questions is not simple. Theoretically, the level of a nutrient reported on a soil analysis form is indeed the amount available for plant uptake. Soil analysis techniques have been designed to extract only the amount of nutrients available for plant uptake. It’s the whole idea behind soil analysis work — to distinguish between “available” and “unavailable” soil nutrients. And many years of field and greenhouse evaluation studies form the basis of and support modern soil analysis methods.

The answer to the second questions is, yes, it’s certainly possible to observe a response to a nutrient that already tests in the “high” category. The next logical question might then be, “If that’s the case, why bother with soil testing?” The primary answer is that soil testing serves as an excellent tool for evaluating the fertility status of our soils and should form the basis for every farmer’s fertility program. However, soil analysis results are not infallible, in that plant uptake of a given nutrient depends on quite a few factors other than the “available” level of that nutrient in a soil.


When considering magnesium (Mg), several factors, both climatic and otherwise, can have a marked effect on plant absorption of this nutrient.

· Temperature: As temperatures decrease, plant absorption of Mg likewise decreases. This can be said for most plant nutrients, but is especially true of Mg. Field trials in Canada dramatically demonstrated this effect in the late 1970s, as shown in Table 1.


It’s obvious that cool temperatures depressed absorption of Mg, especially in hybrids A and B. Also, all but one hybrid exhibited varying degrees of Mg-deficiency symptoms during the cool 1976 season. No symptoms were observed on the three hybrids during 1977 — Mg levels were considerably above critical levels. This tells us that a response to Mg probably would have been observed in 1976, but not in 1977 — on the same soil, under the same Mg soil test level.

· Soil physical conditions: The physical condition of soils can affect the availability of many nutrients. With respect to Mg, the oxygen (O2) level of a soil, or the degree of aeration, is very important (Table 2).


Magnesium uptake was markedly reduced in the same soil (and, thus, at the same Mg soil test level) as O2 level decreased from 21 to 2 percent. Any condition that reduces soil aeration, such as flooding or compaction, will negatively affect Mg absorption.


Magnesium isn’t the only nutrient that decreases in availability because of soil compaction. Results from Wisconsin (Table 3) showed that potassium (K) in the starter fertilizer overcame the negative effects of soil compaction.


In this study, soil compaction made a marked difference in the response of corn to K.

Other factors affecting nutrient utilization:

· Soil pH.

· Nutrient interactions. High levels of fertilizer K depress Mg absorption.

· Hybrid or variety. Note in Table 1 that temperature had a much greater effect on Mg absorption by hybrids A and B than by hybrid C.

· Fertilizer placement.

· Soil moisture. Nutrients in general absorb more readily under optimum soil moisture conditions than under excessively wet or dry conditions.

While soil analysis should play an important and basic part of every fertility program, growers should also know that soils sometimes respond to fertilizers even when soil analysis results indicate they shouldn’t. The numbers in Table 4, published by a leading Midwestern university, support this phenomenon.

As stated, it’s certainly possible to observe a yield response to a nutrient that already tests in the “high” range. In soils testing high in Mg, under cool, wet, early spring conditions, and when high amounts of K have been applied, it’s not unusual to observe a yield response to Mg, especially from starter fertilizer.

Related Topics: