|Could a soil test show a nutrient, such as Phosphorus(P) or Potassium(K), in sufficient amounts
in the soil, yet when a plant tissue analysis is taken,
there is a deficiency of the same nutrient in the plant? Yes, this situation can and does happen.
For example, when a drought occurs, the
plant is not able to take up sufficient soil nutrients, yet a soil test shows that the nutrient is in a sufficient level in the
soil. The same thing may occur in flooding conditions.
However, there are other factors which we have control over that can cause this same
problem. For example, not maintaining a proper pH can stop or reduce plant uptake of nutrients. In most cases,
high pH (7.0 or higher), reduces plant uptake of nutrients, and low
pH (5.0 or lower) can do likewise.
the exceptions may be for those crops which
prefer the high or low pH environment. It is important to understand that limestone
(Calcium(Ca) and Magnesium(Mg)) plays a major role in the availability of other nutrients.
Nutrients do not act alone. All nutrients interact with at least one other nutrient, yet all nutrients are
tested as though they stand alone. In the topic "Fertilizer Efficiency", the example
of a Phosphorus(P) : Zinc(Zn) relationship is given. In order for the plant to be able to
take up Phosphorus(P),
there must be available Zinc(Zn) in the soil. When the plant is unable to find any Zinc(Zn),
it will stop taking up Phosphorus(P).
Over 60 nutrient relationships are discussed in the
- "A & L Laboratory Agronomy Handbook"
- "Micro Nutrients" by Frit
- "Micro Nutrients and Secondary Nutrients" by Stoller
Finally, we know that the soil particles are negatively charged colloids,
while nutrients, including Calcium(Ca), Magnesium(Mg), Sodium(Na), Potassium(K),
and Hydrogen(H) are all cations, or positively
charged elements. Of these elements, we know Calcium(Ca) is the strongest cation, Magnesium(Mg)
is second strongest, Sodium(Na) and Potassium(K) are basically equal in strength at third strongest,
and Hydrogen(H) is the weakest cation. (pH is a measurement of the amount of Hydrogen
ions in the soil).
When limestone is applied (Calcium and Magnesium, the two strongest cations) it has a great effect on the availability of Potassium, a much
weaker nutrient. This relationship of nutrients is very important in
analysis of soil tests and in making recommendations.