The limiting values often decrease in course of time, the highest values are generally found in very young plants. This is due to the “dilution” effect, since the concentrations are based on the dry matter (which contains in course of time less active cell substance and more cell wall material, etc.). This problem is avoided by establishing limiting values for different growth stages or for special plant part of the same age etc.
The limiting values are normally total nutrient contents, whereas in fact they should represent only the active portion of the total content. For most nutrients, however, total and active contents have high correlation.
Problem may arise in case of highly immobile elements such as Fe and Ca of which the active part should be considered. The activity of Fe could be measured indirectly by measuring the Fe-dependent enzyme activity in the plant.
The limiting values for a special nutrient are based on the assumption of an optimum supply of all the other nutrients, it must be emphasized that certain nutrient ratios are very important but not as basic as the limiting value concept.
Read Also: How Nutrients Get to the Root Surface into Plant Cell
Examples of Appropriate Nutrient Ratios Effects
The different ratios of the treatment which gave the highest alkaloid content of henbane medicinal plant are 30, 58, 50, 136, 24, 2, 2, 3 and 1 for P/Zn, P/Fe, P/Cu, N/Zn, Zn/mg, Zn/Fe, Zn/Cu, Fe/Mn and Fe/Cu respectively.
Application of P caused a relative increase in Zn concentration and contents but decreased Fe and Cu.
Corn (Zea may L.)
Maximum accumulation of each nutrient (N, P, K, Fe, Mn, Zn, Cu) during the growth period differed according to the different treatments.
Awa (1976) found that the application of 20kg Zn SO4 fed (48 kg/ha) significantly increased the grain yield by 16% over the control and also increased Zn concentration in leaves and grains application of 48kg Zn, SO4 with 36kg P2 05/ha increased the grain yield by 28%.
There was evident response to Zn when P/Zn ratio in corn leave at tasseling stage was greater than 68. Ratios less than 54 resulted in deleterious effect of Zn accumulation.
