Grain yield - plant Cl relationships

    Montana and South Dakota field research  has shown that plant tissue (whole above ground portion of plant) concentrations at heading are good predictors of responsiveness by wheat to Cl fertilization.   A compilation of grain yield vs. plant Cl data-bases shows similar relationships (Figure 1) for both states where yield is expressed in relative terms.  Relative yield is the ratio of yield for an individual Cl rate over the plateau yield for a site (mean yield over highest producing treatments as determined by F test and 0.10 significance level). Three zones of varying Cl status are evident.  A deficient or low Cl status zone, plant Cl < 0.1%, where significant yield responses to applied Cl occur approximately 65 to 70% of the time (e.g. 59 of 86 events in Montana; 7 of 11 events in South Dakota).  A zone of adequate Cl status, plant Cl > 0.4%,  where few significant response to Cl fertilization are observed. A transition, or critical range, between these two zones which contains a mix of responsive and non-responsive events to Cl fertilization.  Within the critical range response probability to Cl fertilization drops considerably compared to the low Cl status zone.  The Montana and South Dakota data-bases shows that significant responses occurred 28% and 50% of the time, respectively.   

Figure 1.  Relative yield vs whole plant Cl relationships for wheat in Montana and South Dakota.   Montana data-base consists of 221 cultivar x year x site events (148 winter wheat, 73 spring wheat, 1 durum) and South Dakota data-base consists of 35 spring wheat sites.

Grain yield - plant nutrient relationships for Cl and other nutrients

     The relationships in Figures 1 are generally consistent with classical nutrient responses curves described by Bates (1971) and Ulrich and Hills (1967).  However, relative grain yield vs. plant Cl relationships derived from field data do not exhibit the steep drop in yield that is frequently observed for other nutrients.   Also, the critical range is rather broad. The lower limit of plant Cl concentrations observed in Montana and South Dakota studies have been in the range of .02% (200 ppm) and .03% (300 ppm), respectively.  It is possible as plant Cl decrease below this concentration grain yield may drop more precipitously. Biochemical functions of Cl in plants are presumed to require a concentration of no more than 100 ppm (Fixen, 1993). If correct, the benefits from applied Cl observed in field studies are more likely due to its osmoregulatory role in the plant (Flowers, 1988). The importance of this function on plant growth and grain yield is highly dependent on the growing environment (e.g. water and temperature), and other ions which may potentially substitute for Cl in this role. These factors may explain the frequency of  moderate yield responses to applied Cl in the field and the broad transition range with its mix of responsive and non-responsive episodes

Grain yield deficits from inadequate Cl nutrition

Yield deficits which result from inadequate Cl are often modest in size (< 5.0 bu/a) even when plant Cl concentrations at head emergence fall below 0.10%.    Hence,  a low plant Cl concentration at a site is not a guarantee that a large yield deficit from inadequate Cl has resulted.  Nevertheless, yield deficit vs. whole plant Cl relationships(Figure 2) illustrate that overall yield losses from inadequate Cl and response frequency to Cl fertilization increases as plant Cl concentrations drop below 0.4%