Soil chloride calibration

   South Dakota research has shown that soil chloride analyses are a diagnostic tool for identifying sites where spring wheat will potentially respond to chloride fertilization.   A compilation of results from 34 experiments (Figure 1) indicated three interpretive categories of varying chloride status based on soil chloride levels in the upper 2' of the profile (Fixen et al.,1987).  A low category (0-30 lbs/a) where significant yield responses occurred in 11 of 16 experiments (69%); a medium category (31-60 lbs/a)  where yield responses occurred in 4 of 13 experiments; and high category (>60 lbs/a) where no yield improvement from applied chloride was observed.  Several factors were considering in establishing these categories.  First, the medium category brackets the Cate-Nelson critical level of 38 lbs/a.  Second, the probability of response to applied chloride in the low category was high, while the response frequency was very low in the high category.  Third, a sufficient number of observations are located in each category to provide reasonable estimates of response frequency.

Chloride requirements and fertilizer recommendation              

    Chloride requirements developed from this data-base were set at 60 lbs/a as this was the amount of fertilizer plus soil chloride found necessary to maximize yields.   The fertilizer recommendation proposed was:

An assumption in this recommendation is that soil and fertilizer chloride  are equally available to the plant, and that only soil choride in the upper two feet of the profile contributes to the requirement.  The justification for using a two foot sampling depth was based on regression analysis of plant chloride concentration vs. soil chloride levels.  This analysis revealed the highest R² values result when soil Cl in the upper 24" of the profile were considered (Table 1).