Chloride requirements and fertilizer recommendation

    Montana uses an indirect approach to estimate soil and fertilizer chloride requirements for wheat. The foundation of this approach is based on the strong relationship between yield response frequency to chloride fertilization and plant chloride at head emergence (see Critical tissue concentrations section).

The data-base from Montana indicates three interpretative zones of varying chloride status based on whole plant chloride concentrations at head emergence, including a low nutrition zone (< 0.1%) where there is high probability (69%) of yield losses due to inadequate chloride supply; a transition zone (0.1-0.4%); and an adequate nutrition zone (>0.4%) where there is little chance that yield losses have occurred from inadequate chloride supply.  Chloride requirements are estimated by determining the amount of available chloride necessary to bring a plant into the adequate nutrition zone (see explanation below ¶). Regressing plant chloride concentration on the sum of soil (0-24" depth) plus fertilizer chloride indicates 30 lbs/a is needed to achieve this objective under most environments (Figure 2). 

Figure 2.  Relationship between plant Cl and soil + fertilizer Cl for 22 locations (32 experiments) conducted in Montana (1988 to 1995).

   Most wheat production in Montana occurs in semi-arid areas with annual rainfall of 10 to 16".    Although, significant chloride leaching losses can occur they are not very frequent in occurrence.  Chloride fertility studies were conducted at 22 locations between 1988 and 1995.  Only at one location was there evidence of appreciable leaching losses of chloride as apparent from the very different plant vs. soil plus fertilizer Cl relationship (Figure 2, lower curve).

Fertilizer recommendation assumptions

   An assumption of the proposed fertilizer recommendation is that soil chloride in the upper two feet of the profile and fertilizer chloride are equally available to the plant.  The chemistry of chloride in the soil is fairly simple.   It is found in moist soils as soluble monovalent anion.  Its is not readily adsorbed by soil particles, is not assimilated to any great extent by soil microbes, and when applied to soil as a fertilizer salt (e.g. KCl) is readily dissolved.  There is no plausible reason why chloride in the soil or from fertilizer (e.g. KCl) should differ in their availability to the plant.  Most wheat crops in Montana will root to at least a three foot depth assuming moisture is present.  Historically, fertilizer recommendations for the state typically give equal credit to soil NO₃-N (0-24" depth) and nitrogen derived from fertilizer.  Chloride should be as available to the plant as NO₃-N, hence equal credit is given to soil (0-24" depth) and fertilizer chloride.

   ¶Chloride requirements for wheat are defined as having been met if the plant has a chloride concentration >0.4% at the head emergence stage.  Although, yield improvements from chloride diminish in size and frequency from the low (< 0.1%) to the transition zone (0.1-0.4%), the >0.4% plant chloride level is used in establishing the fertilizer guideline.  This is justified because the material cost of applying chloride is comparatively small and the potential exists for carryover of fertilizer chloride from one season to the next in semi-arid wheat growing areas. Chloride fertilizer, most frequently sold as granular KCl (0-0-62 or 0-0-60), is currently priced at $ 0.16 to 0.17/pound (U.S. dollars).  Therefore, the material cost of a 30 lbs/a application is only $ 4.80 to 5.10/acre, and a 1.5 bu/a yield response covers this expense at a grain price of $3.30/bu. In addition, only small quantities of applied chloride (< 3 lbs/a) are translocated to the grain, and removed at harvest even under high yield potential conditions (> 70 bu/a). A large percentage of applied chloride at low testing sites appears to be found in the straw. Chloride in the wheat straw should be recycled and released to the soil as the residue decomposes.  As rainfall is comparatively low in many important North American wheat growing areas, leaching events are infrequent even though chloride is mobile in the soil.