Chlorine Contents of Soil

In the Great Plains of the United States, soil tests are performed to assess the soil chloride level as a factor to be considered in decisions regarding application of chloride fertilizer. The relative responsiveness of the various wheat and barley cultivars to chloride is also considered. Some cultivars of spring wheat and barley frequently exhibit responses to chloride, while others seldom exhibit a response (41,66,70,71). Chloride response trials conducted at 36 locations found that a critical level of 43 kg Cl ha^-1 in the top 60 cm layer of soil would generally separate responsive sites from nonresponsive sites (66,70). On the basis of this research, soils were classified as low (≤34 kg Cl ha^-1), medium(35 to 67 kg Cl ha^-1), or high (>67 kg Cl ha^-1) in relation to the probability of observing a response to chloride addition. Chloride fertilization is recommended according to this equation: Cl^- to apply (kg ha^-1) = 67 – Cl^- (kg Cl ha^-1 to 60 cm sampling depth). This recommendation is specific to wheat and barley crops grown in the region, and it should not be extrapolated to other areas under different climate, soil, and cultural conditions.

Soil test calibration data on chloride are unavailable for most crops and soils around the world. However, an observation of chloride deficiency in Australia provides some insight into concentrations of chloride in soil that may limit growth of some plants (72). In this instance, it was found that subterranean clover (Trifolium subterraneum L.) exhibited poor growth when the soil contained only 3 to 5 µ eq of Cl per 100 g (1 to 2 mg kg^-1).

When other factors limit crop yield potential, the potential for a response to chloride fertilization is also limited. For example, corn grown in high-yield environments in New Jersey (18 miles from the Atlantic Ocean) exhibited yield increases from chloride addition on soils that held 20 kg Cl ha^-1 in the top 60 cm layer of soil (45,46). In other studies with corn under less favorable conditions, yield increases due to chloride fertilization were either small or nil (2,42-44).

In many instances, chloride is frequently supplied to crops as a consequence of the widespread use of KCl-based fertilizers that are applied with the intention of providing potassium. Recommended application rates of potassium, when applied as KCl, will generally supply sufficient chloride to most crops. It is possible that the supply of chloride is sometimes limiting for crops grown on a wider range of soils but that the crop responses to chloride go unrecognized because they are attributed to potassium.

Chloride is widely distributed in soils. Concentrations normally range from 20 to 900 mg kg^-1 with a mean concentration of 100 mg kg^-1 (68). Because igneous rocks and parent materials in general contain only minor amounts of chloride, little of this nutrient arises from weathering. Most of the chloride present in soils arrive from rainfall, marine aerosols, volcanic emissions, irrigation waters, and fertilizers (4).

Chloride is not adsorbed by minerals at pH levels above 7.0 and is only weakly absorbed in kaolinitic and oxidic soils that have positive charges under acid conditions (68). Chloride accumulates primarily in soil under arid conditions where leaching is minimal and where chloride moves upward in the soil profile in response to evapotranspiration. Poorly drained soils and low spots receiving chloride from runoff, seepage, or irrigation water also may accumulate chloride (57). Near the ocean, soils have high levels of chloride, but with increasing distance from the ocean, chloride concentration in soils typically falls (2,4).

How a crop is harvested influences the amount of chloride in soil. When harvested only as seed, the amount of chloride removed is limited (<8 kg ha^-1 for a corn yield of 11.3 Mg ha^-1), but when harvested as green biomass the amount of chloride removal may be substantial (81 kg ha^-1 for corn as silage) (25). Because chloride leaches from aging leaves, harvest of mature biomass may remove only about half as much chloride as does harvest before the onset of senescence (59,61).