Chlorine in soils and plants
There have been claims in the press and elsewhere that the addition of chloride in muriate of potash is harmful in a number of ways. This article provides a factual review of this element in plants and soils.
Forms of chlorine
Chlorine (Cl) is an essential element for humans, animals and all plants. It is a component of common salt and found in seawater. The element exists in the plant soil system as the chloride anion Cl- . This must not be confused with other forms of the element such as chlorine gas (highly toxic and unstable), chlorine in swimming pools, hypochlorite (a sterilant and bactericide), hydrochloric acid (corrosive and dangerous liquid), etc. It is important to recognise that none of these forms can occur in soils as the result of the addition of chloride in fertilisers, manures, or rainfall.
In the context of this article the term chloride will be used throughout.
Chloride in soil
Soils in some parts of the world are deficient in chloride and additions of this element are associated with yield responses and improved growth as for any other limiting nutrient. In the UK chloride is not a limiting factor because of the nature of our soils and climate.
There are relatively few data on chloride levels for UK soils but average content is around 50 mg/l Cl (compared to chloride responsive Californian soils with 12-18 mg/l). The chloride ion is only held weakly in soils and leaches in a similar manner to nitrate and this mechanism is the natural regulator of chloride in our soils.
Soils under covered crops can suffer chloride (and other "salts") build-up because they are not subject to through drainage and the normal natural regulation. Similarly soils in arid climates with little rainfall are vulnerable to salt build up, but such conditions are not applicable to the UK and principles relevant to arid soils cannot be applied to temperate conditions such as ours. Problems may also occur where crops are irrigated with water containing high levels of salts, but again this is not a common problem in this country.
Chloride inputs to soil
Inputs to soil derive from rainfall, fertilisers (mainly muriate of potash - potassium chloride), manures, irrigation water and with seawater flooding. The chloride from all these sources is the same - there is no difference between chloride added from manure or rain than from fertilisers.
Atmospheric deposition from rainfall averages around 20 kg/ha Cl per year with higher levels in coastal districts (over 700 kg/ha per annum has been measured in the Shetland Isles.)
Potassium chloride (muriate of potash - MOP) adds 76 kg Cl for every 100 kg of potash (K2O). The average usage of potash in England & Wales at around 50 kg K2O/ha thus supplies 38 kg Cl/ha.
Organic manures contribute varying levels of chloride with higher concentrations in pig, turkey and poultry wastes than in cattle manures/slurries. Applications of manures to provide the maximum advised rate of 250 kg N/ha will add between 10 and 150 kg Cl/ha.
Large quantities of chloride can be added where irrigation water contains high levels of chloride but this is not a common problem in this country.
Soils flooded by seawater receive very large quantities of chloride which may take a long time to disperse naturally but these are extreme circumstances.
Chloride in plants
Chloride is an essential element for all plants but is only required in small quantities similar to other trace elements. Actual concentrations of chloride content of plants can vary greatly with species and stage of growth. Like many other substances (including innocuous materials such as water), chloride is not harmful to plant animal or microbiological life in normal quantities but is undesirable in excess.
Wilting is a common symptom of chloride deficiency and transpiration is also affected and plants are often chlorotic. Chloride toxicity symptoms include burning of leaftips and margins, bronzing, premature yellowing and abscission of leaves. Seedlings and tubers will exhibit root and shoot scorch. Damage from excess chloride normally results from osmotic effects (moving water across nutrient concentrations) which are associated with the above toxicity symptoms. Other physiological effects are not well defined but can involve reduced carbon dioxide assimilation and reduced protein synthesis. Chloride is easily absorbed by leaves and scorch can result in coastal districts from sea spray and saline drift.
Plant species differ considerably in their sensitivity to chloride excess with sugar beet, barley and rape, being highly tolerant, wheat, grasses and potatoes intermediate, whilst peas, beans, clover and other legumes are sensitive. Because the effect is one of osmotic pressure, the sensitivity also varies with the moisture holding capacity of the soil and soil moisture content.
It has been calculated that safe levels of chloride range from around 180 mg/l for sensitive crops on light soils to 1800 mg/l for tolerant crops on heavy textured soils.
Chloride and biological activity in soil
It has been suggested that biological activity of soil is adversely affected by chloride additions in fertilisers. Soil biology is immensely complex and its measurement at present is very imperfect but there is no reliable scientific evidence to support this contention. The successful use of muriate as a potash source for 150 years to produce flourishing and increasingly productive crops appears to be clear practical evidence refuting such claims. The existence of healthy ecosystems in coastal regions of the UK which, as the figures above indicate, receive enormous quantities of chloride from rain is further evidence that chloride addition in the UK is not a problem.
Antagonists of muriate of potash suggest the use of sulphate of potash (SOP) as an alternative. SOP is of course a very important and satisfactory fertiliser which provides both potash and readily available sulphur. Where there is a requirement for sulphur, SOP may be the best choice of fertiliser, but not because it does not contain chloride.
It has also been emphasised that muriate is prohibited in organic production and some claim this is because of its chloride content. However this is totally contradicted by the allowable use of crude salts such as Sylvinite and Kainit within the organic rules, as these materials respectively contain 2.8 and 4.8 times as much chloride as muriate per unit of potash.
Economics
Recommendations to use sulphate of potash (SOP) instead of muriate for reasons of chloride effects should be opposed on the grounds that MOP is not harmful and because of economics. SOP contains 45% SO3 in addition to 50% K2O and is thus a more costly fertiliser than MOP especially if the value of the sulphur content is excluded. To use SOP instead of MOP as a potassium source for a typical crop of winter wheat, would add an extra £24 per ha to costs. To justify such a move at current prices, nearly half a tonne of extra grain would need to result. Such advice is clearly not sound.
Conclusions
Muriate of potash is a perfectly safe fertiliser and fears of chloride problems in UK agriculture are incorrect. Muriate is the best and most economic source of potash for the vast majority of situations. Correct practice in terms of rates and timing of muriate should be observed as for all other fertilisers or manures. Sulphate of potash is a more suitable fertiliser for a minority of specific crops situations and these are clearly defined in PDA leaflet 15 and MAFF Fertiliser Recommendations RB209 7th edition (2000).
For further information please contact:
info@pda.org.uk
Potash Development Association
update: Dec 2002
