Page 30 - ALG Issue 2 2015

Feature on... trace elements
Trace Elements
Selwyn Richardson is a retired Soil Scientist and Chairman of Hale Road A&GA at Swavesey in Cambridgeshire. He spent over 40 years with ADAS, the Government’s former advisory service for farmers and growers. His philosophy on crop nutrition is “keep it simple”.
Crop composition
Plants are quite simple things really because they only require about 16 elements for healthy growth. Furthermore, they get most of the elements for free, such as carbon in carbon dioxide, absorbed from the air during photosynthesis, hydrogen from water and oxygen from both water and carbon dioxide. Of the other major nutrients, nitrogen phosphorus, potassium, calcium, sulphur and magnesium, the most important is nitrogen because it has the biggest effect on crop vigour and quality. The offtake of potash (K2O) from a whole hectare of a well-grown crop ranges from about 40 kg for peas to 400 kg for potatoes (35 to 350 pounds per acre).
Trace elements
Seven trace elements, sometimes called minor nutrients, are also essential for crop growth; boron, copper, manganese, zinc, iron, molybdenum and chlorine. Typical offtake of copper in crops is around 75 grams per ha (1.07 ounces per acre). Molybdenum offtake is the lowest at around 5 grams per ha (0.07 ounces per acre) but without it, crops would die because it acts as a catalyst in the critical step of converting nitrogen to protein within the crop.
However, although trace elements are essential to make crops grow, you probably don’t need to think about them because they largely take care of themselves. The vast majority of land in the UK contains sufficient trace elements to supply crop needs for many hundreds of years. Thus molybdenum deficiency is very rare and only ever occurs on sandy soils where pH has fallen to 5.0 or less, which is poor husbandry in the first place. At Hale Road allotments our soil type is on the slightly heavy side, which means we
can ignore trace elements because the soil will supply
plenty each year, forever.
limestone soils. Manganese is also “locked up” by large soil organic matter content which is why the problem is worst on peaty soils. It is also worse in dry summers and it is important to water little and often during prolonged dry spells. The deficiency can seriously restrict crop growth.
Beetroot and parsnips are very sensitive to manganese deficiency. The photo shows good and poor beetroot leaves with yellow/ bronze speckling on the affected leaf along with spots of dead tissue surrounded by a red band – parsnips show similar symptoms. Symptoms are always worse on the outer, older leaves. Lettuce and potatoes are moderately sensitive, with lettuce simply showing small dead, brown speckles on the outer leaves, whereas potatoes show small black spots alternately on either side of the veins on the old and mid leaves. The deficiency is fairly common in peas where it is difficult to notice because the crop simply looks pale green, except
So can I stop reading now?
Not quite, particularly if you are on the most
susceptible soil types of sandy, chalk/limestone
or peaty. Even then the only ones that might be
a problem are manganese and iron deficiency.
In commercial practice where maximum yield
is important, some farmers and growers find it
economically worthwhile to add boron, mainly for
root crops such as sugar beet, swedes and turnips and copper is occasionally used on cereals and linseed. But maximum yield is less important on allotment plots so forget boron and copper. Zinc deficiency has been recorded in onions growing on very gravelly, glacial soils in Scotland and Northern Ireland but not in England or Wales. Chlorine deficiency in crops doesn’t occur anywhere in the world.
Manganese – the main concern
On sandy, chalk/limestone and peaty soils, manganese deficiency is fairly common in a wide range of crops. Unlike molybdenum deficiency, manganese deficiency occurs at high soil pH, typically above 6.5 on peaty land and above about 7.0 on sandy and chalk/
that in serious cases when the shelled pea can have a black centre known as “marsh spot” which is not found until the crop is eaten.
There is no suitable soil treatment, but control is easily achieved with a foliar spray of manganese sulphate - note manganese, not magnesium. Although not normally stocked on garden centre shelves, this is readily obtained from specialist suppliers to allotment societies, chrysanthemum and dahlia societies etc. Manganese sulphate should be sprayed onto the leaves following the
instructions on the container. Stop spraying when runoff begins. One spray is normally sufficient and crop colour will improve within 5 days. To make it stick to the foliage, a small amount of proprietary wetter (wetting agent or surfactant) should be used and shaken in with the water – this is particularly important on brassica crops and peas since these have very waxy leaves. Spray early morning or in the evening to minimise scorch risk.
Iron – but deficiency only affects fruit
Iron deficiency is not a problem in vegetable crops, but in the leaves of fruit crops. It can look spectacular on chalk/limestone soils and on sandy soils where pH is above about 7. Crops most affected
are apple, pear, cherry, raspberry and blackcurrant. In contrast
to manganese deficiency, the symptoms show up in the young, topmost/outer leaves. The photo shows good and poor raspberry leaves with interveinal yellowing on the affected leaf, which can be almost white in severe cases, but the veins always remain green. Although it is possible to treat the crop using one form of chelated iron as a foliar spray and another form as a soil treatment, the response is often slow or disappointing or both. In truth, you are probably as well off just living with it and admiring the symptoms.
In summary
Trace element disorders in crops are uncommon. Manganese deficiency is the only one worth giving thought to and even then only on specific, susceptible soil types. However, if you need some reassurance the simplest way to add trace elements to soil (plus valuable organic matter of course) is to use a dressing of 5 to 10 kg/ sq metre (9 to 18 lbs/sq yard) of horse manure, cattle manure, pig manure or compost (soil improver) applied every 4 or 5 years in the rotation, preferably in front of potatoes or brassicas.
Beetroot and parsnips are very sensitive to manganese deficiency
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