How Common Is Mn deficiency
By: Doug Penny, Dr. Ieuan Evans and Elston Solberg
On a world scale Zn, Fe, Mn and B are the most common micronutrient deficiencies in crop production. Victor Shorrocks with the Micronutrient Bureau, UK, has stated that, “It is surprising that it (Mn) is not as well catered for as other micronutrients, either with commercial technical service or research associations”. In the Prairie Provinces, Mn deficiency is observed occasionally but its extent and frequency is largely unknown. In the Great Lakes Region, Mn deficiency is common on high pH, organic (muck) soils.
Deficiencies in the Prairie Region tend to be sporadic; occurring some years and not others. In addition to the variation that occurs from year to year, Mn availability can also fluctuate markedly within a growing season. These characteristics, plus large differences in among species and varieties in their tolerance to low Mn, make it difficult to determine when Mn fertilization would be beneficial.
Soils Prone to Mn Deficiency
- Highly weathered tropical soils are often low in total Mn. Liming these soils often induces Mn deficiency. The total Mn content of soils ranges between 20 and 3000 ppm (average – 600ppm).
- Mn deficiency also occurs on high pH soils and high organic matter soils in semi-arid, temperate regions. Mn deficiency in these soils is not a result of low total Mn but low plant availability. Mn2+ is the principle species in soil solution and the form that is taken up by plants. Solubility decreases 100 fold for every unit increase in pH, similar to other divalent metal cations. Low Mn availability in high organic matter soils is attributed to the formation of unavailable chelated Mn compounds.
Effects of Soil Moisture, Aeration and Biological Activity
In addition to pH and organic matter, plant available Mn is strongly influenced by soil moisture, aeration and biological activity. Work lead by Don Huber, Purdue University, West Lafayette, Indiana has identified the important role of microorganisms in Mn availability and crop diseases.
An important characteristic of Mn affecting its availability to plants is that it is very readily oxidized from Mn2+ (available) to Mn3+ and Mn4+ (unavailable) by soil microorganisms. Except in very dry and very wet (saturated) soils, both oxidizing and reducing conditions exist. At saturation, all pore spaces are filled with water and reducing conditions dominate, producing high levels of Mn2+. As the soil dries, air replaces water, first in the larger pores and then in smaller and smaller pores, until oxidizing conditions dominate. The high levels of Mn2+ that are generated when the soil is wet provide large amounts of substrate for Mn oxidizing microorganisms. As the soil dries, the population of Mn oxidizers can build up to a high level, depleting the supply of Mn2+. This can result in a period of Mn deficiency that can be quickly reversed by rainfall and warm temperatures. This strong influence of soil wetting and drying and biological activity on Mn availability tends to make the occurrence of Mn deficiencies sporadic and make soil tests less reliable than for other micronutrients.
Deficiency Symptoms and Diagnosis
Prediction of when Mn fertilization would be beneficial is difficult given the sporadic nature of deficiencies. This is further complicated by large differences among species and varieties in their susceptibility to Mn deficiency. While large differences in susceptibility are known to occur, the susceptibility of specific varieties is seldom known and producers usually acquire this information by trial and error. Barley, oats, wheat, peas, potato and soybean are some of the crops listed are being susceptible to Mn deficiency.
About 15 years ago, severe Mn deficiency was identified on some varieties of oat in a regional variety trial at the U of Alberta Research Farm near Edmonton (Evans, Penney and Solberg). While some varieties were severely affected, others were essentially free of deficiency symptoms (Figure 1). On the severely affected varieties, two narrow strips of much better growth were evident on the tractor tracks made when the plots were harrowed after seeding (Figure 1). Two varieties least affected were Waldrin and Calibre, both developed at the Agriculture Canada Research Station, Lacombe, Alberta. Dr. L. J. Piening, Plant Pathologist at Lacombe (retired) indicated that he commonly saw ‘gray speck’ (Mn deficiency) in the area where oat breeding and selection was conducted. This indicates that at least some varieties developed there are selected for tolerance to Mn deficiency.
Better growth of barley and wheat on wheel tracks has been observed quite frequently in the Prairie Region. At maturity, crops on the wheel tracks are typically taller, and lighter in color (less disease) than off the wheel tracks (Figure 2). When lodging occurs, the crop on the wheel tracks tends to stand.
There has been some concern for negative interactions between copper deficiency and some herbicides. In areas where copper deficiency is a concern, Dr. leuan Evans has postulated that these strips may be tracks created during crop spraying for weed control and that the impact of the herbicide is reduced on the wheel tracks. Given some of the more recent insight into Mn deficiency and its interaction with diseases, it seems plausible that some of the observed better crop growth, and resistance to lodging and diseases on wheel tracks is a result of increased Mn availability. Soil compaction on the wheel tracks slows drying, reduces aeration, thus maintaining reducing conditions and Mn availability for longer during dry periods.
The main manifestations of Mn deficiency on crop growth are:
- Reduced cell division and cell elongation (cell elongation is affected more than cell division). It has been reported that Mn deficient barley plant take twice as long to reach the boot stage as non deficient plants; and
- Reduced lignification, which makes plant more susceptible to lodging and disease.
- Low pollen fertility, resulting in fewer kernels per head.
- Reduced 1000 kernel weight resulting from a shortage in carbohydrate supply for grain filling.
The fungus that causes ‘take-all’ root rot of wheat (Gaeumannomyces graminis), and other root pathogens, has been shown to be strong Mn oxidizers. The organisms reduce Mn availability, thus reducing lignification of roots, making them more susceptible to infection. This suggests that Mn deficiency may be more of an issue when disease pressure is high in continuous wheat rotations.