The Micronutrient and Trace Element Status of Forty-Three Soil Quality Benchmark Sites in Alberta: Results and Summary

5.0 Results and Discussion

5.1 Background
Soils are the primary source of trace elements for plants and through feed and food crops, for animals and humans. Trace element problems (deficiencies or toxicities) are therefore generally associated with soil properties on a regional or local basis.

The availability of trace elements to plants is influenced by both parent material and soil development, but soil properties can operate very differently for different trace elements. For example, high concentrations of Mo are found only in plants growing on poorly drained soils, whereas poorly drained or poorly aerated soils tend to produce plants with low Se concentration.

The trace element requirements and tolerances of plants differ from those of animals and humans, to the extent that, deficiencies or toxicities in plants do not necessarily translate into deficiencies or toxicities in animals or humans. For example:

1. Higher plants can produce optimum growth and have Se and I concentrations that are deficient for animals and humans.
2. Some plants growing on high Se soils can accumulate Se concentrations that are toxic to grazing animals.
3. B toxicity in plants is common in some arid regions of the world but B toxicity is rare in animals or humans.
4. The trace elements I, F, Se, Co, As, Li, Cr, Si, Sn, and V are considered essential for animals and humans but not for plants.

1. Trace elements are not often analysed in routine soil testing;
2. For many of the trace elements, deficiency and toxicity criteria are not well defined.

5.3 Boron (B)
B is essential to plants and is taken up as H₃BO₃ (boric acid). There is increasing evidence that it may be required for normal mineral metabolism in mammals (Welch et al., 1991). B deficiency does not follow well-defined geographic patterns related to soils. Deficiencies are widespread throughout the world and typically occur on well-drained light textured soils that are low in soil organic matter (SOM).

Crops vary widely in their susceptibility to B deficiency. Monocotyledons generally have lower B requirements than dicotyledons. Crops with high B requirements include alfalfa, beets, turnips, canola and apples. B deficiency may occur when plant B content is < 15 mg/kg on a dry matter basis but critical values ranging from 5 to 30 mg/kg have been reported.

Toxicity can occur when the B content of plants exceeds 200 mg/kg although there are reports of corn tolerating in excess of 1000 mg/kg and barley seedlings being impaired at 80 mg/kg. B toxicity in plants is fairly common in semiarid regions with alkaline soil but B toxicity has seldom been reported in Canada. B toxicity in animals is primarily an experimental phenomenon but Underwood (1977) reported B toxicity in lambs grazing high-B plants growing on solonetz and solonchak soils in Russia.

Agronomists in Alberta have occasionally reported seeing B deficiency symptoms on alfalfa and canola, but documented responses to B fertilization are rare. Nyborg and Hoyt (1970) obtained response to B fertilization of turnip rape in greenhouse studies with Grey Luvisols in the Peace River region. Poor seed set of turnip rape on some Grey Luvisols in the region was attributed to B deficiency. B deficiency symptoms on alfalfa and response to B fertilization were observed on a light textured Luvisol in the Barrhead area (Dowbenko, R., personal communication).

5.3.1 Boron results from benchmark sites -The hot water extractable B for the 129 sampling locations (43 sites x 3 slope positions) in this study:

	B	Cl	Mo	Ni	Co	Si	Cr	Cd	Se	Sand	Silt	Clay	CEC	CaCO3	OC	P	K	pHw	pHc
B	1.0														0.53
Cl		1.0
Mo			1.0															0.72	0.62
Ni				1.0	0.55			0.69					0.63		0.46
Co				0.55	1.0													0.55
Si						1.0
Cr							1.0
Cd				0.69				1.0
Se									1.0				0.54		0.55
Sand										1.0
Silt											1.0
Clay												1.0
CEC				0.63					0.54				1.0
CaCO3														1.0
OC	0.53			0.46					0.55						1.0
P																1.0
K																	1.0
pHw			0.72		0.55													1.0
pHc			0.62																1.0

Table 4. Effect of slope position on selected micronutrients and trace elements from 43 benchmarks.

5.4 Chlorine (Cl)
Cl has been recognized as an essential plant nutrient for many years (Romheld and Marschner, 1991), but since it is abundant in the environment (both lithosphere and atmosphere), deficiencies in the field were generally thought to be rare. However, more recent work by Christensen et al. (1981), Fixen et al. (1986), and others has found prophylactic and beneficial properties of the element. Low rainfall, little or no need for K fertilizer (KCl), and the large land locked geography of the Northern Great Plains region have been cited as reasons for relatively frequent reports of response to Cl fertilization. (Note: land locked regions receive low levels of Cl in precipitation compared to coastal regions).

Plants utilize chlorine as the Cl^- anion. In soils, common Cl salts are highly soluble and Cl^- is highly mobile. Less than 45 kg/ha Cl in the 0-60 cm depth (approx. 12 mg/kg in the 0-15 cm depth) has been reported to be deficient in the Northern Great Plains region (Fixen et al., 1986). Crop response to Cl in the Northern Great Plains Region appears to result from prophylactic or beneficial effects rather than its function as an essential plant nutrient.

5.4.1 Chlorine results from benchmark sites - The 0.01 M Ca(NO3)2 extractable Cl^- for the 129 sampling locations:

5.5 Molybdenum (Mo)
Mo is an essential element for both plants and animals. In plants, it is taken up as MoO₄^2- and is involved in NO₃ reduction, protein synthesis and biological N₂ fixation. In animals, both deficiencies and toxicities have been reported. Excess Mo causes diarrhea of cattle (teart scours) in England. In the Inter-lakes Region of Manitoba, high Mo in association with low Cu has been reported to cause Mo toxicity (molybdenosis) in cattle (Corbett, R., personal communication). More recent work has indicated that Mo supplementation in human diets may have anticarcinogenic benefits (Miller et al., 1991).

Mo in soils has not been extensively researched, in large part because deficiencies are rare. Mo availability to plants is strongly affected by soil pH. Availability increases as pH increases. Therefore, Mo deficiency in acid soils is often corrected by liming.

Acid ammonium oxalate (AAO) is the soil test extractant most commonly used for Mo (Sims and Johnson, 1991). A level of >0.2 mg/kg AAO extractable Mo has been indicated as adequate. However, Johnson and Fixen (1990) state that, "Critical levels of extractable available Mo remain unknown for lack of suitable extraction reagent or Mo deficient soil. Identifying critical levels is made more difficult by the fact that plant response in marginally deficient soils appears to be more influenced by liming than by application of Mo". Critical levels for hot water or DTPA extractable Mo, used in this study, have not been established.

5.5.1 Molybdenum results from the benchmark sites - The hot water extractable Mo for the 129 sampling locations:

5.6 Nickel (Ni)
The main interest in Ni has concerned its toxic effects on plant, but more recent research has indicated Ni requirements for both plants and animals. Asher (1991) discusses the role of Ni in a chapter in "Micronutrients in Agriculture", entitled "Beneficial Elements, Functional Nutrients and Possible New Essential Elements". Depending on the reference, Ni is considered as either a functional or essential nutrient. Havlin et al. (1999) stated that Ni is the latest nutrient to be established as essential for plants. It is taken up by plants as Ni²⁺ and has been shown to be required for urea reduction (Dixon et al., 1975) and for grain viability of barley (Brown et al., 1987).

Critical soil levels for Ni deficiency were not identified. However, Asher (1991) lists toxic levels (DTPA extractable) for various grasses ranging from 6 mg/kg for Bermuda grass (Cynodon dactylon) to 112 mg/kg for pangola grass (Digitaria decumbens). Toxic symptoms have been described as being similar to Fe deficiency. In cereals and grasses, variation in chlorosis along the length of the leaf gives a characteristic transverse banding effect. Risser and Baker (1990) state that, "most background levels in soils and plants are of little concern for human health, since inhibition of plant growth and development by excess Ni places a limit on Ni entering the food chain".

5.6.1 Nickel results from the benchmark sites - The DTPA extractable Ni for the 129 sampling locations:

5.7 Cobalt (Co)
Cobalt was established as an essential nutrient for animals in the late 1930’s and for plants in 1960 (Asher, 1991). Co is essential for ruminant animals. Rumen microflora require Co for synthesis of vitamin B12. Co deficiency in ruminants is usually corrected by using Co fortified salt licks, adding it to drinking water, or by using Co boluses. Co fortified salt is commonly used in Alberta, although there are no reports of Co deficiency (Corbett, R., personal communication).

In plants, Co is essential for N₂ fixation by leguminous plants. Co has not been shown to be essential for plants, which are not dependent on N₂ fixation, but beneficial effects have been reported for several crops. Plants take it up as Co²+.

Total Co content of soils typically ranges from 1 to 10 ppm and average about 8 ppm. Forages produced on soils containing < 5 ppm of total Co often results in Co deficiency in ruminants. Soils prone to Co deficiency include (1) acidic, highly leached sandy soils; (2) some calcareous soils; and (3) some peaty soils (Havlin et al., 1999). A critical deficiency level for DTPA extractable Co was not identified.

5.7.1 Cobalt results from benchmark sites - The DTPA extractable Co for the 129 sampling locations:

5.8 Silicon (Si)
Welch et al. (1991) stated "since 1970, eleven additional trace elements have been proposed to be required by animals and possibly humans". They include As, B, Br, Cd, F, Pb, Li, Ni, Si, Sn and V. Except for Ni and B, none are considered to be essential for higher plants. Si is regarded as an essential trace element for normal growth and development in animals, where it is involved in the formation of bone and cartilage.

While an essential role for silicon in plants has not been established, many beneficial effects have been reported. Si is absorbed by plants as silicic acid (H₄SiO₄). Cereals and grass typically contain 0.2 to 2 % Si on a dry weight basis. Accumulator plants such as horsetail can accumulate up to 20 % Si (DW). Rice is often supplemented with Si when the Si content in the straw falls below 11 %.

5.9 Chromium (Cr)
The metals generally considered as toxic include: Cd, Cr, Hg, Ni, and Pb. Cr and Ni are the most phytotoxic. These metals are economically important and used in substantial quantities, thus providing the possibility of soil and plant contamination from anthropogenic sources such as mining and waste disposal. Toxic metals are also added to soils in organic forms. The leather tanning industry is a large source of high Cr organic waste. Toxic levels can also occur naturally in soils developed on serpentine high in Cr and Ni.

5.10 Cadmium (Cd)
As referred to earlier (Welch et al., 1991), As, B, Br, Cd, F, Pb, Li, Ni, Sn, and V have been proposed as being required by animals and possibly humans. Beneficial effects, such as suppression of powdery mildew in crop, have also been reported for Cd. However, the main interest in Cd has been toxic concentrations in food and feed resulting from application of sewage sludge and phosphate fertilizers high in Cd.

Reports of Cd uptake by crops are highly variable, depending on crop type and soil conditions. Most field crops do not absorb appreciable quantities of Cd, but leafy vegetables such as spinach and swiss chard are known to accumulate Cd. One of the earliest reports of Cd toxicity in people was traced to the flooding of rice with water that drained from a mine (Welch et al., 1991). The concentration of Cd in corn forage was not affected by Zn fertilizer, containing from 1 to 2165 mg Cd kg^-1 (Mortvedt, 1991).

Soil pH is a dominant factor affecting Cd availability. Availability increases as pH decreases. Sludge application is usually restricted to soils of pH 6.5 or above and rates are generally based on CEC.

Although some significant correlation of Cd uptake with DTPA extractable Cd have been reported, no critical values were given. Limitations for Cd application to soils are generally based on maximum accumulative loading. Maximum loading limits range from 2.5 to 5.0 lb Cd ac^-1 based on soil texture (Risser and Baker, 1990). For fertilizers, risk based concentrations have been established. For diammonium phosphate, the safe level is 736 ppm (The Fertilizer Institute, 2000). Reported levels of Cd in diammonium phosphate in the U.S.A. show levels no higher than 200 ppm.

5.11 Selenium (Se)
In the early 1930’s, high concentrations of Se were identified as being responsible for animal production problems in parts of the western USA. The location of high Se areas was facilitated by collection and analysis of Se accumulator plants. The high Se soils were generally developed on shales of the Cretaceous or Permian geologic age. Crop plants and range grasses containing > 4 to 5 mg Se kg^-1 (potentially detrimental) were generally confined to parts of fields or farms. High levels of Se in irrigation drainage water are also a concern in some areas (e.g. The Kesterson Wildlife Refuge in California).

Small areas of Se toxic soils have also been identified in Alberta, Saskatchewan and Manitoba (Fleming, 1980). Livestock deaths from Se toxicity are rare in Alberta, but have been reported in the Cypress Hills and Crowsnest Pass areas (Corbett, R., personal communication).

Se accumulator plants, such as Astragalus (milk vetch), the Xylorhiza section of Machaeranthera (woody aster), Haplopappus (goldenweed) and some Stanleya (Prince’s plume) can contain >50 mg Se kg^-1. Secondary Se absorbers include Aster, Atriplex, and Grindelia (Fleming, 1980).

While high levels of Se in plants dominate the literature, beneficial effects to symbiotic N fixation have been reported (Munson and Nelson, 1990).

6.0 Summary and Recommendations

6.1 Boron
There is no evidence of toxic levels of B at any of the sites. However, a few sites have marginal to deficient levels (< 0.5 mg/kg). Plant analysis would be useful on a few sites to determine if deficient levels occur in plant tissues. Sites were there are significant differences between slope positions should be examined.

6.2 Chlorine
If the critical level of 45 kg Cl ha^-1 (0-60 cm), identified for the Northern Great Plains Region is valid, many (approximately 40 %) of the benchmark sites are in the potentially deficient range. Limited trials in Western Canada have shown relatively small and inconsistent response to Cl fertilization (personal experience and personal communication with several researchers). Until more definitive results are available, follow-up on the benchmark sites does not appear warranted.

6.3 Molybdenum
There are no reports of Mo deficiency or toxicity in Alberta. No critical values for hot water extractable or DTPA extractable Mo were found in the literature for comparison with the values reported in this study. If further examination of soil Mo is deemed warranted, the Acid Ammonium Oxalate extraction method should be used since it is the most commonly used method reported in the literature. (Note: Mo toxicity in livestock (molybdenosis) has been reported in Manitoba).

6.4 Nickel
There was a fairly wide range in DTPA extractable Ni among the benchmark sites (0.51 to 5.87 mg/kg). The highest value (5.87) approaches the toxic level of 6 mg/kg reported for Bermuda grass. Plant analysis may be warranted on this site to determine if phytotoxicity is an issue.

6.5 Cobalt
Although there are no studies in Alberta that indicate Co deficiency in livestock (Corbett, R., personal communication), salt-licks supplemented with Co are commonly used. Further investigation of soil Co does not appear to be warranted, unless future research indicates Co deficiency for legume crops.

6.6 Silicon
The mean hot water extractable Si values of 137 mg/kg (0-15 cm) and 192 mg/kg (15-30 cm) are much higher than typical values of 3 to 37 mg/kg given for soil solution Si. This indicates that hot water extractable Si may not be comparable to soil solution Si. Plant analysis could be used to determine if plant Si is low at sites with low hot water extractable Si.

6.7 Chromium
None of the benchmark sites have received industrial or municipal wastes. Therefore, Cr toxicity is not a concern. However, given the recent reports of low Cr levels in human diets, Cr analysis of plant materials may be warranted.

6.8 Cadmium
Cadmium contamination of soils from industrial sources is not an issue, and given the Cd content of phosphate fertilizers and the relatively low rates of application used in this region, high Cd levels in crops from these sources is not a concern. However, it would be valuable to examine the Cd content of crops from a few sites with the highest DTPA extractable Cd levels.

6.9 Selenium
All of the total Se values in this study fall within the normal range of 0.1 to 2 mg/kg and the mean of 0.48 mg/kg is above the reported normal mean of 0.3 mg/kg. However, given the prevalence of Se deficiency in livestock in Alberta, it is likely that some of the sites are deficient. Analysis of plant material from some of the sites with low soil Se is recommended.

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