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A Troubling Tissue Test: Micronutrient Toxicities

Given the crazy growing season weather year we are having, we are seeing some very interesting, great learning scenarios cropping up. The following question came in from Tanner Pollack, agri-coach and owner of Solar Agri-Services Ltd at High Prairie, Alta.:

“I have copied a tissue test (see Figure 1) and the soil test from a field of mine. Plants are small, struggling a little bit in very wet ground. I was surprised by the decent nitrogen (N), phosphorus (P), potassium (K) and sulphur (S) levels and totally blown away by the extremely high iron (Fe), aluminium (Al), sodium (Na) and manganese (Mn) levels in the tissue analysis. At what point are these elements becoming toxic in the plant? Is this something we should be concerned about and can we fix this?”

This prompted a response from Doug Penney, senior agri-coach and soils specialist, “This is on an acid soil, so you would expect Fe, Mn and Al to be high but Fe is very, very high so it may indicate some dust (dirt) contamination, which is a common issue in testing for Fe because it doesn’t take much. Contamination is one of the reasons we need to wear latex gloves when sampling to minimize contamination from soil or other contaminants that may be on our hands like salt, ash, etc. In research trials, tissue samples are often washed with deionized and distilled water to be sure potential contamination is minimized.

“Back in 1971, I had Mn toxicity show up on canola in an acid soil research site (pH 5.3) near Delia (I think canola was still rapeseed then). Canola is a bit more tolerant to Al toxicity but less tolerant to Mn toxicity than barley and wheat. Manganese toxicity is characterized by raised inter-veinal areas giving a puckered appearance, leaves become chlorotic and distorted and an uneven distribution of chlorophyll.”

In many crops, tissue Al is not reliable for identifying toxicity because translocation from roots to shoots is poor, especially in cereals. In other words, you can have toxic levels in the roots and moderate levels in the shoots. Having both soil and tissue information help a lot with interpretation.

Chris Solick, agri-coach and owner of Elite Ag Services Inc. at Lacombe, Alta., has also found fairly severe manganese toxicity symptoms in canola this year with about 650 ppm Mn in plants growing in the affected area as compared to 250 ppm Mn in plants growing in the seemingly unaffected areas. “I think this toxicity is more common in very low pH soils than people think. It presents very similar to herbicide injury and I think manganese toxicity is often misdiagnosed as a herbicide residue problem,” Solick says.

As others have pointed out, the primary causative factor here is low soil pH, which is likely amped up by the wet soil conditions. This keeps the surface roots quite active, thereby “residing” in the acid zone for longer than normal. There will be a layer in the top six inches that has a pH even lower than the average 5.4 and this may be where all the toxic effects are coming from. In drier years, the root systems would not be so active in the top six inches and less exposed to the acidity. If we think like a plant, what are your roots experiencing under these scenarios?

If the average pH of this field is 5.4 (in the top six inches) there are areas in this field where the pH could be as low as 4.5 and as high as 6.5. This means that there will be patches where plants are more severely affected than others. An inexpensive portable digital pH meter will confirm what your eyes are telling you.

From the soil test, we can see that the pHs in this field are higher at lower depths. So for the short-term, we need to figure out how to create root dominate crops. Once a crop is deeply rooted, it can mostly ignore the factors in the upper layers that caused them grief. I’d be inclined to give this crop a foliar fertilizer feed to help it through some stress in the hopes the soil will dry some so that roots can penetrate deeper and avoid the toxic surface soup.

The long-term solution to fixing this problem is to adjust soil pH and this can likely be done with liming or wood ash using variable rate applications as this problem is usually variable across the field. Once addressed, this fix will last for many, many years.

While liming is the ultimate solution, if we look at the six-to- 12-inch depth, the pH is 5.7 which is still quite acidic, but remember that the pH scale is logarithmic, so that a pH of 5 is 10 times more acid than pH of 6. When comparing the two pH levels, the plant sees two-times less acidity in the bottom six inches (six to 12 inch depth) than in the top six inches. My guess is that the pH in the 12-to 24-inch depth will be higher and even less acidic again. Roots will gravitate to favourable pH zones.

There are more than 12,000,000 acres of acidic soils in Alberta alone that could respond to liming and as Chris so rightly pointed out the resulting toxicities are likely being misdiagnosed as a damage due to a herbicide residue. The real question for all of us is, “What are your plants telling you?” Think like a plant and you will get some amazing answers!

ElstonSolbergisasenioragri-coachand presidentofAgri-TrendAgrology

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Figure 1.

5.4 5.7

0.21 0.10

TISSUE ANALYSIS REPORT -2011

Sample ID:

Stage: Seedling to bud development

DateSampled N(%) NN(%) S(%) P(%) K(%) Mg(%) Ca(%) Na(%)

07/04/2011

Normal Range

Actual Ratio

Expected Ratio

5.63

5.19 7.00 N/S

6.57

7.90

0

N/K 1.54

1.40

SOIL TEST REPORT -2011

Depth

0-6” 6-12”

Depth 0-6”

6-12”

OM

3.9 2.2

S 18

18

P

26 19

N 12

4

0.86

0.64 0.90 P/S

0.50

0.41 0.69

3.66

3.49 5.10

0.59 118.86 7.25 163.19 6.57 596.48

0.70 135.40 10.10 614.30 3.20 300.00

P1

43 0 27 0

Zn 3.1

nd

Plant Part: Upper matured leaf blades prior to flowering

0.50

0.24 0.60

1.53

0.69 2.00

0.47

0.03

P/Zn K/Mg K/Mn Fe/Mn Ca/B

P2

Mn 6

nd

K

Mg

233 340 135 430

Fe

145

nd Cu 0.6

nd Ca

1400 1470

B 0.7

nd pH

B (ppm)

25.6

29 60

pH B

Zn (ppm)

42.32

32 49

CEC

nd

6.5 6.6

SS Sat P% 0.3 8 nd 4

Mn (ppm)

224

39 100

%K

16.5 3.6 16.2 2.1

Fe (ppm)

1472 1.77

99 350

%Mg %Ca

17.1

22.1 Al

711 769

Crop Type: Canola

Cu (ppm)

4 25

42.3

Al (ppm)

765

300

Cl

(%)

0

%H %Na

36.3

29.6 Cl

13 nd

0.7

0.9 Na

25 33

Mo 0.65

45.3 K/Mg

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