Soil Science
The living foundation beneath your feet — what soil is, how it works, how to improve it
Chapter 6: Micronutrients and Secondary Nutrients
Why this matters: Secondary nutrients and micronutrients are needed in smaller amounts but are just as essential as NPK — a deficiency in any one of them can completely stop plant growth. Texas soils have specific micronutrient challenges, particularly around iron, zinc, and manganese at high pH.
6.1 Secondary Macronutrients
THREE SECONDARY MACRONUTRIENTS
From: 6.1 Secondary Macronutrients
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6.2 Micronutrients (Trace Elements)
ESSENTIAL MICRONUTRIENTS
From: 6.2 Micronutrients (Trace Elements)
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6.3 The Organic Matter Solution to Micronutrients
WHY ORGANIC MATTER SOLVES MOST MICRONUTRIENT PROBLEMS
From: 6.3 The Organic Matter Solution to Micronutrients
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1. Micronutrients are needed in smaller amounts than macronutrients but are equally essential.
2. Iron (Fe) is involved in chlorophyll synthesis.
3. Manganese toxicity can occur in very acidic soils.
4. Zinc deficiency can cause stunted growth and small leaves.
5. Boron is essential for cell wall formation and pollen tube growth.
6. Copper deficiency can cause wilting even in well-watered plants.
7. Molybdenum is required for nitrogen fixation in legume root nodules.
8. Chlorine is required in very small amounts as a micronutrient.
9. Adding too much of a micronutrient can create toxicity problems.
10. Micronutrient deficiencies are rare in soils with good organic matter.
11. Iron chlorosis in alkaline soils is a micronutrient deficiency problem.
12. Boron deficiency causes hollow heart in beets and black heart in celery.
13. Chelated iron fertilizers are more available to plants in alkaline soils.
14. Zinc is often deficient in heavily leached sandy soils.
15. Foliar application can quickly correct micronutrient deficiencies.
16. Manganese deficiency looks similar to iron deficiency.
17. Molybdenum availability decreases in acidic soils.
18. Texas caliche soils commonly cause multiple micronutrient deficiencies.
19. Organic matter generally improves micronutrient availability.
20. All micronutrient deficiencies cause yellowing (chlorosis) of leaves.
1. What are the seven essential micronutrients for plants?
2. How does alkaline pH affect micronutrient availability?
3. What is chelated iron and when is it used?
4. Describe the symptoms of iron deficiency and how to distinguish it from nitrogen deficiency.
5. Why are boron deficiencies serious?
6. How does organic matter help prevent micronutrient deficiencies?
7. What is the risk of over-applying micronutrient fertilizers?
8. Why are micronutrient deficiencies more common in Texas than in eastern US soils?
9. What does foliar application mean and when is it preferred?
10. How do you test for micronutrient deficiencies?
1. The plant nutrient showing yellowing between leaf veins (interveinal chlorosis) on young leaves in alkaline soils is .
2. Manganese toxicity causes brown spots on leaves and is associated with very (acidic/alkaline) soils.
3. Boron deficiency causes growth points and distorted young leaves.
4. Chelated iron has the metal bound to an organic molecule making it in alkaline soils.
5. Molybdenum is essential for the enzyme used in nitrogen fixation and nitrate reduction.
6. Zinc deficiency causes leaves to be small and crowded — a symptom called .
7. Foliar application means spraying nutrients directly onto for rapid uptake.
8. The micronutrient most commonly deficient in Texas alkaline soils is .
9. Organic matter improves micronutrient availability by pH and providing chelating compounds.
Complete in the field. Check each off as you go.
1. Iron Chlorosis Survey: Walk your property looking for yellowing between green veins on young leaves. Photograph examples. Test pH in that location. Are they related?
2. Chelated Iron Trial: Find a plant showing iron chlorosis symptoms. Apply chelated iron per label instructions to half the plant via foliar spray. Observe recovery over 2-4 weeks.
3. Soil Test for Micronutrients: Request a full micronutrient analysis with your soil test from Texas A&M. Compare results to standard recommendations.
4. Boron Application: If growing brassicas (broccoli, cabbage) or beets, apply borax at recommended rate (1/4 tsp per gallon water, 1 gallon per 100 sq ft). Note any improvement in head formation.
5. Organic Matter and Micronutrients: Compare micronutrient availability in: (1) high-organic compost-amended bed, (2) unamended garden bed, (3) bare compacted soil. If you have a soil test, do all three locations.
Chapter Summary
SECONDARY NUTRIENTS
From: Chapter Summary
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Next Chapter → Soil Biology — Bacteria, Fungi, and the Soil Food Web
Connections to Other Topics
→ C03 Ch04: Soil pH — pH controls micronutrient availability
From: Connections to Other Topics
Image file: ../../../images/s01-foundation/c03-soil-science/ch06/c03-soil-science_ch06_micronutrients_fig05.png
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