Soil Science
The living foundation beneath your feet — what soil is, how it works, how to improve it
Chapter 5: Macronutrients — Nitrogen, Phosphorus, Potassium
Why this matters: NPK — these three letters appear on every bag of fertilizer. Understanding what each does, how each moves through soil, and how each is made available naturally gives you the ability to diagnose plant problems, reduce or eliminate purchased inputs, and build a self-sustaining fertility system.
5.1 Primary Macronutrients Overview
THE FERTILIZER BAG
From: 5.1 Primary Macronutrients Overview
Image file: ../../../images/s01-foundation/c03-soil-science/ch05/c03-soil-science_ch05_macronutrients_fig01.png
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5.2 Nitrogen (N)
WHAT NITROGEN DOES
From: 5.2 Nitrogen (N)
Image file: ../../../images/s01-foundation/c03-soil-science/ch05/c03-soil-science_ch05_macronutrients_fig02.png
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5.3 Phosphorus (P)
WHAT PHOSPHORUS DOES
From: 5.3 Phosphorus (P)
Image file: ../../../images/s01-foundation/c03-soil-science/ch05/c03-soil-science_ch05_macronutrients_fig03.png
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5.4 Potassium (K)
WHAT POTASSIUM DOES
From: 5.4 Potassium (K)
Image file: ../../../images/s01-foundation/c03-soil-science/ch05/c03-soil-science_ch05_macronutrients_fig04.png
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5.5 The NPK Balance
COMMON MISTAKES WITH NPK
From: 5.5 The NPK Balance
Image file: ../../../images/s01-foundation/c03-soil-science/ch05/c03-soil-science_ch05_macronutrients_fig05.png
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Chapter Summary
NITROGEN (N): green growth, chlorophyll
From: Chapter Summary
Image file: ../../../images/s01-foundation/c03-soil-science/ch05/c03-soil-science_ch05_macronutrients_fig06.png
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1. Nitrogen (N) is the nutrient most often deficient in crops.
2. Phosphorus (P) promotes root development and flowering.
3. Potassium (K) is called the "quality nutrient" — it improves disease resistance.
4. NPK fertilizer labels show percentages by weight of actual nutrients.
5. Nitrogen deficiency first shows as yellowing of older (lower) leaves.
6. Excess nitrogen causes excessive vegetative growth at the expense of fruit.
7. Phosphorus becomes unavailable in both very acidic and very alkaline soils.
8. Potassium is readily leached from sandy soils by rain.
9. Calcium is considered a secondary macronutrient.
10. Magnesium is the central atom in the chlorophyll molecule.
11. Sulfur gives brassicas and alliums their distinctive smell.
12. Legumes can supply their own nitrogen through root nodules.
13. A fertilizer labeled 10-10-10 contains 10% each of N, P2O5, and K2O.
14. Nitrogen deficiency causes purple coloration of leaves.
15. Excess phosphorus in soil runoff causes algal blooms in water bodies.
16. Potassium deficiency shows as scorched or brown leaf margins.
17. Magnesium deficiency shows as yellowing between veins on older leaves.
18. Calcium deficiency causes blossom end rot in tomatoes.
19. All plant-available nitrogen must first be converted to ammonium or nitrate.
20. Slow-release fertilizers feed plants over months rather than weeks.
1. What are the three primary macronutrients and their main functions?
2. What are the secondary macronutrients and why are they less commonly deficient?
3. Describe nitrogen deficiency symptoms and how to correct it.
4. Why does excess nitrogen reduce fruit production?
5. How do legumes supply nitrogen to themselves and neighboring plants?
6. What is blossom end rot and what nutrient deficiency causes it?
7. Explain the nitrogen cycle briefly.
8. How does phosphorus affect root development?
9. Why should you test soil before adding fertilizer?
10. Describe three organic sources of nitrogen available to homesteaders.
1. The three primary macronutrients are nitrogen, , and potassium.
2. On a fertilizer label, the three numbers represent N, , and K₂O percentages.
3. Nitrogen deficiency first appears on leaves (older/newer).
4. Potassium deficiency causes brown (scorching) around leaf edges.
5. Magnesium is the central atom in the molecule.
6. Calcium deficiency in tomatoes causes end rot.
7. The process where bacteria convert atmospheric N₂ to plant-usable ammonia is .
8. Excess soil nitrogen promotes leaf and stem growth at the expense of .
9. The secondary macronutrients are calcium, magnesium, and .
Complete in the field. Check each off as you go.
1. Soil Test First: Send soil samples from 3 areas of your garden to the Texas A&M soil testing lab. Review results — what nutrients are deficient, adequate, or excess?
2. Deficiency Identification: Walk your garden and look for: yellowing old leaves (N), purple tinting (P), scorched edges (K), yellowing between veins (Mg). Photograph and record.
3. Legume Green Manure: Plant a bed of crimson clover or cowpeas this season. At flowering, incorporate into the soil. Test nitrogen levels before and 4 weeks after incorporation.
4. Compost vs Synthetic: Apply compost to one half of a bed and a synthetic 10-10-10 to the other at equivalent nutrient rates. Monitor plant growth and soil health over one season.
5. Tomato Blossom End Rot Prevention: If growing tomatoes, maintain consistent watering and add calcium source (gypsum or lime). Monitor first fruits for BER symptoms.
Practice Exercises
- Your vegetable garden plants look generally pale and stunted. The lower leaves are turning yellow while the upper leaves remain green. What nutrient is deficient and what are three organic ways to supply it?
- A soil test shows very high phosphorus levels but your plants show P deficiency symptoms (purple leaves). What are two possible explanations?
- Why do repeated applications of chemical fertilizer tend to require larger and larger amounts over time to achieve the same results?
- A gardener covers a raised bed with 6 inches of wood chips as mulch. The following spring, plants in that bed grow very poorly despite the wood chips decomposing. What is happening nutritionally and what is the fix?
- Name two cover crops well-suited to Texas summer heat that fix nitrogen.
Next Chapter → Micronutrients and Secondary Nutrients
Connections to Other Topics
→ C03 Ch04: Soil pH — pH controls nutrient availability
From: Connections to Other Topics
Image file: ../../../images/s01-foundation/c03-soil-science/ch05/c03-soil-science_ch05_macronutrients_fig07.png
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