Plant Nutrition
INTRO TO SOILS
CH 12
Plant Nutrition
Many soil factors affect plant growth
Difficult/expensive to improve
However . . .
Supply of soil nutrients can be controlled . . .
Plant Nutrition
Essential Elements
Nitrogen
Promotes rapid growth and dark green
color
Needed for chlorophyll production
Key component in protein
Plant water efficiency
Plant Nutrition
Essential Elements
However, too much Nitrogen . . .
• Produces soft, weak, easily injured growth
(lodging)
• Prone to disease/insect injury
• May slow maturity and ripening
• Delays hardening-off
• May impair flavor in some vegetables
• Nitrates may accumulate
Plant Nutrition
Essential Elements
Nitrogen is most important for crops
grown for their vegetation, such as
leafy vegetables, hay or turfgrass
Nitrogen Cycle
• Nitrogen moves around . . .
• Atmosphere is ~ 78% N2
• Unfortunately, N2 (gas) is an unusable
form for plants
However . . .
Nitrogen Cycle
Bacteria can use this form; they may . . .
• Use it to form protein for their own bodies,
or . . .
• Supply it to host plants
• Eventually, both microbes and host plants
do die;
• This allows other microbes to . . .
Nitrogen Cycle
Mineralize the protein to ammonium ions
(NH4+)
• NH4+ may be . . .
– taken up by plants
– converted by bacteria (Nitrification) to NO3– lost to the atmosphere (Volatilization) as NH3
• See reaction p. 198 (NH4+ with Hydroxyl ions)
Nitrogen Cycle
• Study the Nitrogen Cycle (simplified)
on p. 199 and Nitrogen Cycle handout
• Additionally, lightning and fertilizer
factories fix N
Nitrogen Cycle
Non-biological losses of nitrogen include:
– Leaching of NO3– Ammonia Volatilization*
*may occur in dry, alkaline, or recently limed
soil
Nitrogen Cycle
Losses through production:
•
•
•
•
Crop harvest
Erosion
Irrigation
Liming
Nitrogen Cycle
Additions for production
• Manuring
• Growing legumes
• Fertilizing
Forms of Nitrogen in Soil
97-99% of soil nitrogen resides in
organic matter
• only a small percentage is mineralized to
useable forms
• average of 90 lbs/acre/yr . . .
far short of typical crop needs
• e.g. 150-bushel/acre corn crop
contains ~ 190 lbs of nitrogen
• See fig. 12-6, p. 201
Know this chart!
Nitrogen Deficiency
Symptoms:
• Slow growth and stunting
• Lack dark green color
• Exhibit chlorosis
• Lower leaves first affected
• Grasses begin yellowing at blade tips
• Extreme cases dry up – called “firing”
Phosphorus
•
•
•
•
•
“Partners” with Nitrogen
Part of genetic material
ATP stores and transfers energy
Spurs early and rapid root growth
Helps plants use water more efficiently
resist cold and disease and improve
quality of grains and fruits
Improves efficiency of Nitrogen uptake
Phosphorus
“Balances” Nitrogen
•
•
•
•
•
Hastens maturity
Aids blooming and fruiting
Important in crops flowers, fruits, seeds
Promotes early and rapid root growth
Major element in starter fertilizers
Forms of Phosphorus in Soil
• Provided by weathering of mineral . . .
Apatite
(calcium phosphate)
• Anions are:
H2PO4HPO4-2
(referred to as phosphate ions)
Forms of Phosphorus in Soil
• Much phosphorus is unavailable to plants
• “fixed” in insoluble forms
– Iron phosphates in strongly acid soils
– Aluminum phosphates in moderately acid soils
– Calcium phosphates in alkaline soils
Maximum availability between 6.0 to 7.0
Phosphorus Deficiencies/Excesses
•
•
•
•
P-deficient plants often have a purple tint
Older, lower leaves affected first
May exhibit darker than normal green color
Delayed maturity and poor root systems
• Excess P ties up nutrients such as iron (Fe)
Potassium
• a.k.a. – Potash
• Elemental symbol “K”
• Plants consume more K than any other nutrient
except N
• Cell walls and stems strength
• Regulates stomates
• Fruit development and ripening
• Root and tuber crops
• Potassium promotes tougher growth
Potassium Improves:
•
•
•
•
Stem strength; less prone to lodging
Fight disease
Increase winter hardiness
Influences transpiration rate; conserves
water supplies
• Caution: excess/luxury consumption may
inhibit uptake of calcium or magnesium
Potassium in the Soil
• Weathering releases Potassium ions (K+)
• Stored on CEC and fixation in clays
(fig. 12-11, p. 206)
• Moves in soil: more than P, less than N
• Most plant uptake occurs by diffusion
Potassium Deficiencies
• Less of a problem than other primary
nutrients; some occurrence in sandy,
leached soils
• Excess N can inhibit uptake of K
• Dry, cold, poorly aerated soil inhibits
uptake; neutral pH is desirable for uptake
• Deficiencies are seen as “marginal scorch”
on edges of lower, older leaves