Plant NutritionINTRO 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 NutritionEssential Elements

NitrogenPromotes rapid growth and dark green

colorNeeded for chlorophyll productionKey component in proteinPlant water efficiency

Plant NutritionEssential 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 NutritionEssential 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:

H2PO4-

HPO4-2

(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