Chapter 14 Summary: Soil Phosphorus and
Potassium
Phosphorus:
Phosphorus in pant nutrition and soil fertility:
 Second most likely of essential elements to limit
plant productivity. Commonly applied as fertilizer.
 Neither plants nor animals can live without
phosphorus. Is an essential component of ATP
(energy current), DNA, RNA, and phospholipids.
 Enhances the processes of photosynthesis,
nitrogen fixation, flowering, fruiting, and
maturation.
 Encourages root growth, strengthens structural
tissues, and improves crop quality.
 Phosphorus-deficient plant is usually stunted, thin
stemmed, and spindly and characterized by
delayed maturity, sparse flowering, poor seed
quality, yellowing and senescence of leaves.
The Phosphorus problem in soil fertility:
 Total phosphorus content of soils is relatively low.
 Phosphorus compounds commonly found in soils
are mostly unavailable to plants and highly
insoluble.
 Soluble sources of phosphorus such as fertilizers
and manures are fixed when added to the soil.
Effects of phosphorus on environmental quality
 Land degradation caused by too little available
phosphorus
 Accelerated eutrophication (nutrient enrichment of
lakes, ponds and other waters leading to oxygen
deficiency in the water body) caused by too much
phosphorus.
Phosphorus gains and losses:
 Gains: fertilizer, plant residues, manure, sludge.
Losses: plant removal, erosion, surface runoff, and
leaching.
Organic and inorganic phosphorus in soils
 Organic: 1) inositol phosphates or phosphate esters
of a sugar like compound 2) nucleic acids 3)
phospholipids.
 Inorganic: 1) those containing calcium 2) those
containing iron and aluminum.
Practical control of phosphorus in soils:
Neil Schibbelhute
 Enhance availability: adjust fertilizer application
to soil, localized fertilizer placement, combine
ammonium with phosphorus, cycling of organic
matter, control of soil, and enhance mycorrizal
symbiosis.
 Reduce losses to water: avoid excess
accumulation, minimize loss in runoff and
sediment, capture phosphorus from runoff and
sediment, tie up phosphorus with inorganic
amendments.
Potassium:
Nature and ecological roles:
 Third most likely of essential elements to limit
plant productivity. Commonly applied as fertilizer.
 Present in the soil solution only as a positively
charged cation, K+.
 Its behaviour in the soil is influenced by soil cation
exchange properties and mineral weathering rather
than by microbiological processes. No off-site
environmental problems.
Plant and Animal Nutrition:
 Activator of cellular enzymes. Is essential for
photosynthesis, protein synthesis, nitrogen fixation
in legumes, starch formation, and translocation of
sugars.
 Especially important in helping plants adapt to
environmental stresses.
 Improves flavour, colour and strengthens stems of
flowers, fruits, and vegetables.
The potassium problem in soil fertility:
 1) A very large proportion is unavailable to plants
( held rigidly as part of primary material or fixed)
 Subject to leaching losses
 Removal by plants is high. Plants will take up far
in excess of their needs if available (luxury
consumption).
Forms and availability of potassium in soils:
 K in primary mineral structure (unavailable).
Nonexchangeable K in secondary minerals (slowly
available). Exchangeable K on soil colloids and K
solution in water (readily available).
Factors affecting potassium fixation in soils:
 Four soil conditions that influence the amounts of
potassium fixed: 1) the nature of soil colloids 2)
wetting and drying 3) freezing and thawing 4) the
presence of excess lime.
Potassium gains and losses:
 Gains: plant residues, animal manures,
atmospheric deposition, commercial fertilizers,
slowly available potassium minerals. Losses: plant
removal, leaching losses, erosion losses, fixation.