soil nutrition - sulfur and phosphorous

soil fertility management tends to focus on:

N, P, K

sulfur plant and animal nutrition

essential amino acids, accumulates in plant seeds, component of vitamins and enzymes, component of aromatic oils, healthy plant foliage

essential amino acids with sulfur

methionine, cysteine, cystine

sulfur healthy plant foliage content

0.15-0.45%

sulfur environmental issues

acid precipitations, forest decline, acid sulfate soils, acid mine drainage, toxic effects on drinking water

sulfur sources

mineralization of soil organic matter, desorption of sulfate held by anion exchange capacity sites (lower horizons of highly weathered soil), dissolution of sulfate minerals such as gypsum, adsorption of sulfur dioxide gas from atmosphere (plant foliage and soils)

plants uptake sulfur as

sulfate (SO4 2-)

sulfur deficiency

thin petioles, spindly stems, maturity may be delayed, growth is slowed, chlorosis, low sugar.

causes of sulfur deficiencies (in the last 30 years)

enforcement of clean air standards (reducing SO2 emissions), lower concentration of sulfur in NPK fertilizers, increased in crop harvests (more sulfur removed)

areas of sulfur deficiencies

soil parent material are sandy or inherently low in sulfur, weathering and leaching removed this element, little replenishment from the atmosphere. 

sulfur cycle moves forward in ______ conditions, and backward in ______ conditions.

aerobic, anaerobic

sulfur cycle

hydrogen sulfide (H2S) is most reduced form of S→elemental sulfur is intermediate→sulfate ion is most oxidized.

sulfur cycle is done by:

microorganisms

every mol of S oxidized results in _____ mols of acidity (H+ ions)

2

after nitrogen, ____ is the nutrient that most influences plant productivity and ecosystems health

P

phosphorous for plant nutrition

ATP, DNA, RNA protein and enzymes synthesis. cellular membrane (phospholipids).

phosphorous leaf tissue content

0.2-0.4%

phosphorous environmental issues

degredation of land (too available phosphorous, erosion from poorly protected land due to sparce vegetative growth), degredation of water (eutrophication)

phosphorous soil problem in soil fertility

total phosphorous content is relatively low, most of the P found in soils is in forms quite unavailable for plant uptake, soluble P added to soils is rapidly transformed into unavailable or fixed forms

sources of phosphorous

organic matter, inorganic P in parent materials, deposition (dust)

phosphorous deficiency

stunted, thin, spindly stems. mature may be delayed, growth is slowed, foliage dark bluish-green (not pale), purple colors, very mobile (older leaves show deficiency first)

plants uptake phosphorous as:

hydrogen phosphate in alkaline soils, dihydrogen phosphate in alkaline and acid soils, organic phosphorous compounds

_______ in low P soils may only allow a small fraction (10-15%) of the phosphorous applied in fertilizers and manures to be taken up by plants in the year of application

fixation reactions

fixation reactions

the chemical being fixed is bound, entrapped or otherwise held tightly by soil solids in a form that is relatively unavailable to plants

phosphorous soil solution is typically much more _______

dilute

phosphorous form in alkaline soils

HPO4

phosphorous form in acid soils

H2PO4

often the process limiting the uptake of P by plants is:

the movement of P to the surface of the plant root. diffusion of phosphate ions, mass flow of the soil solution, or strong fixation of the P by iron, aluminum, and calcium compounds lining the soil pores in which the soluble P must flow

______ growing in the rhizosphere assist in P uptake

transport of soluble P happens through:

the hyphae of symbiotic mycorrhizal fungi

plant strategies to obtain more phosphorous

increased root absorptive surface area, chelate iron or aluminum to release P, dissolve Ca-P compounds with acid exudates, exude phosphatase enzymes to release P from organic compounds, exude substances to stimulate P-solubilizing rhizobacteria, encourage colonization by mycorrhizal fungi that help plants take up P

practical control of phosphorus availability

optimize P availability while using minimal amount of soluble P

optimize P availability while using minimal amount of soluble P includes:

enhance mycorrhizal symbiosis, use or breed P-efficient plants, adjust P application to soil status, localized placement, combine ammonium with phosphorous, control of soil pH, cycling of organic matter, use of P-efficient cover crops or rotation/agro-forestry plants

localized placement of phosphorous:

P fertilizer concentrated in a localized zone of soil is less likely to undergo fixation reactions than fertilizer mixed into the bulk soil 

phosphorous availability usually best at pH=

6-7

cycling of organic matter for phosphorous

during the microbial breakdown of organic materials, phosphorous is released and P fixation is reduced

use of P-efficient cover crops or rotation/agro-forestry plants

highly P-efficient plants grown as cover crops solubilize and take up phosphorous from the soil profile and release it into the surface soil when their residues decay. 

management strategies for controlling over-enrichment of soils and water pollution by P

avoid excess accumulation, minimize loss in runoff and eroded sediment, capture P from runoff, capture P from drainage water, tie up P with inorganic iron, aluminum, or calcium containing materials