nutrients chapter 6. soil analysis ch 62 plant nutrients  the species that they require to obtain...

Download Nutrients Chapter 6. Soil Analysis Ch 62 Plant nutrients  the species that they require to obtain from outside the plant (air, water, soil) in order

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  • NutrientsChapter 6

  • Plant nutrientsthe species that they require to obtain from outside the plant (air, water, soil) in order to grow and survive

    this chapter: the nutrients that plants gain from contact with the soil

    carbon (as atmospheric carbon dioxide) and water (from the soil) not considered nutrientswithout them, the plant would not grow at all

  • Nutrients in plantsmove around in the plant through the course of its growtheg cornthe major point of concentration initially is in the leavesmove towards the stalks and cobsthe grain (fruit) develops lasta major requirement for the nutrientsmobile nutrients move from the leaves, stalks and cobs into the fruitdeficiency symptoms develop in the leaves as they drop below the necessary nutrient content

  • Mobilitydifferent nutrients have different abilities to move through the plantN is very mobile, and will move easily to points where a deficiency or need occurswhen a N deficiency occurs, it will move from the older growth to the new tissuesame applies to P, K & MgCa & S are much less mobilewhen a deficiency occurs, the symptoms will appear in the new growthclassified as macro or micro on the basis of their content in normal plants

  • Different nutrients have different abilities to move through the plant. Nitrogen is very mobile, and will move easily to points where a deficiency or need occurs. When a nitrogen deficiency occurs, it will move from the older growth to the new tissue. The same applies to phosphorous, potassium and magnesium. Calcium and sulfur are much less mobile, and when a deficiency occurs, the symptoms will appear in the new growth.Nutrients are classified as macro or micro on the basis of their content in normal plants: macronutrients have levels of greater than 500 mg/kg. Tables 6.1 and 6.2 lists the macro- and micronutrients and a brief description of their role in plants.

  • Nitrogen

    plants absorb all this nutrient from the soil

    the most important plant nutrient

    the major limitation to plant growth

  • N cycle

  • Fixationnitrogen gas in the air is not soluble in watercannot be absorbed directly by any part of the plantfixation converts N2 to ammonia by an enzyme called nitrogenasenot all plants can do thisthose that can are known as legumes, and use a bacteria called rhizobiumit resides in the plant roots and produces absorbable N for the plantother bacteria reside in the soil and fix nitrogen for uptake by non-fixing plants.

  • Mineralisationmost nitrogen in the soil is in the form of organic nitrogen, held in organic matternot available to plantsbout 2% of the organic nitrogen will decompose in a year to form inorganic (or mineralised) nitrogen as ammonia/ammoniumsome plants (eg rice) are capable of absorbing ammonium ionsmost prefer nitrate

  • Nitrificationammonium converted by soil bacteria to nitrite and then to the useful form nitraterequires oxygen; will not readily occur in compacted or water-saturated soilspH should also be greater than 6 to encourage nitrificationnitrate is not retained on soil minerals ammonium is; provides a small reserve of nutrient in depleted soils

  • Immobilisationafter uptake, the plant uses the N in one of the many organic compounds that requires itmost particularly protein and chlorophyllcovalently boundnot be available elsewhere until the death of the plant or the metabolism of that compound

  • Decompositiondead plant matter becomes available as food for organisms such as worms, and micro-organisms such as bacteria in the soilreleases nutrients such as nitrogen bound up in the plantC:N ratio of decaying organic matter affects decompositiondry, woody material (high ratio, eg straw) isnt consumed by bacteriain the absence of this source of N, will use the soil reserves

  • Denitrification

    some bacteria convert nitrate to nitrogen gas or nitrogen oxidesshould be a balance between fixation and denitrificationthis prevents significant runoff into groundwateradding more N (as fertiliser) creates imbalance some of this excess N ends up where it is not wantedin the waterways, producing algal blooms and eutrophication

  • N problemsLow levelsreduced growthyellowed leaves

    Excess levelsrapid growthdark leavesreduced flowering/fruit

  • Phosphorusprincipal source of new phosphorous in its cycle is from some rock mineralsit is found in the form of phosphateorganic phosphorous after bacterial conversion

  • P cycle

  • phosphate, is not found in soil solution to any great extentformation of very insoluble compounds with Ca, Fe & Aladsorption onto clayprocesses which are slow in reverseuptake by the plant is needs to be efficient Australian native plants have adapted to soils that are relatively low in Pintroduced species (grain, fruit and vegetable crops) need addition of phosphate in the form of fertilisertemporary increase only

  • Testing for P

    can give misleading results if the purpose of test is not made clear

    total phosphorus is very different to available P

    various extracting solutions have been devised to simulate the availability of the element

  • P deficiencyreduced growthpurpling of green leavesdeath of older leaves

  • Potassiumno organic form, so simpler

    Mineral (bound)Soilsolution

    Plant

    Mineral (adsorbed)123456solubilisationleachingsolubilisationimmobilisationimmobilisationdecomposition

  • In the plant

    Deficiencystalks are relatively weak and break easilyyellowing and death occurs around the edges of older leaves

    Excesstakes up too much, at the expense of Ca & Mg

  • Calcium & magnesiumsources and cycling similar to potassiumadsorption of Ca and Mg to cation sites is greater than of K

    90% of adsorbed cations in neutral or alkaline soils will be Ca & Mgreserve supplies are likely to be goodthe plants need for these elements is much less than that for potassium, so deficiencies are less common

  • Sulfursimilar to nitrogen except:atmospheric source already useable by plantsonly one form of inorganic S

  • Exercise 6.2

  • Class Exercise 6.3In each of the nutrient cycles in this chapter, one exit point for the element is not included. It is one of the major reasons that extra nutrient must be added to domestic and commercial soils. What is it?

    removal of plant material for human consumption

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