Nutrients Chapter 6

Plant nutrients the 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 nutrients without them, the plant would not grow at all Soil Analysis Ch 6

Nutrients in plants move around in the plant through the course of its growth eg corn the major point of concentration initially is in the leaves move towards the stalks and cobs the grain (fruit) develops last a major requirement for the nutrients mobile nutrients move from the leaves, stalks and cobs into the fruit deficiency symptoms develop in the leaves as they drop below the necessary nutrient content Soil Analysis Ch 6

Mobility different nutrients have different abilities to move through the plant N is very mobile, and will move easily to points where a deficiency or need occurs when a N deficiency occurs, it will move from the older growth to the new tissue same applies to P, K & Mg Ca & S are much less mobile when a deficiency occurs, the symptoms will appear in the new growth classified as macro or micro on the basis of their content in normal plants Soil Analysis Ch 6

Different nutrients have different abilities to move through the plant 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. Soil Analysis Ch 6

Nitrogen plants absorb all this nutrient from the soil the most important plant nutrient the major limitation to plant growth Soil Analysis Ch 6

N cycle N as N2 (air) N as organic N (living plant) inorganic N (bacterial fixing) (soil) fixation denitrification decomposition (dead plant) mineralisation nitrification immobilisation Soil Analysis Ch 6

Fixation nitrogen gas in the air is not soluble in water cannot be absorbed directly by any part of the plant fixation converts N2 to ammonia by an enzyme called nitrogenase not all plants can do this those that can are known as legumes, and use a bacteria called rhizobium it resides in the plant roots and produces absorbable N for the plant other bacteria reside in the soil and fix nitrogen for uptake by non-fixing plants. Soil Analysis Ch 6

Mineralisation most nitrogen in the soil is in the form of organic nitrogen, held in organic matter not available to plants bout 2% of the organic nitrogen will decompose in a year to form inorganic (or mineralised) nitrogen as ammonia/ammonium some plants (eg rice) are capable of absorbing ammonium ions most prefer nitrate Soil Analysis Ch 6

Nitrification ammonium converted by soil bacteria to nitrite and then to the useful form nitrate requires oxygen; will not readily occur in compacted or water-saturated soils pH should also be greater than 6 to encourage nitrification nitrate is not retained on soil minerals ammonium is; provides a small reserve of nutrient in depleted soils Soil Analysis Ch 6

Immobilisation after uptake, the plant uses the N in one of the many organic compounds that requires it most particularly protein and chlorophyll covalently bound not be available elsewhere until the death of the plant or the metabolism of that compound Soil Analysis Ch 6

Decomposition dead plant matter becomes available as food for organisms such as worms, and micro-organisms such as bacteria in the soil releases nutrients such as nitrogen bound up in the plant C:N ratio of decaying organic matter affects decomposition dry, woody material (high ratio, eg straw) isn’t consumed by bacteria in the absence of this source of N, will use the soil reserves Soil Analysis Ch 6

Denitrification some bacteria convert nitrate to nitrogen gas or nitrogen oxides should be a balance between fixation and denitrification this prevents significant runoff into groundwater adding more N (as fertiliser) creates imbalance some of this excess N ends up where it is not wanted in the waterways, producing algal blooms and eutrophication Soil Analysis Ch 6

N problems Low levels reduced growth yellowed leaves Excess levels rapid growth dark leaves reduced flowering/fruit Soil Analysis Ch 6

Phosphorus principal source of “new” phosphorous in its cycle is from some rock minerals it is found in the form of phosphate organic phosphorous after bacterial conversion Soil Analysis Ch 6

compounds and adsorbed phosphate P cycle Organic P (in plant) Mineral P Phosphate (soil solution) Insoluble phosphate compounds and adsorbed phosphate weathering immobilisation decomposition desorption solubilisation leaching (in solution) mineralisation fixation Soil Analysis Ch 6

phosphate, is not found in soil solution to any great extent formation of very insoluble compounds with Ca, Fe & Al adsorption onto clay processes which are slow in reverse uptake by the plant is needs to be efficient Australian native plants have adapted to soils that are relatively low in P introduced species (grain, fruit and vegetable crops) need addition of phosphate in the form of fertiliser temporary increase only Soil Analysis Ch 6

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 Soil Analysis Ch 6

P deficiency reduced growth purpling of green leaves death of older leaves Soil Analysis Ch 6

Potassium no organic form, so simpler Plant decomposition 3 2 decomposition immobilisation 4 Mineral (bound) immobilisation Mineral (adsorbed) Soil solution 1 solubilisation 5 solubilisation 6 leaching Soil Analysis Ch 6

In the plant Deficiency stalks are relatively weak and break easily yellowing and death occurs around the edges of older leaves Excess takes up too much, at the expense of Ca & Mg Soil Analysis Ch 6

Calcium & magnesium sources and cycling similar to potassium adsorption of Ca and Mg to cation sites is greater than of K 90% of adsorbed cations in neutral or alkaline soils will be Ca & Mg reserve supplies are likely to be good the plant’s need for these elements is much less than that for potassium, so deficiencies are less common Soil Analysis Ch 6

Sulfur similar to nitrogen except: atmospheric source already useable by plants only one form of inorganic S Soil Analysis Ch 6

Exercise 6.2 S as SO4 (air) S as organic S (living plant) inorganic S (solution) fallout decomposition (dead plant) mineralisation immobilisation Natural cycle (adsorbed) solubilisation Soil Analysis Ch 6

Class Exercise 6.3 In 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 Soil Analysis Ch 6