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Plant Nutrition

Like all living organisms plants require the raw materials essential to keep their metabolism functioning. Apart from carbon dioxide from the air and a supply of water - which provide Carbon (C), Oxygen (O) and Hydrogen (H) - they require thirteen essential nutrients. Three Major and three Secondary Macronutrients, and seven Micro-nutrients, which are taken up from the soil or any other growing medium.

As plants grow they remove these essential elements to a varying degree and rainwater leaches out more, so from time to time they need to be replenished usually in the form of fertilizers. In uncultivated ground the plants are naturally recycled and the nutrients return to the soil, but as gardeners we harvest crops and keep the place tidy taking most of the topgrowth away at some time, so they are gradually depleted.
Historically animal manures and byproducts of slaughter have been used to supply additional nitrients to crops. As agriculture developed, more sources were needed to satisfy the needs of intensive cultivation. In the early nineteenth century the extraction of bird guano which had built up on isolated offshore islands over many centuries, became the most important source of fertilizers. Later as demand grew for relatively cheaper sources various manufacturing processes have been discovered and natural gas has become a major source of Nitrogen in particular. These are sometimes called inorganic fertilizers as they do not come from animal or plant material.
The Haber-Bosch process converts atmospheric Nitrogen to ammonia by combining it with Hydrogen under pressure and high temperature. Unfortunately Fritz Haber who discovered it blotted his copybook later when he became known as the father of chemical warfare as he developed the weapons used in the First World War.

There is some evidence that fruit and vegetables produced today are lower in the micronutrients and vitamins than half a century ago. Although growers add the Major nutrients in fertilizers, they may not be adding the Secondary and Micro-nutrients. Also the balance of the Major nutrients and soil condition can affect the availability of the others to plants. They are helped in this by a special relationship with fungi called mycorrhizae which are found in the rootballs of most plants. This symbiotic relationship improves the uptake of nutrients.

The three Major Nutrients are, Nitrogen (N), Phosphorous (P) and Potasium (K).

The three Secondary Nutrients are:-

The seven Micronutrients are required in lesser amounts, but are just as essential for healthy growth. They are usually present in adequate amounts in most soils, but can be leached from sandy soils. Soil-less composts can run out of them if not supplemented, but at high levels they can be toxic to plants:-

    Boron (B) taken up as H2BO3- ions. It is involved in the transport of sugars, in cell division and the production of amino acids. At a pH greater than 6.8 it is leached from peat-based composts. Borax is the usual compound to use as a supplement.
    Chlorine (Cl) in the form of Cl- ions. It is needed for photosynthesis and keeping the cells turgid.
    Copper (Cu) in the form of CU++ ions. It is involved with the enzymes in photosynthesis. Applied as Copper Sulphate or Copper Oxide.
    Iron (Fe) in the form of Fe++ ions. Can be displaced by Calcium ions in chalky soils making them unavailable for uptake, leading to Iron deficient chlorosis as it is used in the making of Chlorophyll and in other metabolic pathways concerned with photosynthesis. Applied as a chelated form such as Fe-EDTA to the foliage.
    Manganese (Mn) in the form of Mn++ ions. It is involved in the production of chloroplasts. Can be deficient in slightly acidic to neutral soil. At low pH it becomes more available and can reach toxic levels. Applied as a Manganese Sulphate foliar feed.
    Molybdenum (Mo) taken up as molybdate (MoO4=) ions. It is needed for the conversion of Nitrate ions to Ammonia in the plant before it is included in amino acids. Also it is used in nitrogen fixation, so this is why it is particularly important to legumes which have a symbiotic retationship with bacteria on their roots that carry out this process. At low pH it is less available and liming usually solves the deficiency by raising the pH. Otherwise it can be applied to the soil as Sodium Molybdate or to the foliage as Ammonium Molybdate.
    Zinc (Zn) in the form of Zn++ ions. It is used in enzymes particularly in the hormone balance of auxins. Applied as Zinc Sulphate or Zn-EDTA chelate.

When plants are deficient of nutrients they show symptoms which can usually be seen in the foliage. The essential nutrients are involved in the metabolic processes so the same symptoms can be caused by a number of deficiencies and cannot be taken as definitive, but they are a warning of possible problems. Chlorosis is caused by deficiencies of more than one nutrient as photosynthesis involves many of them. Some leaf colours are due to the presence of componds which are unseen when chlorophyll is present, but if it decreases they become visible - the converse occurs when poor light conditions causes more chlorophyll to be present and normally coloured foliage becomes greener. An analysis of the soil or the tissues of the plant is needed for proof of a deficiency - a pH test will show if conditions are correct for nutrients which are present, to be absorbed. Other stresses can show up in a similar way, eg. air pollution can cause scorching of leaf tips.

On older foliage

Deficiency  Leaf Symptoms
Magnesium yellowing between the veins which remain green
Manganese brownish-whitish-greyish spots
Nitrogen yellowing from the tips
Phospherous reddish colour on green leaves and stems
Potassium dying at the edges

On younger foliage

Deficiency   Leaf Symptoms
Boron   brownish or dead youngest leaves
Copper   white tips to youngest leaves
Iron   mottled yellow and green with green veins
Sulphur   mottled yellow and green with yellowish veins
Manganese   brownish-black spots

Where a particular deficiency is present a single supplement or 'straight' can be applied, but usually a multinutrient fertilizer is used. The analysis of multinutrient fertilizers usually gives the ratio of the major nutrients, N:P:K that they contain. A general fertilizer such as Growmore has a NPK ratio of 7%:7%:7% or equal amounts of each, calculated according to their availability to plants. Fish, Blood and Bone is a general organic fertilizer with a typical analysis of 6:6:6, another is Pelleted Chicken Manure with a typical NPK of 4.3:3.2:3.2, plus trace elements and magnesium.
The Secondary and Micronutrients may also be included and some fertilizers are prepared for plants with different requirements, eg. Ericaceous fertilizers for acid-loving plants has a typical N:P:K of 13:5:9 plus 2% Mg.
To extend the duration of an application the granules can be coated with a resin which has microscopic pores to release the fertilizer. This coating also reacts to temperature as the pores contract during cold spells, cutting off the 'leaking' of the contents. An example is Osmocote, which is a general fertilizer. Bonemeal is a slow-release organic fertilizer as it has to break down to free the nutrients it contains.
A fertilizer for growing tomatoes and fruit has a higher level of K with a typical NPK ratio of 4:4.5:8, there is a lower N level so that leaf and stem growth is better controlled. Similarly a fertilizer for cereals has a ratio of 10:10:20 to produce shorter stems and better seeds.

Garden compost made from leaves and fading stems tends to have relatively low levels of nutrients. Before a tree sheds its leaves it extracts useful nutrients (a process known as Senescence) and the organisms involved in the rotting process use up more of them. The main benefit of compost and leaf mould is adding organic matter which holds moisture, nutrients and improves drainage, by coating soil particles.

The timing of the application is important. There is no point in applying Nitrogen before the soil warms up in spring as the plants will not be taking it up and the rain will leach it away. Also any applied at planting time may not be needed until later, but it can have leached away in the meantime. Fertilizers can be caustic to the fine roots, so should be well mixed in the growing medium. Seed composts do not need much added nutrition although Phosphorous is needed for good root development.

The most common way to add fertilizers is as a powder or pelleted form which is washed down into the soil. It can be used as a solution for more immediate availability, applied to the soil or to the foliage. A foliar feed is limited in that a concentrated solution may scorch the leaves and they can only take in small amounts, so this method is better suited to the application of the Secondary and Micronurtients. Such sprays should only be applied in shady conditions to allow the solution to dry slowly and avoid scorching by strong sunlight.

Too much added fertilizer can be detrimental as it affects the concentration gradient and the plants are unable to take up water. The soil water has a high level of dissolved salts so retains the moisture or can draw it from the root cells. This is more of a problem with container-grown plants and shows up as white flecks on the surface of the planting medium, particularly if tap-water is used, as in hard water areas it contains Ca++ ions.
Applying too much fertilizer on open ground can cause environmental damage if it runs off or leaches into ground water or water-courses. Excess nutrients also cause soft growth which is more susceptible to diseases, frost and wind scorch. Mycorrhizal fungi can also be damaged by excess fertilizer, particularly if these organisms are used at planting time to enhance establishment.

As mentioned earlier some of the ingredients in artificial fertilizers such as Ammonium Nitrate are prone to a chemical reaction known as deliquescence where water vapour is absorbed from the air. This can occur to such an extent that they become liquefied. The process also leads to loss of active ingredients as they are converted to less available chemicals and gases which escape, eg. ammonia. Once opened it is a good idea to decant the fertilizer into an air-tight container. Rinse out a two or three litre, plastic milk carton and allow to dry. Make a funnel from a piece of card or a cut off carton with a narrower spout. A mask should be worn as the dust may not be healthy. Label the carton clearly with the contents and store safely in a cool dry place.
Organic fertilizers also need to be kept in a closed container. Bonemeal attracts mice and rats, and flies and moths lay their eggs in it so it becomes infested with maggots.
Wormeries produce a liquor which can be drained off and diluted by ten, to be used as a liquid feed. Also fresh shoots of Nettles or Comfrey can be rotted in water to make a concentrated plant feed. Even troublesome weeds such as Ground-elder can be rotted this way to provide some nutrition, the fibrous waste can then be added to the compost heap if it is checked carefully for signs of life.

During the great expansion of the agricultural industry as it tried to feed the population of the towns and cities, the demand for fertilizers increased. The discovery of massive deposits of seabird guano off the coast of Peru started large-scale mining and shipping of it and vast fortunes were made. The Incas used it to enrich their soil and guarded it with the penalty of death. It also contains saltpetre which is used to make gunpowder. Peru and Bolivia fought Chile in the war of the Pacific from 1879 to 1883 partly due to the attempt of Bolivia to tax Chilean guano miners. The United States of America enacted the Guano Islands Act in 1856 which entitled US citizens to take possession of unclaimed islands which had deposits of guano. By the end of the nineteenth century artificially produced fertilizers started to take over from it.

Artificial fertilizers are manufactured using fossil fuels and have been responsible for massive increases in the yield of crops achieved in the last century. Some would estimate that yields could fall by about 75% if we stopped using them, so to grow the same amount of food would require much more land than is available. It would also take more management which needs fuel and labour. However, when applied to the soil some of the Nitrogen is converted to Nitrous Oxide which is a greenhouse gas, so it adds to the global warming process.


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