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Soils : Plant nutrients

Nitrogen | Phosphorus | Potash | Minor nutrients | Calcium | Magnesium | Iron | Boron | Other minor nutrients

Plants get their food from the soil. Rock material and decaying organic matter contain tiny quantities of the nutrients essential for plant growth. As they are released gradually, they become bonded to clay and humus in the soil. From this reservoir, nutrients are slowly released back into the soil solution from which plant roots can absorb them easily.

Leafy rhubarb needs high nitrogen levels

Leafy rhubarb needs high nitrogen levels

Nitrogen

Nitrogen (denoted by the letter N) is the scarcest of plant nutrients but among the most important, being essential for protein production. Dead plant and animal material, and animal waste, are sources of nitrogen as they break down. Bacteria in nodules on the roots of certain plants – particularly the pea family – are capable of extracting nitrogen from the air which is 80% nitrogen.

Lightning is another important source; the powerful electrical discharge ionises unreactive elemental nitrogen in the air, changing it to oxides of nitrogen that bond with water and fall in the rain of a thunderstorm.

Nitrogen is a vital part of green chlorophyll, which is essential for plant growth because it traps the energy of the sun. Without enough chlorophyll, plants turn pale, yellow-green all over and produce weak spindly growth. This often occurs on soils that are sandy and dry, low in organic material, or constantly wet.

Too much nitrogen causes leafy, rich green growth, at the expense of flowers and fruit. Nitrogen is especially important for grass and leafy vegetables.

Phorphorus

Phosphorus (denoted by the letter P) is involved in root growth, and the development of buds and growing points. Plants short of phosphorus show stunted growth and a purple tinge to stems and leaves. Deficiency occurs most often on acid soils – especially in high rainfall areas, and in peat composts.

Potash

Potash (denoted by the letter K) is essential for flower and fruit production, and for balancing the vigorous leafy growth caused by nitrogen. Plants short of potash tend to be leafy, and often have pale, or brown, margins to the leaves. Deficiency occurs most commonly on light sandy soils, and strongly limy soils.

Minor nutrients

The minor nutrients are not needed in as great quantity, nor are they as scarce as the major nutrients, N, P and K.

Calcium

Calcium (Ca) is important in its own right as a nutrient, apart from its neutralizing effects on soil acidity and its availability to bond with other nutrients, making them unavailable in high-calcium soils.

Calcium is involved in the building of plant structures. Shortage can cause browning within the tissue of plants such as tomatoes, apples and brussels sprouts. Deficiency occurs in peaty soil, acid soil and sandy soil. Apply lime as a remedy.

Magnesium

Magnesium (Mg) forms part of chlorophyll, making it essential for good growth. A shortage of magnesium causes yellow ‘netting’ of the leaves. Deficiency occurs on acid soils, or where too much potash has been given, because potash bonds with magnesium and makes it unavailable. Apply a spray of Epsom salts.

Iron

Iron (Fe), too, is part of chlorophyll. Shortage causes yellow leaves at the growing points, because iron is not mobile in plants. Deficiency is common on limy soils and it is the main reason why lime-hating plants suffer in such soils. Apply Sequestrene or sulphate of iron.

Boron

Boron (B) is closely associated with the actions of calcium in building the structure of plants. Similar symptoms of shortage appear – internal browning of brussels sprouts, swedes, cauliflowers and apples. Shortage occurs on limy soils where boron is bound up by calcium. Apply a spray of borax diluted at 30 grams in 10 litres of water to 10 square metres.

Other minor nutrients

Other nutrients that are important for plant growth include manganese, copper, sulphur and molybdenum, but plant deficiency symptoms rarely appear because most Irish soils contain adequate amounts.

Plants on very peaty soil can suffer copper deficiency and show shoot tip dieback. Lawns on very acid soil can show pale colour due to sulphur deficiency, because sulphur is part of green chlorophyll. Sulphate of iron applied to a lawn for moss control also greens up the grass by adding sulphur.

The application of organic material, especially farmyard manure, supplies additional quantities of most of the minor nutrients required. Organic material also provides plants with other useful food. As it is broken down, a vast range of organic chemical compounds is formed – protein derivatives, amino acids, sugars, oils, enmes and growth-promoting hormonal substances.

Plants can make these compounds, starting from the basic plant nutrients, but they obviously benefit from an energy saving if they can take in these derivatives, part-formed.

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