Soil nutrients

Phaseolus vulgaris with nitrogen deficiency Credit: Rasbak, shared under CC BY-SA 3.0

At least 17 elements are essential for plant growth. Soil nutrients can limit plant growth if they are not available in sufficient quantity, ratio, or the correct form. Some soils are naturally low in certain elements.

Macronutrients

Three macronutrients (carbon, oxygen and hydrogen) are obtained by plants from air and water. The rest of the macronutrients are obtained from the soil. The ‘big three’ are nitrogen, phosphorus and potassium - often known as NPK (from their elemental symbols).

Nitrogen (N) makes up a large part of plant tissues and is essential for photosynthesis. It has to be provided from the soil ‘fixed’ (combined) with other elements as nitrate or ammonium.  Nitrogen deficiency results in stunted growth, slow growth, and yellowing (chlorosis).

Phosphorus (P) is a major structural component of DNA. Phosphorus is highly reactive and is often limited to plants. Many plants make symbiotic relationships with mycorrhizal fungi to increase phosphorus uptake. Phosphorus deficiency causes intense green colouration or reddening in leaves.

Potassium (K) is involved in forming carbohydrates and proteins. Potassium is particularly important for fruit formation. Potassium fertilisers are called potash and is mostly from mining; there are only small amounts in manure. Potassium deficiency may cause yellowing between leaf veins, stunted growth and dieback.

Sulphur, calcium and magnesium are classed as 'secondary macro-nutrients', and are all essential for photosynthesis in plants.

Micronutrients

Micronutrients, or trace elements, are essential for plant growth but in much smaller quantities. There are eight micronutrients that plants obtain from the soil: iron, boron, chlorine, manganese, zinc, copper, molybdenum and nickel

Feeding your soil

Try some organic methods. Soil nitrogen content can be improved by planting legumes. Incorporating compost into your soil will increase nitrogen, phosphorus and potassium. Compost also contains many essential micronutrients. Applying organic mulches can increase your soil fertility over time as they break down.

Soil pH

pH is a measure of how acidic or alkaline a soil is. The pH scale runs from 0 (acid) to 7 (neutral) to 14 (alkaline). Acidic soils have a pH below 7, and alkaline soils have a pH above 7. The optimum pH range for most plants is between 5.5 and 7.5., while most annual vegetable crops prefer soils with pH 6.5-7. Soil pH is determined by the underlying parent rock material, vegetation type and land management. Soils tend to become more acidic over time.

To make soils more alkaline (increase pH) lime is often added. You can also use wood ash from hardwood trees and eggshells. To make soils more acidic (lower pH) sulphur is added. Adding organic matter including compost and manure can also lower pH.

 

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This content is derived from work created within the GROW Observatory, which received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 690199.

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