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Phosphorus is a primary macronutrient (it's the P in NPK). It is required for plant respiration and photosynthesis as well as cell division and growth (and thus plant growth). It is also vital for the production of fruit and seeds and is an important part of proteins, enzymes and DNA. When plants can uptake sufficient quantities of phosphorus, their roots grow early on and overall plant growth is better in cold temperatures. More flowers and seeds are also produced and fruit and grains mature more quickly. Furthermore, these products tend to be of a higher quality. Adequate phosphorus also ensures that plants use water efficiently. In legumes, phosphorus is important for nodule development and function (which is important for nitrogen fixation).

A wide variety of factors can influence the availability of phosphorus including:

Ideally, for healthy and productive soil you should aim for a phosphorus concentration of 20‑100 mg/kg.

Phosphorus Deficiency

Plants are most likely to suffer from phosphorus deficiency when they are grown in compacted, poorly aerated soil that is low in organic matter. Symptoms are also more prevalent early in the season when the soil is cold.

Symptoms of Phosphorus Deficiency

Phosphorus is a fairly mobile element in plants so deficiency symptoms are more often seen on older leaves (because the phosphorus from old leaves is more easily redirected to new leaves as they're produced). The symptoms of phosphorus deficiency vary widely between species but most plants will show the following symptoms:

Some symptoms that are only displayed by certain species include:

Treating Phosphorus Deficiency

When plants display symptoms phosphorus deficiency, your first steps should be to neutralise the soil pH, loosen and aerate the soil (with a fork) and reduce the amount of digging (to prevent disruptions to mycorrhizas). Aim for a pH if between 6.5 and 7.5 but as long as your soil's pH is between 6 and 8, most plants should be able to uptake sufficient phosphorus provided the soil contains enough. Excess phosphorus may be needed when plants require quite acidic soils however (azaleas and blueberries require acidic soil for instance).

If deficiencies are experienced early in the season, cloches can be used to warm the soil. If you have a large property, you may also consider installing an automatic watering system in order to maintain optimal soil moisture levels.

Regardless of the size of your property you should also add lots of organic matter to your soil. A good, thick application of organic mulch will be useful but initially you may need to dig some organic matter into the soil as well (make that the last time you dig your soil ‑ except when planting or transplanting or when harvesting tubers). In order to prevent future deficiencies, you'll need to continue to add organic matter periodically. Home grown green manures are a good source of organic matter and if you have the space you cal so grow your own straw.

If this doesn't fix the problem and a soil test confirms that a low level of phosphorus in the soil is the culprit, high phosphorus fertiliser can be used to boost levels. Organic fertilisers that contain a decent amount of phosphorus include bone meal and animal manures; poultry manure in particular. Rock dust and rock phosphate are also good sources of phosphorus (rock dust contains a variety of other minerals as well). There are a variety of high phosphorus inorganic fertilisers including superphosphate. Be aware that many of these contain however and this toxic heavy metal can be difficult to remove from the soil once it's been added.

Phosphorus Toxicity

Excess soil phosphorous is concerning because it can easily flow into waterways when it rains or if excessive irrigation water runs off a property (excess phosphorus causes algal blooms and excessive vegetative growth when it enters waterways). Excessive levels of soil phosphorus are not particularly detrimental to plant health however, though they can impede the uptake of iron and zinc. As a result, symptoms of phosphorus toxicity when they do occur are actually symptoms of iron and/or zinc deficiency.

It is difficult to remove phosphorus from the soil but excess levels can be remediated by using low phosphorus (or phosphorus free) fertilisers until the levels drop (as a result of crop harvest and leaching). It is also a good idea to cease using phosphorus accumulating plants as mulch.

If your garden is watered with grey water, ensure that all detergents used around the house are low in phosphorus.

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