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:
- pH - phosphorus becomes less available when the soil pH increases above 7.5 and when it decreases below 6.5. This is as a result of it binding with calcium and aluminium respectively.
- Compaction - compacted soil makes it difficult for roots to spread quickly in order to obtain phosphorus from new locations.
- Aeration - poorly aerated soil (from compaction and/or poor drainage) reduces oxygen flow to plant roots and this can reduce phosphorus uptake by as much as 50%.
- Moisture - lack of water reduces phosphorus availability and uptake. Too much moisture can reduce the amount of oxygen getting to roots however and this can reduce uptake.
- Soil temperature - cold soil reduces the uptake of phosphorus.
- Cation exchange capacity - the more clay and organic matter in the soil, the more phosphorus it holds. Soils with high cation exchange capacities also tend to hold more water which facilitates the movement of phosphorus to plant roots.
- Organic matter - organic matter supplies phosphorus and can increase the amount of phosphorus that is available to plants.
- Mycorrhizas - fungi can form symbiotic relationships with plants and these are called mycorrhizas. Mycorrhizas can increase the amount of phosphorus uptaken by plants.
Ideally, for healthy and productive soil you should aim for a phosphorus concentration of 20‑100 mg/kg.
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:
- Leaf tips appear burnt
- Sometimes the rest of the leaves will become spotted with necrotic tissue
- Older leaves will turn a darker blue‑green or a reddish‑purple; in some cases they may go yellow first or the leaves may go yellow and drop off
- Plants will be dwarfed or stunted
Some symptoms that are only displayed by certain species include:
- The stems and underside of leaves of some plants such as tomatoes (and other fruiting nightshades), lettuce, corn and brassicas will often turn a purple colour
- In corn, the purple colour usually begins along the margins of the leaves
- Strawberry leaves will turn red or red‑brown, usually from the outside in
- Legumes will often show symptoms of nitrogen deficiency because a lack of phosphorus affects their ability use nitrogen fixing bacteria to produce nitrogen
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
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.