Soil phosphorus
Physiological processes of crop and pasture plants are sensitive to concentrations of nutrients.
Native pastures are adapted to low soil phosphorus levels, but introduced crops and pastures are not. In soil, the plant-available concentration and/or rate and timing of supply from the soil can be inadequate to meet the demands of agricultural plants. Root growth after emergence and tiller development for example, are particularly sensitive to phosphorus concentration in the soil.
Historically, profitable production of grain and pasture crops in many regions of the south-western agricultural region relied on the application of phosphorus on new land, particularly in high rainfall (> 800 mm) areas and on highly weathered soils with very low levels of natural phosphorus. Several decades of continuous application, combined with positive phosphorus balance in most crop and pasture systems, has resulted in gradual accumulation of plant-available and total phosphorus in soil. Currently, inadequate levels of phosphorus are generally confined to soil with very high phosphorus buffering index (PBI) such as forest gravels, or deep pale sands where leaching depletes topsoil phosphorus.
Why is phosphorus important for soil and plants in their early stages?
Phosphorus is required for cell growth during early plant development, with demand increasing during plant establishment after the seed’s reserves are exhausted.
Phosphorus plays a key role in all metabolites dealing with energy acquisition, storage, and utilisation: sugar phosphates, adenosine phosphate (ATP, ADP, AMP), nucleotides, and nucleic acids. Phytic acid and its calcium and magnesium salts act as phosphorus storage compounds in plants and seeds.
Phosphorus often reacts rapidly with other elements such as calcium and iron when applied to soil, and can result in less than 5% of the total phosphorus applied being taken up by plants in the year of application. This adsorption effect increases with declining soil pH.
Phosphorus cycle
The capacity of soil to store and supply phosphorus to crops and pastures depends on its inherent properties. Phosphorus is stored in soil in inorganic and organic forms; and taken up by plants and microorganisms primarily from the soil solution as inorganic phosphate ions.
Clay content, clay mineralogy, presence of aluminium or iron-bearing minerals and the presence of gravel influence how much reactive surface area exists. These properties influence the amount of phosphorus that can be stored in soil and how readily they are released in plant-available form. The chemical nature of phosphorus forms along with their stability and binding energy, microbial activity, clay mineralogy, soil pH, intensity of land-use and management and inputs influence phosphorus equilibrium in soil.
Phosphorus is present as two different molecular ions dissolved in soil solution known as orthophosphates: dihydrogen phosphate (H2PO42-) and monohydrogen phosphate (HPO4–). This phosphorus is plant-available and has been made soluble through the action of soil microorganisms. There is usually a relatively small amount of phosphorus in the soil solution, about 0.05-0.30 micrograms of phosphorus per millilitre. The concentration of phosphorus in solution reflects a dynamic equilibrium between inputs and outputs. This equilibrium is influenced by a complex interplay of factors, including soil properties, microbial activity, and external inputs like fertiliser application.
Plant roots acquire phosphorus from the soil solution as inorganic phosphate (Pi), and at neutral pH the predominant form dihydrogen phosphate (H2PO4–) is transported into plant cells. The full pathways of phosphorus supply to plant roots and uptake by the plant are relatively complex and influenced by many interdependent factors. The accessibility of soil phosphorus for plants greatly depends on their specific phosphorus -acquisition strategies.
Phosphorus uptake principally occurs at the young root tip as they grow into the soil environment and are exposed to phosphorus. Root tips take in phosphorus and transport it into the cells of root hairs and the outer layer of the root cortex. A phosphorus depletion zone of 0.2–1.0 millimetre forms around the root whilst the root surface actively absorbs phosphorus, especially due to the presence of root hairs that increase its surface area. In the case of nonmycorrhizal plant species without any specific strategy, they only can access phosphorus that is close to the surface of roots or root hairs.
In contrast, arbuscular mycorrhizal and ectomycorrhizal plants have the capacity to access not only the inorganic phosphorus present in the soil solution surrounding the roots but also that beyond the immediate root depletion zone. Mycorrhizal hyphae from fungi that are associated with plant roots can also transfer phosphorus to roots. In addition, ectomycorrhizal plants can assess the organic phosphorus in solution and a portion of sorbed phosphorus is mobilized through the release of organic acids. The released carboxylates enhance phosphorus availability for uptake by displacing phosphorus that is adsorbed onto other compounds.
The organic phosphorus pool comprises soil microorganisms (e.g. bacteria, fungi, archaea, protozoa, nematodes, macrofauna) and plant residues and in the majority of soil makes up 20–80% of total soil phosphorus. These soil organic phosphorus forms are produced when plants and microbes take up orthophosphate, incorporating it into essential organic molecules for life functions like DNA, phospholipids, and ATP. These compounds are deposited in soil when organisms die. Since plants can only take up inorganic orthophosphate, the role of soil microorganisms in mobilising organic phosphorus forms into plant-available phosphorus is crucial. The source compounds of organic phosphorus include phospholipids, nucleic acids and phytin.
The transformation of organic phosphorus into inorganic phosphorus occurs through the action of soil microorganisms in a process called mineralisation, which is conducted by microbial phosphatase enzymes. Organic phosphorus can be categorised into four main fractions: labile, moderately labile, moderately resistant (fulvic-acid phosphorus), and highly resistant (humic-acid phosphorus) – and reflects the varying degrees of bioavailability and the potential for crop uptake. All those fractions can be transformed into plant available inorganic forms but at different rates based on their chemical complexity and susceptibility to microbial decomposition. Labile and moderately labile organic phosphorus fractions are easily mineralised for crop uptake, while moderately resistant fulvic-acid phosphorus can serve as a medium-term nutrient source. Highly resistant humic-acid phosphorus is considered to be stable.
The reverse of this process, where inorganic phosphorus is converted into organic phosphorus, is called immobilisation. Through immobilisation, microorganisms incorporate phosphorus into their cells. Mineralisation and immobilisation occur at the same time in soil and the net rate is influenced by the carbon to phosphorus ratio of the soil. Both these processes are affected by abiotic factors, such as soil moisture, aeration, temperature, pH and the presence of mycorrhizal associations. Indicators like microbial biomass and enzyme activity can be used to measure the rate of utilisation of organic phosphorus.
The sorbed phosphorus pool consists of phosphorus ions or phosphorus-containing molecules adsorbed on the surfaces of soil particles, including clays or minerals such as aluminium and iron oxy-hydroxides and calcium carbonates. The process of adsorption is relatively fast, and is influenced by both soil mineral and clay content. Soil rich in iron and aluminium oxides or clay have greater capacity to adsorb phosphorus onto these surfaces.
Over time, sorbed phosphorus is released slowly by desorption (the reverse of adsorption) into soluble phosphorus for plant uptake. The balance between adsorption and desorption is dynamically controlled by the interaction of phosphorus binding to and being released from the soil matrix. Consequently, during the plants growth period, as plants absorb dissolved phosphorus, phosphorus ions are increasingly desorbed from soil particles to establish equilibrium. Conversely, increasing the concentration of phosphorus in the soil solution through fertilisation increases phosphorus sorption.
The mineral phosphorus pool consists of phosphorus compounds in solid-phase. About 150 such phosphorus-containing minerals are known, including compounds such as calcium-phosphorus minerals and iron/aluminium-phosphorus minerals, which are formed by the precipitation of metal ions with phosphate ions. These compounds have a range of structures, and in turn surface areas, from unclearly defined to crystalline. Mineral phosphorus compounds can be formed by pedological processes in the soil, such as the formation of apatite crystals by the precipitation of phosphate ions reacting with calcium ions. Other mineral phosphorus compounds such as di-calcium phosphates are formed as a product of the reaction of phosphorus fertiliser like superphosphate being applied to the soil. The release of mineral phosphorus is very slow and is caused by weathering whereby minerals break down over time. When these compounds weather, the phosphate ions are released back into the soluble phosphorus pool – this is referred to as dissolution.
Chemical fertilisers supply an inorganic source of phosphorus to the soil system in a form which enters the soluble phosphorus pool and can therefore be taken up by plants. However, different fertilisers have different percentages of available phosphate – for example, superphosphate has 20% available phosphorus whereas diammonium phosphate (DAP) has 46%.
Phosphorus fertility varies widely across soil types and while starter phosphorus fertilisers are applied routinely at planting by many producers, the economic yield responses to applied phosphorus can be inconsistent and difficult to predict. Phosphorus reacts with clays and oxides very soon after application, so that it is fixed in the soil. In many instances, as little as 5% of the total phosphorus applied becomes available to crops in the year of application due to adsorption and precipitation – forming a range of insoluble compounds in the soil. Therefore soils with a high phosphorus sorption or retention index are likely to require fertiliser application to meet crop requirements, even where soil testing suggests sufficient levels of phosphorus.
Fluid forms of phosphorus (such as polyphosphates and DAP) have been shown to be more effective than granules in supplying crop needs in alkaline soils with free calcium carbonate, and choice of fertiliser should also take account of the actual cost per unit of phosphorus applied. Fertiliser placement is also an important consideration in optimising plant uptake as well as avoiding toxicity during early stages of growth.
Residual fertiliser phosphorus in soil declines over time and in highly calcareous and oxidic soils, phosphorus availability can be very low. Highly weathered soils such as those in Western Australia have low levels of Colwell P (< 10 mg/kg in sandy soils and < 15 mg/kg on other soils), and profitable production of crops and pastures has only been achieved by applying phosphorus fertilisers. However, in Victoria, New South Wales and Queensland increasing ‘available’ soil phosphorus (as measured by bicarbonate) has been observed in some Vertosols, despite increasing phosphorus removal in products. This is due to the mobilisation of insoluble phosphorus or redistribution of phosphorus from deeper in the soil profile. Soils with a long history of phosphorus fertilisation may build up luxury levels of this nutrient beyond crop or pasture requirements.
After harvest, crop residues within the paddock retain a significant amount of phosphorus. This crop residue phosphorus in a form that is not available to be taken up by plant roots. Soil microorganisms (e.g. bacteria, fungi, archaea, protozoa, nematodes, macrofauna) convert this phosphorus from decomposing crop material into an inorganic form that plant roots can access. Agronomically substantial amounts of phosphorus are potentially present in both crop residues and the microbial biomass associated with their decomposition. The key factors that influence the mineralisation and accessibility of phosphorus derived from crop residue include the quality of the residues, the activity of the soil microbial biomass, and the sorption reactions of mineralised phosphorus within soil.
Manure provides an organic source of phosphorus to the soil system. However, some phosphorus in manure will also be in an inorganic (and plant-available) form. The major difference between manure sources of phosphorus and chemical fertiliser sources is the varying ratios of organic to inorganic phosphorus.
Phosphorus is removed from the soil system by harvesting of crop biomass containing phosphorus or when plants are grazed by livestock. The amount of phosphorus removed in grain, hay or by grazing should be calculated by grain or plant tissue testing as the level of phosphorus removal varies with crop type, soil fertility and climate factors like rainfall. On average, around three to four kilograms of phosphorus are removed for every tonne of harvested grain, and this rate should be added through the application of manure or fertiliser inputs.
Soluble phosphorus is lost through leaching as water moves deeper down the soil profile. Losses of phosphorus from the soil system through leaching are often less severe than by surface runoff. The amount of phosphorus leached strongly depend on the concentration of phosphorus in soil solution and the kinds of phosphorus binding sites within the soil matrix. Leaching can increase if the soil approaches its maximum phosphorus holding capacity which can be caused by the overapplication of phosphorus fertiliser. Sandy soil is one of the most vulnerable soil types to phosphorus leaching due to its low phosphorus binding capacity caused by low clay content.
Runoff is one of the major loss pathways of phosphorus from the soil system. Soluble phosphorus dissolved in water is removed and soil particles containing phosphorus are also removed.
Annual phosphorus balance in wheat
Integrated nutrient management utilises a number of sources to meet crop demand including fertiliser, crop residues and soil nutrient supply. This example provided below illustrates the annual cycle (source and fate) of phosphorus for an average Australian wheat crop. The numbers accompanying the arrows indicate the quantity of nutrients (kg/ha) cycling through the wheat crop: crop yield is 2.1 t/ha; grain protein concentration 10.5%; phosphorus removal in grain 3.3 +/- 0.7 kg/t; potassium removal in grain 4.6 kg/t.
Phosphorus and arbuscular mycorrhizal fungi
Arbuscular mycorrhizal fungi are a well-known example of a symbiotic relationship in which fungi gain photosynthates from the plant and increase phosphorus uptake in the plant by extending the effective root system of the plant and enabling it to explore a greater volume of soil. The effectiveness of arbuscular mycorrhizal fungi can be reduced in soils with high phosphorus concentrations, and their presence of in some legume crops can result in yield losses.
What are the symptoms and problems caused by phosphorus deficiency?
Phosphorus deficiency can result in poor early growth with few small leaves and death of older leaves. Plants not getting enough phosphorus will often take longer to reach maturity, and it is typical to see differences in crop ripening and maturity across a paddock associated with variations in soil phosphorus.
Plants not getting enough phosphorus will often take longer to reach maturity, and it is typical to see differences in crop ripening and maturity across a paddock associated with variations in soil phosphorus.
Assessing phosphorus in the paddock
Plant testing
The concentration of phosphorus in plant tissue can be used as an indicator of yield regardless of the type, placement or timing of fertiliser application (the exception to this is in south-western Australia). Tissue testing is not generally used as a tool for in-season fertiliser application as economic responses to phosphorus application are generally confined to early in the growing season. However, they will determine whether there is a deficiency in supply.
Soil testing
Get a soil test done. The plants may not be responding because the phosphorus levels are already high enough. If the test shows the soil is low in phosphorus, apply fertiliser. Banding of phosphorus fertilisers below the seed when sowing can decrease the amount of fertiliser being used (sometimes up to half the fertiliser saving) compared with broadcasting. However, placement of fertiliser too close to the seed may result in fertiliser toxicity (particularly in canola and lupin). Also, broadcasting increases the risk of fertiliser being washed into waterways.
- Test the pH of your soil and, if it is acid, lime it. This will increase the availability of phosphorus.
- Monitor available phosphorus levels regularly, as plants can take up only a small proportion of what you apply
The response to phosphorus fertiliser must be defined for each soil type and environment due to the different capacities of soils to adsorb phosphorus, and can in fact vary from year to year because of seasonal conditions. Hence different regions have adopted different methods of soil testing, but the two most common methods are based on a bicarbonate soil extraction, i.e. Colwell P and Olsen P.
Phosphorus sorption influences the capacity of the soil to supply phosphorus – as soils that sorb more phosphorus require larger amounts of fertiliser to produce the same yield. Typically soils that sorb large amounts of phosphorus include those with abundant iron and aluminium oxides (e.g. Ferrosols), as well as some calcareous soils. Calcareous soils can also fix large amounts of phosphorus. These soils would therefore require more phosphorus fertiliser than sandy soils with the same soil phosphorus concentration. The phosphorus sorption index should be measured simultaneously to estimate phosphorus fixation, and also the potential leaching of phosphorus as some sandy soils in southwest Western Australia have such low sorption capacity that leaching of phosphorus into water ways is a problem.
PAGE REFERENCES AND ACKNOWLEDGEMENTS
Material on this page adapted from:
- Hoyle FC (2007). Soil Health Knowledge Bank.
- Scanlan C, Weaver D, Bell R, Borrett R and Cheng M (2023). Soil Quality: 10 Plant Nutrition. SoilsWest, Perth, Western Australia. [Access]
Last updated July 2024.
