- Phosphorus fixation in soils
- The role of organic matter (OM) with phosphorus
- Soil phosphorus analysis
- Critical concentration
- Optimising P fertiliser applications
- Model inputs
- Monitoring soil phosphorus
Phosphorus for potatoes
Phosphorus for yield
Foliar application of phosphorus
Phosphorus dynamics in soil
Phosphorus is present in soil in four ‘pools’, each of which is differently available to plants and the relative sizes of which depend on soil characteristics. Figure 1 shows these four pools and indicates the approximate relative speed of movement between them. When a soil has had no perturbations for some time, P is distributed in these pools at dynamic equilibrium.
If P fertiliser is applied, it will initially be present in the solution P (or ‘organic P’, if applied as an organic amendment) pool, and the total amount applied will be gradually distributed to the others to reflect the equilibrium proportions (though absolute concentrations will change, the proportions will ultimately end up the same). The speed at which P moves into and out of soil pools depends on the individual soil, as indicated in Figure 1.
Figure 1. Distribution of phosphorus in soil (from Phosphorus application theory 2019).
Since phosphate is rapidly fixed to soil, it becomes associated with the soil particles to which it was initially applied. The implication of this is that if the soil moves for some reason, be that erosion transferring it horizontally (to other fields or water courses) or ploughing transferring it vertically, the P will move accordingly. However if the soil is undisturbed, the P will mostly remain in the location to which it was applied. Therefore if P fertiliser is consistently broadcast without any incorporation there will be a high concentration on the surface of the soil. This is illustrated in Figure 2 and becomes particularly relevant when sampling for P concentration in minimum-till or no-till soils.
Figure 2. Illustration of likely ‘high concentration’ band of P fertiliser when applied to surface of soil and not incorporated. (From Phosphorus application theory 2019)
Phosphorus fixation in soils
- Depending on soil pH, phosphate can exist as the anions HPO42- or H2PO4–. Since these are both negatively charged molecules they interact with positively charged soil components (and are not relevant in consideration of cation exchange capacity, CEC).
- In acidic soils (sandy or organic), phosphates may become bound to Al/Fe (aluminium/iron) oxides by three mechanisms:
- Adsorption to surface of metal oxides – initial reaction;
- Forming complexes with metal cations and organic matter;
- Occlusion within Fe oxides (takes longer time).
- In soils with higher pH, interaction with calcium is the important factor in fixation. Initially, the complexes which form are less stable, more amorphous and more soluble (therefore more plant available), which transform through time (rate depending on other soil components) to more stable, less soluble forms. These tend to be not very plant available.
- An important distinction to note is that phosphates in calcareous soils (i.e. ones naturally high in calcium carbonate) are generally less available than those in limed, previously acidic soils (the most-stable Ca complexes may not yet be the dominant form).
- Calcium phosphates form in soil as a result of calcium (particularly present in chalky soils) binding to phosphate molecules, which may have been added recently as fertiliser. The results of this are:
- There is often lots of phosphorus (P) in calcareous soils, but low P plant availability.
- Low response to P fertilisers in calcareous soils is common (as fertiliser becomes fixed).
- Residual P in soil can be important for plant supply (depending on the type of calcium phosphate formed and other soil components).
The role of organic matter (OM) with phosphorus
- Organic matter can be a direct source of P for plants; however its presence can also improve plant-availability of P in soils where fixation to metal oxides or calcium may otherwise reduce its availability.
- Organic matter (e.g. manures, composts, digestates) applied as fertiliser contains P in organically bound forms, the precise amounts and composition of which depends on the material in question. This P is gradually released as the OM breaks down, therefore OM can be considered a source of slow-release P (quantity and speed or release being material-dependent).
- There are also other, more indirect ways in which higher OM content can improve plant use of P from soil, including:
- More OM promotes better water retention in soil and thus improved availability to plant via diffusion through soil;
- More OM tends to promote healthier microbial communities in soil, which in turn break down OM faster and therefore release P into the plant available pool;
- High OM content may be associated with better mycorrhizal systems, which can promote availability of P to plant.
- On acid soils, phosphates and OM components both bind to Al/Fe oxides, but OM binds faster / more strongly than phosphates, therefore higher OM contents in soil can effectively ‘block’ the binding sites to phosphates and thus leave a greater concentration of phosphates in solution and available to plant.
- On calcareous soils, OM disrupts the formation of crystalline calcium phosphates and instead favours the formation of amorphous forms of the complex, which tend to remain more soluble for longer.
- The three-way interaction between calcium, phosphate and organic matter in soil has implications for:
- Application of P fertilisers (amounts and timing);
- Use of organic matter (to potentially improve use efficiency of P fertiliser);
- Use of lime (consider in relation to P fertiliser applications).
Soil phosphorus analysis
- There are several ways that ‘available’ P can be measured in soil, and different countries use different standards. In Scotland this is Resin P; but in England, Wales and Northern Ireland, Olsen P is used.
- Different extraction techniques measure different portions of each of the pools described in Figure 1.1, and may be slightly better suited to different soil types as a result, in terms of reflecting actual availability to plant.
- Whichever method is used, this should be kept constant when comparing results (e.g. between years for the same field or between sites in the same year); although correlations between results from different extractions can be found, they are always generalisations and exceptions may be hard to identify in advance.
- There can be a significant difference between results of Olsen P when measured gravimetrically (mg/kg) compared to volumetrically (mg/L).
- Notice that mg/L and mg/kg can both be (correctly) represented as ‘ppm’.
- Lab results should always express either mg/L, i.e. the mass in milligrams of P per litre of soil; or mg/kg, i.e. the mass in milligrams of P per kilogram of soil. If they don’t: ask!
- It is likely that the volume taken up by a given mass of soil will change between being in situ in the field and by the time it arrives at the lab, and certainly by the time stones have been removed and the sample is sieved and ground ready for analysis. Therefore volumetric measurements can be used for comparison with other volumetric measurements, e.g. to see whether a treatment has had an effect, but volumetric concentrations should be used with in situ measurements of soil bulk density if a gravimetric concentration is required. This may be the case if an estimate of absolute amount of nutrient present is required.
- Olsen extraction gives an indication of the ability of a soil to provide P (at a given time), rather than a measure of the total amount present.
- It measures the intensity / quantity immediately available, not the buffer capacity.
- Buffer capacity may depend on how ‘full’ binding sites are.
Use this link for definition of critical concentration.
- Once an extraction method has been defined, it can be used in conjunction with field experiments to determine a critical concentration of nutrient for a given crop.
- Work undertaken in the UK suggested values of 10 – 51 mg kg−1 (dependent on soil texture and P content) as a critical concentration of phosphorus in soil for potatoes. Worldwide literature provided a similarly broad range of values.
- There has been a recent set of (UK-based) experiments and reviews published, which have shown that critical P values (for various crops), where they can be calculated, are often outside Index 2; some higher, often lower.
- Good yields often obtained from Index 1 soils with fresh application of P fertiliser.
- Variability in yield depends weakly on measured soil P concentration but strongly on other factors.
- The concept of ‘running to Index 1’ (rather than aiming for index 2 as recommended in RB209) is likely to be more/less appropriate in particular situations; consider:
- Potatoes – may be a benefit to having generally slightly higher concentration throughout soil profile, due to root/uptake factors.
- High pH, high chalk soils – if higher concentration is maintained there is a greater risk of longer term loss due to slow fixation. In this case, careful positioning and locally high concentration is more practical.
- Sandy soils cannot retain P so there is no point trying to raise overall concentration. Instead, targeted applications are likely to be more efficient.
- ‘Little and often’ is likely to be a good approach for P application in many cases.
Optimising P fertiliser applications
- The lack of mobility of P in soil, coupled with potato crop’s high demand for P yet relatively shallow rooting depth and inefficient nutrient and water uptake systems means that P fertiliser applications should be carefully considered to optimise uptake.
- In terms of timing, application of pre-plant P is beneficial because it allows the fertiliser to be positioned in the right place for the growing crop. If application is necessary while the crop is growing, the difficulty is enabling P to reach the roots rather than becoming fixed in the surface of the soil. Care should be taken to incorporate the fertiliser as close to roots as possible, while not damaging the roots. For this reason it is best to apply when the crop is young and roots are small (but take care not to raise salt content of soil too close to growing roots).
- By applying P fertiliser in the months leading up to the crop needing it (e.g. the autumn before), there is very little risk of leaching (though erosion by wind or water should be avoided), and the timescale is too short for significant ‘lock up’, e.g. with calcium, to occur.
- Over longer term (years, decades), sorption and precipitation do occur, and are likely to be worse if large amounts of fertiliser are applied in one application. Therefore ‘little and often’ is a better approach to P fertiliser rather than applying a whole rotation’s worth in one go.
- Banding can be very beneficial in terms of enabling potato roots to access as much P fertiliser as possible (compared to broadcasting), however, it must be put in the correct position in the first place and not disturbed by future soil manipulations.
- Field experiments have shown shallow placement (3 – 6 cm above seed) at planting; or deep placement (7 – 9 cm below seed) at row formation to be effective.
- Banding may be particularly useful on calcareous soils because it creates a band of particularly high concentration P which may therefore be able to overwhelm the effects of precipitation with carbonate.
Phosphorus interaction with water
- Phosphorus uptake efficiency and use efficiency are both likely to be better if the soil is adequately irrigated.
- However, if amounts of available P in the soil are low, poor soil moisture content is less likely to have a negative effect than if sufficient P is also supplied (crop is already stressed/not growing well?).
- Also, if P provision is low, this can have a detrimental effect on water use efficiency.
- It is important to provide both sufficient water and sufficient nutrient.
- In a set of Canadian field trials, phosphorus harvest index and uptake efficiency were more affected by site and year than by P application.
- Water, soil type, temperature may dominate P use efficiency;
- If P nutrition is not limiting factor, then changing P inputs will not affect outcome.
For further information see Phosphorus update 2018.
Phosphorus interaction with zinc (Zn) and manganese (Mn)
- Nutrient interactions are complex and interpreting symptoms can be tricky. Specifically:
- Apparent deficiency of one nutrient may in fact be due to an excess of another;
- Correcting the limiting deficiency may mean that a larger uptake of other nutrients is required to prevent ‘dilution’ due to greater biomass;
- There’s no benefit of applying excess of one nutrient if other nutrients are limiting growth – problems may be exacerbated.
- Excess P fertilisation has been linked to Zn deficiency in crops including potato. This may be due to the attraction between cationic Zn2+ and anionic phosphate (HPO42- or H2PO4–) making both nutrients less available to plants. However, it is unclear whether the issue occurs in soil or in plant (or both).
- Hydroponic studies have shown that micronutrient (Mn and Zn) deficiency can be at least as detrimental as macronutrient deficiency (P); the ‘micro’ and ‘macro’ refer only to the relative amounts of each which are required, rather than their relative importance.
- Remember that sufficient supply of P to produce larger, more healthy plants, will cause a greater requirement of Zn and Mn.
- Apparent Zn deficiency may at least in part be due to P toxicity, depending on their relative availabilities. Also Zn tissue concentrations (also in hydroponic studies) were more robust against influence of P supply, when Zn supply was adequate as opposed to deficient.
- Sufficient provision of Mn may regulate tissue P concentration and therefore prevent P toxicity. However excess provision of Mn may mask effects of P deficiency.
For further information see Phosphorus update 2018.
PRG Phosphorus Guidance Model
In designing the phosphorus model for potatoes there were two possible structures, namely:
- Critical concentration and buffer factor
- Requires knowledge of critical concentration for the given crop in the given soil.
- Buffer factor describes how much fertiliser to apply to effect a 1 mg L-1 increase in available (Olsen) P.
- Replacement and fertiliser efficiency
- Apply the amount anticipated to be removed by crop.
- Amend ‘replacement’ amount according to soil characteristics.
Despite thorough literature searches, there was little evidence to support a single critical concentration. See Critical concentration for more information. Also, although soil concentrations of organic matter and calcium are likely to be vital for determining the soil’s ‘buffer factor’ (the amount of P fertiliser (kg/ha) to effect a 1 mg/L change in Olsen P), at the time of writing there is insufficient evidence to create a quantitative model from a theoretical basis. Therefore the model as it stands advises users in terms of sampling, monitoring and calculating relevant values for their own soils.
The 2019 PRG phosphorus model suggests the amount of P fertiliser to apply to a soil in a given year to provide sufficient fertiliser for that year’s crops. Unlike the 2019 PRG potassium model, it does not recommend a target concentration of the nutrient in soil, but instead requires the user to decide that for themselves. The model uses information about anticipated offtake of P in the forthcoming potato crop, the amount by which the soil concentration is required to be increased (to reach the user’s target amount), and the calculated buffer factor of the individual soil.
For further information see Phosphorus application theory 2019
The model does not provide a target concentration of P in soil, however the ‘key steps’ tab in the model suggests considerations when choosing a target concentration. Also be aware of the information provided in this Growers’ Guide ‘phosphorus dynamics in soil’ section [link]. Sampling and analysis of the soil will provide useful information about factors such as organic matter content and calcium carbonate content.
Measured soil concentration
The current concentration of P in soil will be required to calculate the ‘build-up amount’ (if any). The following details are also important to follow as part of standard P monitoring and record keeping, which will be necessary to determine a buffer factor for the soil in questions.
- This is the actual concentration of ‘Olsen P’ measured by extraction with sodium hydrogen carbonate according to BS 3882: 2007 (the standard method used by NRM and Lancrop – and probably other labs – and specified in RB 209 at the time of writing). Quoted in mg L-1 soil (which should be the units obtained directly from the lab), it should be noted whether this refers to mg of P or P2O5.
- Requires sampling to be undertaken correctly and to be representative of the area in question. The value used in the model is that obtained by the user for that specific field/area.
- Sampling must be undertaken according to best practice (see also RB209), namely:
- At a time when soil nutrient status is steady, i.e. not immediately after an application of fertiliser or organic matter (leave at least 3 months between application and sampling);
- At the same time of year as previous samples (determined by calendar but with necessary modifications for unusual weather);
- Using a bulked sample with composites taken in a ‘w’ shape across the area in question;
- Ensuring that areas likely to have different P concentrations and/or with compositions likely to affect the availability of P are sampled and analysed separately;
- Sample 0 – 30 cm depth of soil, to best represent the soil volume reached by actively growing roots during the majority of the growing season.
- Samples should be taken / sent to the laboratory as soon as possible after sampling and if possible should not arrive at the laboratory on a Friday. This is so that samples are as fresh as possible when analysed.
- The anticipated tuber yield is required (t/ha) to calculate anticipated offtake. This should be based on previous years’ information and a reasonable prediction for the year in question.
- Tuber P concentration should be a mean value from previous years, using as many (representative) years’ data as possible. There are two versions of the model: one uses dry matter concentration and the other uses fresh weight concentration. It is important to ensure that the correct version of the model is used.
The model provides a space in which buffer factor for the soil can be calculated (by the model). This requires several years’ data on ‘before’ and ‘after’ Olsen P concentrations in soil; see model for more details. At the time of writing, it is not possible to provide a generic buffer factor for all soils, as there are likely to be many, complex influences involved and current published literature does not provide enough information for these to be robustly determined.
Monitoring soil phosphorus
- A vital aspect of phosphorus management is monitoring the soil. As with all nutrients in soil, monitoring the system in question and interpreting results alongside chemical theory is likely to be the best way to understand what has happened and predict what may occur in future. This is particularly true for P, where theory provides ideas about general relationships but published data are insufficient to determine quantitative, generalised relationships.
- In particular, growers making decisions about phosphorus fertilisation should be aware of the following scenarios:
- When P fertiliser input > P offtake, calculate buffer factors for your own soil: How much P (kg / ha) is required to increase soil Olsen P by 1 mg / kg? –> see ‘calculate buffer factor’ tab in 2019 PRG Phosphorus Guidance Model.
- If P fertiliser input = P offtake, is Olsen P approximately constant over several years, or does it change?
- If Olsen P is approximately constant then the system is in dynamic equilibrium (i.e. ‘balanced’).
- If Olsen P increases, this indicates that the binding sites on the soil are ‘full’ and input amounts could potentially be decreased.
- If P fertiliser input < offtake, does Olsen P change?
- If Olsen P doesn’t change, despite a negative balance, this indicates that the soil’s ‘less available’ reserves are being used. This is not necessarily a bad thing.
- If Olsen P decreases, P application may need to be increased to maintain it; this decision would be made in conjunction with plant observations e.g. health and petiole P concentrations.
- If Olsen P decreases (consistently over several monitoring points and to a greater degree than may be expected from analytical variation), this indicates that applied fertiliser is being fixed into less available forms. If this is ‘normal’ for this soil, consider careful P management (timing, placement, protection?).
- If Olsen P has been approximately constant for several years and suddenly starts decreasing (DESPITE no change in input / offtake balance), this indicates that the ‘less available’ reserves are being used up and management needs to change in order to replenish them.
ALWAYS CONSIDER PLANT HEALTH OBSERVATIONS AND TISSUE SAMPLE RESULTS TO ENSURE ADEQUATE FERTILISER IS BEING PROVIDED.
Notes on phosphorus chemistry
Phosphorus application theory 2019 (Principles of the guidance model)
Phosphorus update 2018 (Influences on phosphorus availability and interactions in soil)
Phosphate and phosphite for potatoes 2014 (Phosphorus interactions in soil; phosphate is a nutrient, phosphite aids disease suppression by increasing plant resistance)
PRG Phosphorus Guidance Model
Download the PRG Phosphorus Guidance Model (Excel spreadsheet) here: