- Model calculations
- Factors considered
Potassium chemistry background
Potassium for potato plant health
Internal enzymatic blackening of potatoes is more likely to occur in tubers which are K deficient.
Potassium dynamics in soil
Information about potassium fertilisation in Potassium application to potatoes 2014 (slide 50 onwards) has been superseded by the presentation Potassium application model – background 2019 and other notes in this Grower’s Guide.
Relevant information from Potassium application to potatoes 2014
- Potassium in soil is present at ‘dynamic equilibrium’ in four ‘pools’, as shown in Figure 1.
- Dynamic equilibrium means that although individual K+ ions move between pools, overall there is a steady concentration of K+ in each of those pools.
- Structural K is unavailable to plant and is very slowly released into fixed K by weathering of clays. This is not a reversible process.
- Fixed K is present within layers of clay. It can be (relatively slowly) released into the exchangeable pool; and K applied as fertiliser can (relatively slowly) move into the fixed pool. It is not immediately available to plant.
- Exchangeable K is attached to the surface of soil particles, on the negatively charged sites which make up the cation exchange capacity (CEC). It can rapidly attach/detach from these sites and therefore contributes to the pool of ‘available’ K as measured by the standard laboratory test.
- Solution K is the pool into which K applied as fertiliser initially moves, whether that is through dissolution of granules, release from organic matter, or directly when added as a liquid fertiliser. Potassium in this pool is immediately available for uptake by plant but is also subject to leaching if soil moisture content and chemistry promote it. This is the main pool which contributes to ‘available’ K as measured by the standard laboratory test.
Figure 1. From Moody & Bell (2006) in ‘Potassium for potatoes 2015 V3’
- The amount of K in the ‘solution’ and ‘exchangeable’ pools determines how much is available to the plant over the short term. However, once those pools are exhausted, plant relies on release from the ‘fixed’ and ‘structural’ pools; this process is relatively slow but can be very important over the course of a growing season since they have greater overall capacity (except in very sandy soils) to store K.
Potassium ‘critical concentration’
Information from Potassium for potatoes 2015
There tends to be a positive correlation between yield and available potassium in soil (up to the point where other factors become limiting). Therefore the suggestion of a ‘critical concentration’ is often made, i.e. the concentration (of nutrient – in this case potassium – as measured by a given method) at which no further response to applied K is seen.
- In practice, the response of a crop to applied K depends on the availability of K already present in the soil, which may be affected by various factors including:
- Soil moisture
- Soil texture
- Other nutrients, particularly cations, present in the soil
- Volume of soil explored by roots
- In particular, Figure 2 shows how the response to applied K fertiliser depends on soil texture. The y axis shows the maximum response recorded in that experiment (experiments collated from published literature) as % increase in yield for each kg K / ha applied. Soils are arranged on the x axis from most clay rich to most sandy.
- Greater response to applied K fertiliser tends to occur in sandy soils, with least response in clay-rich soils. There are two reasons for this:
- Supply from soil: in clay-rich soils, K is released (albeit relatively slowly) from the structural and fixed pools, to become available to the plant through the season. Therefore the relative impact of applied fertiliser is smaller. In sandy soils, this store is not present (low CEC), so the plant relies more heavily on applied fertiliser.
- Fixation of applied fertiliser. When K fertiliser is applied to clay soils, there is the potential for it to become part of the ‘fixed’ pool (see Figure 1, ‘The fate of cations in soil after application as fertiliser’ in Application of cationic fertilisers and Potassium dynamics in soil). Although this means it will be available in the longer term as part of the ‘supply from soil’, its availability to that season’s crop is reduced; in sandy soils this effect does not occur so closer to the whole amount is available to that year’s crop (except any that is leached).
Figure 2. Response of crop to applied K fertiliser depends on soil texture. Data compiled from various literature sources.
- Based on compiled literature to date (2015), a very tentative critical K was proposed: 150 mg/kg as K = 180 mg/kg as K2O, measured by the standard laboratory method (extraction with ammonium nitrate).
- However, critical K concentration depends on a variety of additional factors and therefore further research and analysis were undertaken in 2018/19 to produce the PRG potassium model for potato growers, which tries to account for these; further details about the development of this model are available here Potassium application model – background 2019.
- Slide 33 onwards (‘How much fertiliser to add to raise soil K by 1 ppm?’), of Potassium for potatoes 2015 has been superseded by work undertaken during the creation of the 2019 PRG potassium model for potato growers.
2019 PRG potassium model for potato growers
Information from Potassium application model – background 2019
- It is important to note the scope of application of this model: it is designed to allow farmers to estimate how much K to apply in the given year, immediately before a potato crop, to provide sufficient K for that crop. It does not take into account the rest of the rotation.
- The model accounts for specific soil and plant factors, based on available literature evidence; it is NOT supposed to thoroughly model the whole soil-plant system as this would be too complex and would introduce too much uncertainty into predictions.
- Furthermore, the model is based on specific assumptions and requires particular conditions to be met, namely:
- Good, healthy plant growth, resulting in reasonable root exploration of the soil.
- Adequate soil moisture to enable good availability of K to plant.
- Soil samples are taken during February and analysed immediately for available K – see notes below in Inputs section.
- Apply the amount of K fertiliser as directed by the model, at a suitable time in spring, before planting that year’s potato seeds.
- It is vital that sampling and analysis follow best practice methods, to ensure input values are as reliable as possible.
This section of the Grower’s Guide is not intended to fully describe the model (that information is available in Potassium application model – background 2019), but is written to aid understanding of how it should be used and the inputs / outputs relevant to the users.
Each of these values are required to be known by the user before the model can be used to estimate the amount of K fertiliser to apply.
Measured soil concentration
- This is the actual concentration of ‘available K’ measured by extraction with ammonium nitrate 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 K L-1 soil (which should be the units obtained directly from the lab).
- 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);
- At the same point in the rotation and NOT during or immediately after a crop of OSR (this is specific to K sampling);
- Using a bulked sample with composites taken in a ‘w’ shape across the area in question;
- Ensuring that areas likely to have different K concentrations and/or with compositions likely to affect the availability of K 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.
Target soil concentration
This is the target concentration for ‘available’ K which would (theoretically) be measured if a sample was taken as plants started growing. For the model, a conservative value of 200 mg L-1 has been chosen, in light of literature research and understanding to date. Common to all soils and users, defined in the model: not input by user.
- This is a measure of the soil’s ability to buffer the concentration of ‘available’ K, against applications of fertiliser; in other words, it describes how much fertiliser must be applied in order to effect a 1 mg L-1 increase in available K.
- Its inclusion in the model effectively accounts for real-world ‘loss’ to fixation and leaching (therefore these factors are not explicitly included elsewhere in the model) so is soil dependent. In the model, the user selects the most appropriate soil type, which automatically applies the correct uplift factor.
- Soil type MUST be determined according to Figure 3, which is also shown in the model itself. The definitions may result in a slightly different category to that in which the soil would usually be placed, however for the purposes of this model, it is important to use the correct selection criteria.
Figure 3. Selecting the correct soil type to determine uplift factor.
- In order to use the model, users must select the most appropriate tab in the excel file, based on the texture of the soil in question. This is used to estimate the likely release of K from the soil through the growing season. Due to constraints of literature data available, soils must be defined as clay, sand or loam for the purposes of this model.
- Figure 4 (details also included in the model) should be used in conjunction with laboratory particle-size distribution analysis (of the same sample as used for ‘measured soil concentration’) to select the correct soil texture for this application; as for soil ‘type’, this may be a slightly different category than usually used to describe the soil in question, but it is vital to use the values defined as shown, for the model to work properly. For further guidance on particle-size distribution analysis and on using soil texture triangles, see Using a texture triangle.
Figure 4. Selecting soil texture for the purposes of the model.
After inputs are selected/entered by the user, the model first calculates the ‘provisional application amount’, then applies modifications to that in light of likely release of stored K from the soil during the potato growing season.
The ‘Application amount’ in red text at the bottom of the calculations page for each soil texture (clay, loam, sand) is the recommended amount of fertiliser to apply that year for that potato crop on that soil. The amount is rounded to the nearest 1 decimal place and the model is set to never return a value less than zero kg ha-1. Two numbers are provided: one in kg K ha-1 and one in kg K2O ha-1. This enables users to most easily calculate the amount of fertiliser material that should be applied, based on the nutrient content of the specific fertiliser they are using.
- Various factors were considered during the creation of the 2019 potassium model for potato growers, but only those which could be quantitatively supported with data have been included. This improves the robustness of the model but does mean that there are some factors likely to change the efficiency of fertiliser use that cannot be accounted for at the present time.
- Factors considered but not accounted for in the model (either due to lack of supporting evidence for any link with potassium in soil; or due to complexity and lack of quantitative data) are:
- Cation competition
- Duration of growth
- Tuber dry matter
- Timing of application compared to uptake
- Please note that variation in response to applied K fertiliser is likely to be greater in soils with lower K content due to factors including:
- Soil moisture content affecting movement of K within soil profile;
- Root exploration and ability to reach areas of sufficient K concentration in the soil.
- Since the model accounts for a single crop in a single year, it can be equally applied to owned or rented land. In the case of owned land, the potassium status of the soil should be monitored at least once in every rotation and potentially more frequently, to ensure that soil K stores are not depleted. This is more important on clay-rich soils (as opposed to sandy soils) as these are the soils where the ‘fixed’ K pool plays a pivotal role in provision of nutrition to plant. For further discussion of this concept, see [link to be added].
Potassium application model – background 2019 (Principles of the model)
Potassium for potatoes 2015 (Availability of potassium in soils of different types)
Potassium application to potatoes 2014 (Assessing availability of potassium and potato requirements; application model now outdated)
Download the PRG Potassium Model (Excel spreadsheet) here:
PRG Potassium Model