Checking nitrogen status:
- Nitrogen has a strong influence on the rate of tuber bulking but the rate of tuber bulking can also influence the nitrogen requirement.
- An efficient crop, with a high rate of tuber bulking has a lower nitrogen requirement than an inefficient crop with a low rate of tuber bulking, knowledge of usual rates of bulking for a site / cultivar thus helps calculation of nitrogen fertiliser requirement. More …
- Crops require the optimum amount of nitrogen in soil before the onset of tuber bulking, in order to achieve a high rate of tuber bulking. More …
- However high concentrations in the ridge can reduce the number of tubers formed, as a result of a high salt content close to developing plant organs. See Principles of applications to soil. More …
- The optimum amount of nitrogen for the rate of tuber bulking is the same for long and short duration crops.
- Nitrogen availability may influence canopy duration without necessarily influencing the duration of tuber bulking. More …
- Residual soil nitrogen must be measured before fertiliser requirement can be determined. See Nitrogen Application.
- Applications of nitrogen should be made to soil as foliar applications can be damaging. More …
- Nitrogen applied as a top dressing will become available after rain or irrigation.
- A rate of nitrogen which is appropriate for tuber yield, if applied at the correct time, is also suitable for tuber quality.
- Quality problems are most likely to be associated with excessive or late applications of nitrogen. More …
- High rates of nitrogen application can reduce tuber dry matter concentration, however, tuber dry matter concentration is also related to tuber growth. See Tuber dry matter.
- High rates of nitrogen application can increase canopy size with little influence on tuber yield: this may result in delayed crop maturity.
- Nitrogen application can influence the rate of canopy desiccation: this may be related to canopy size or maturity at the time of desiccation. More …
- There is concern that high rates of nitrogen application may result in delayed skin set.
- Skin growth occurs throughout tuber growth but skin set begins when tubers stop growing, though the rate of skin set is suggested to be related to crop maturity. See Skin set.
- Research for the PRG has shown that rate of nitrogen application can have a greater influence on canopy growth than on senescence: nitrogen availability may therefore influence skin set only of crops which are harvested before maturity. More …
Rate of application
- Use the PRG nitrogen model to calculate nitrogen fertiliser requirement.
- The basic model is based on soils with 1 – 2 % organic matter and accounts for any mineralisation from this.
- The model can also account for mineralisation of nitrogen in organic soils but it is better to apply organic manures to a previous crop.
- Do not exceed any environmental nitrogen restrictions for the site.
- The model requires information on:
- residual nitrogen (nitrate and ammonium) at 0 – 30 and 30 – 60 cm depth.
- stone content.
- expected rate of tuber bulking (use the PRG mean of 6 t / ha / week if this cannot be predicted).
Measurement of soil nitrogen
- Take soil samples close to the time of planting but before soils begin to warm up, e.g. February.
- Sample soil at 0 – 30 and 30 – 60 cm depth.
- Keep samples cool until they reach the laboratory and ensure that the laboratory will keep samples cool until analysis.
- Warming the soil can result in mineralisation of organic nitrogen and unreliable results.
- Have samples analysed for
- nitrate nitrogen
- ammonium nitrogen
- stone content
- organic carbon (on high organic matter soils)
Time of application
- Potato roots “sense” the concentration of nitrogen in soil and this influences the rate of plant growth, most importantly the rate of tuber bulking. For maximum rate of tuber bulking the nitrogen requirement of a short duration crop is therefore the same as that for a long duration crop. More …
- The total amount of nitrogen required must therefore be in the root zone before the onset of tuber bulking.
- It is most efficient to apply all the nitrogen at planting.
- If there are concerns about a high salt content in the ridge or leaching on light soils a top dressing could be made after planting but sufficient moisture will be required to move the nitrogen into the root zone before the onset of tuber bulking.
- Nitrogen applied after the onset of tuber bulking will not influence the rate of bulking but can result in:
- increased canopy growth
- delayed senescence
- delayed skin set
- low tuber dry matter concentration (relevant for processing crops)
Leaching of nitrogen
- The following table indicates how far down the soil profile nitrate will be moved by 10 mm rain or irrigation if the soil is already at field capacity.
- Nitrogen fertiliser added as a top dressing will need to be moved down into to root zone to be available for uptake.
- Nitrogen leached below 60 cm depth in soil is unlikely to be available for uptake by potato roots.
- The earlier in the season leaching occurs, the greater the potential detriment. More …
- If there is likely to be a benefit from adding further nitrogen – and it will not exceed the permitted amount in any nitrogen restriction zones – sufficient moisture will be required to move the nitrogen into the root zone.
Checking nitrogen status
Nitrogen concentration in foliage
- The concentration of nitrate nitrogen in the petiole of the 4th leaf provides a good indicator of availability of nitrogen for growth. Guidance on sampling and identification of the 4th petiole is available here.
- Early season measurements can be used to check the success of fertiliser applications.
- If nitrate is deficient according to the data below, there may be a potential to nitrogen to soil, depending on the stage of crop development (see Time of application).
- Once tubers are larger than 30 – 50 mm, there is little relationship between petiole nitrate and tuber yield and it is too late to make corrective applications.
Nutrition 2021 (Includes results of PRG nitrogen experiments in 2020)
Nutrition 2020 (Includes introduction to / review of nitrogen)
Cover crops in the potato rotation 2020 (Includes influence of nitrogen availability)
Nitrogen prediction models 2016 (PRG N model reviewed)
Measuring soil nutrients 2016 (Summary of sampling requirements)
“Nutrisphere” and “Avail” 2016” (Products suggested to aid availability of N and of P: no effect shown on N)
Nitrogen supply from organic matter 2015 (An addition to the PRG N model)
Nitrogen 2013 (Fate of N prills applied to ridges; tuber dry matter is influenced more by tuber size than by N application)
Potato nutrient model 2011 (Development of an interactive model)
Towards a new PRG nitrogen fertiliser model 2010 (Relationship between rate of bulking and N requirement)
Nutrient interactions 2010 (High concentrations of one nutrient may reduce availability of others)
Nutrition experiments 2010 (N applied to high organic matter soil did not influence yield)
Nitrogen nutrition 2009 (PRG experiment results: optimum N is higher for a stressed than an unstressed crop)
Nitrogen application 2008 (PRG experiment results from a low N site)
Early nitrogen nutrition 2008 (Roots “sense” nitrogen in soil and this influences plant growth; early availability of N is important)
Nutrient combinations 2008 (N, P and K combinations: deficiency of one nutrient may reduce response to another)
Nitrogen application 2007 (PRG experiment results; PRG N model tested with a high bulking rate crop)
Nitrogen application 2006 (PRG experiment results; testing the PRG N model)
Nitrogen nutrition 2005 (PRG experiment results; further analysis indicates relationship between rate of bulking and nitrogen requirement)
Nitrogen requirement 2004 (Analysis of all PRG rate experiments to date: N requirement declines with increasing rate of tuber bulking, i.e. fast bulking crops are more efficient)
PRG nitrogen experiment 2004 analysis (PRG N rate research)
Background to RB209 2004 (Limitations to this method)
Nitrogen research 2003 (PRG N rate research; influence of cultivation on mineralisation)
Nitrogen 2002 (Continuing research on rate of application: PRG and published data)
Nitrogen nutrition 2001 (Influence of nitrogen on tuber dry matter concentration and canopy desiccation)
Nitrogen experiment results 2001 (Uptake to soil with high nitrogen content; uptake of nitrogen)
Nitrogen 1998 (Nitrogen supply, uptake and leaching; use of petiole N)
Tuber dry matter 1998 (PRG research: late application of N can reduce tuber dry matter concentration)
Nitrogen and tuber formation 1996 (Excess nitrogen may reduce tuber formation)
Nitrogen 1994 (The start of PRG research on rate of application, confirms influence of N on rate of bulking)
Foliar application of nutrients 1994 (Foliar application of nitrogen requires careful use)
Notes for 1993 (Chapter 7: N influences rate more than duration of bulking)
Seminar 3 Notes 1992 (Chapter 1: soil sampling method. Chapter 5: time and rate of application. Chapter 6: foliar application)
Download the PRG nitrogen model here:
Estimation of nitrogen in organic manures: external website