P and K management for high-yielding crops


Recently, I drew up a number of medium term nutrient strategies for clients. This inspired me to put down some of my thoughts around this topic in writing.

Across the SOYL client base growers who maintain a robust and thorough approach to phosphate (P) and potassium (K) applications are those who consistently achieve high yields. I sometimes hear some advice from elsewhere that devalues the contribution of P and K, partly I expect because you don't get the visible impact that can be seen from nitrogen.

My approach to robust P and K planning is based on three key questions:

  1. What is the physical and chemical status of the soil through soil type and nutrient data?
  2. What are the nutrient needs of my crops across the rotation?
  3. What sources of nutrient are available locally?

This blog will focus on question two.

What are the nutrient needs of my crops across the rotation?

In order to design an appropriate nutrient strategy it is essential to look at the P and K needs of each crop. We can then match different products to suit the total needs across the rotation.

Yield, straw removal and crop choice will all affect crop nutrient offtake and peak uptake.

Ten tonnes of wheat will remove 78kg of phosphate and 56kg of potash from the land. If straw is removed as well, these levels rise to 84kg and 104kg respectively. Producing a balance sheet of removals and inputs in fertilisers and organic manures throughout the rotation will provide a useful tool when calculating the following year's nutrition needs.

NEW free rotational PK Calculator

To help growers and advisors understand the nutrient needs of a rotation we have developed a freely available online tool that allows growers and advisors to calculate the P and K requirements of a rotation of up to 7 years in length. You can find this free tool here - https://www.soyl.com/services/calculators-intro/pkoc.

The general target indices of mid index 2 - I prefer to work using a PPM (Parts Per Million) measurement of nutrients in the soil or a decimal index as it is more accurate but let's keep things simple for this article - for both phosphate and potash should provide adequate nutrients to support the following crop, but these can be eroded if removals are not replaced.

Of the fields sampled by SOYL last year, 24% had areas that were deficient in phosphorous and 26% deficient in potassium. This presents an unnecessary risk to crop health and subsequent yield, but it is simple to address.  

A simple rotation example

 Table 1 shows how this might be calculated for a typical winter wheat/spring barley/winter OSR rotation, taking into account existing nutrient decisions:

·DAP applied at 165kg/ha to give 30kg/ha N and 76kg/ha of the all important fresh phosphate to encourage rooting in front of the OSR
·FYM at 15t/ha that provides 29kg/ha P and 108kg/ha K for the winter wheat
·CF Fertilisers' KayNitro Sulphur at 320kg/ha giving 80kg/ha N for the first dose plus 42kg/ha of fresh K ready for the spring barley's peak growth period.

Table 1:

​Yield t/ha ​P removal ​K removal​Non bagged fert applied​Bagged fert applied​VR applied
​Winter wheat ​10 ​78kg/ha ​56kg/ha​29kg/ha P
108kg/ha K
(from FYM)
​VR P and K applied using TSP and MOP
​Spring barley​8​62.4kg/ha​44.8kg/ha​42kg/ha K
(from CF KayNitro Sulphur)
​VR P to meet balance of SB and OSR needs
​Winter OSR​4.3​60.2kg/ha​47.3kg/ha​76kg/ha P
(from DAP)
​VR K to meet balance of SB and OSR needs
​Total for rotation​201kg/ha​148kg/ha​29kg/ha P
108kg/ha K
​42kg/ha K
76kg/ha P
​Balance of requirement

​VR = variable rate                    FYM = farmyard manure 

Implementing this plan gives the highest margin crop, winter wheat, the benefit of fresh in-season variable rate P and K. The spring barley and oilseed rape each receive one variable rate application to balance rotational needs, while using FYM as part of the solution reduces the reliance on bagged P and K and boosts organic matter levels. All of this is achieved with just four variable rate applications over three years.

In my next blog I will look at the same principle and the impact it can have on magnesium and soil pH management.

Please have a look at the new calculator, it's FREE - https://www.soyl.com/services/calculators-intro/pkoc and email me at This email address is being protected from spambots. You need JavaScript enabled to view it. to let me know how you get on.

Simon Parrington

SOYL commercial director

For specific advice for your business related to this blog or any other aspect of precision crop production get in touch with SOYL.

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Sunday, 03 July 2022

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