Farm Ireland
Independent.ie

Monday 24 July 2017

Do a soil sample to decide costly compound needs

The practice of applying the crops' phosphorous and potash requirements in a compound with nitrogen in early spring is commonplace. The choice of compound can be critical in contributing to the yield potential, as the N, P and K elements are fundamental to crop yield
The practice of applying the crops' phosphorous and potash requirements in a compound with nitrogen in early spring is commonplace. The choice of compound can be critical in contributing to the yield potential, as the N, P and K elements are fundamental to crop yield

Gerry Bird

Cereal crops are looking well after the recent bad weather, with the exception of some crops of oats, which are showing leaf tipping symptoms, root heave and plant death.

Growers will begin to consider the fertiliser options and strategies for the coming season, against a background of increased fertiliser cost and possible reduced application rates over previous seasons.

A good starting point is to have up-to-date soil analysis results to set out the baseline nutrient levels. The practice of applying the crops' phosphorous (P) and potash (K) requirements in a compound with nitrogen (N) in early spring is commonplace. The choice of compound can be critical in contributing to the yield potential, as the N, P and K elements are fundamental to crop yield.

Nitrogen is the element which gives the most obvious response, controlling the rate of growth, leaf colour and crop maturity, and has the biggest impact on yield. It is applied in several splits in a winter cereal's growth cycle. This way, nutrients are available to the plant over a longer period, which maximises tiller and leaf production. More leaf area means the plant can harvest more light and optimise grain fill. Decisions on nitrogen application can be ongoing during the season, depending on crop appearance, weather, cost issues and the context of the Nitrates Directive regulations.

Phosphorus is the expensive part of the compound fertiliser. However, it is vital for root development, winter hardiness and key to the major processes of water and nutrient movement in the plant. If fertiliser P applications have been reduced or missed over the past seasons, and if soil results show moderate to low levels, crop yields will suffer so P has to be applied at the earliest opportunity to promote root growth.

Potash is essential for grain number and quality. It is also central to straw strength, vital for water movement and retention and a major regulator of plant solution concentrations. The crop response to applied nitrogen is enhanced where adequate potash is available. Reduced applications of potash, or where straw is removed, can impact on the amount of nitrogen the crop takes up. This is an important consideration on heavy, cold, clay soils, which can make potash unavailable.

Decisions on fertiliser applications have to take many factors into account. These include crop potential, soil type and analysis results, seedbed, plant numbers, rotation and the Nitrates Directive regulations.

The nutrient removal attributed to the previous crop is a useful indicator of the minimum requirements. An additional portion of fertiliser is retained by the soil reserves which is not readily available to the crop in the current season -- so this must be factored in (see table, right).


The revised Nitrates regulations have been issued and the main changes in the nutrient section are in the Nitrogen Index in relation to organic manures application. The nitrogen levels for winter wheat have all been increased, so check with any member of the ITCA for details.

Growers of a certain vintage will discuss fertiliser requirements in terms of 'units' and a degree of confusion occurs with the concept of nutrient kilogrammes per hectare.

One unit is one percent, a 50kg bag of 27pc CAN contains 27pc nitrogen, or 27 units. Calculating 27pc of 50kg = 13.5 kg of nitrogen in a 50kg bag -- the balance 36.5kg is filler.

A 50kg bag of CAN contains 27 units (13.5kg) of N, so 1kg of N is equal to two units of N. It is the same process for 10.10.20, 18.6.12 etc. The figure represents the percentage nutrient in the fertiliser or the units. Converting nutrient kilogrammes per hectare to units per acre or vice versa is simple:



  • 140kg N/ha x 0.8 = 112 units/ac
  • 112 units N/ac x 1.25 = 140 kg/ha.


Gerry Bird is a crop consultant and a member of the ITCA. Email: gjbird@eircom.net

Indo Farming