Soil pH and Liming
Spring cereals are sensitive to soil pH. Maintain a soil pH 6.5 for cereal crops grown in rotation. Apply lime as recommended on the soil test report, where greater than 7.5t/ha is required, apply 50% now and apply the remaining 50% in 2 years time. Over liming can cause problems in the form of induced micronutrient deficiencies e.g. manganese in oats, barley and beet crops.
The nitrogen (N) requirement of the crop is met partly by N already in the soil organic matter and the remainder has to be supplied by fertiliser. The biggest difficulty is to accurately estimate the level of N mineralised from the organic matter. Previous cropping and manuring history have a major influence on the level of available N in the soil.
A relatively high level of N is mineralised and available to a crop following a permanent pasture. Conversely, the need for fertiliser N is high in a continuous cereal rotation. Table 1 below shows the recommendations for fertiliser N in medium textured soils. A soil N index is used to relate previous cropping and management to the recommended rates as follows:
Table 1:- Determining Nitrogen Index for Tillage Crops
|Continuous Tillage – Crops that follow short leys (1 - 4 years) or tillage crops|
|Index 1||Index 2||Index 3||Index 4|
|Maize||Sugar beet, fodder
Oilseed Rape, Peas,
Beans, Leys (1-4 yrs)
grazed or cut and
receiving more than
Swedes grazed in situ
|Tillage crops that follow permanent pasture|
|Index 1||Index 2||Index 3||Index 4|
|Any crop sown as the 5th or subsequent tillage crop following permanent pasture||Any crop sown as the 3rd or 4th tillage crop following permanent pasture. If original permanent pasture was cut only, use index 1||Any crop sown as the 1st or 2nd tillage crop following permanent pasture (see also index 4). If original permanent pasture was cut only, use index 2||Any crop sown as the 1st or 2nd tillage crop following very good permanent pasture which was grazed only|
Phosphorus (P) and Potassium (K)
To determine how much P and K to apply you first must test the soil and find out the soil P and K index. This will allow you to calculate the amounts of P and K required. The most reliable way to do this is through a soil test once every 3 to 5 years. There are four soil indexes with Index 1 soils deficit in P & K and Index 4 soils very fertile.
The P and K indices can be interpreted as follows:-
Index 1 (very low) – The soil is deficit. There will be a definite response to applied P and K. Index 2 (low) – Extra P and K required, over and above the quantity the crop is likely to remove.
Index 3 (medium) – This is the optimum soil P and K index. A maintenance dressing of P and K is required to replace the nutrients removed by the crop.
Index 4 (sufficient) – No additional P and K is required to grow a satisfactory crop.
Once you know the soil index simply refer to the following tables below to determine the amount of P and K required.
Phosphorus rates in table 4 are based on a crop yield of 6.5t/ha. Where there is proof of higher grain yields achieved in any of the 3 previous harvests, at 20% moisture content an additional 3.8kgP/ha can be applied for each additional tonne above 6.5t/ha.
|Table 4:-Soil P Index, Soil P test reading and phosphorous requirements for spring cereal crops|
|Soil P Index||Soil P test Result (mg/1)||Rate (kg/ha)|
Table 5 shows the soil K index and K advice for a range of spring crops at crop base yields (t/ha). Potassium advice will differ depending on the crop type, crop yield potential and soil type.
|Table 5:- Soil K index and nutrient advice for spring cereals where the straw is removed (kg/ha)|
|Soil K Index||Soil K test Result (mg/L)||6.5 t/ha||7.5 t/ha||8.5 t/ha|
1. K rates based on: Spring barley 6.5t/ha, Spring wheat 4.5t/ha, Spring oats 6.5t/ha
2.For Wheat & barely crops; increase or decrease K rate by 10kg/ha per tonne increase or decrease in grain yield
3. For Oat crops; increase or decrease K rate by 15kg/ha per tone increase or decrease in grain yield.
Timing of P and K applications
The timing of P and K will depend on the soils fertility status. Index 1 soils (very low), the P and K should be combine drilled / board cast before sowing and incorporated at sowing time. This is beneficial as sufficient P and K is required in the seedbed in the early stages of development (rooting & tillering) in cereal crops. On index 2 (low) and 3 (medium) soils P and K should be applied at sowing and worked well into the seedbed as a compound fertiliser (N, P, K) for example 10-10-20 / 13-6-20 etc.
On K – fixing soils (soils that don’t release soil K and fix applied K) it is recommended to apply K close to when the crop requires it. Apply K in the springtime along with the N applications. It can be very difficult to build up soil K levels on these soils therefore apply maintenance levels to meet annual crop requirements.
P and K off takes in cereal crops
Table 6 below shows off takes of P and K in cereal crops. Theses values are off takes per tonne of grain yield but include the nutrient value of straw where applicable. P and K applications should take account of crop yields and nutrient removed in straw.
|Table 6:-P and K off takes in cereal crops (kg/ha) per tonne of grain yield|
|Crop type||Straw removed||Straw not removed|
|Spring Wheat / Barley||3.8||11.4||3.4||4.7|
Application of Mg is only advisable for cereals if soil is less than 50mg/l (Index 1 & 2). On such soils, use magnesium limestone when liming is necessary. Where there is no lime requirement, Mg should be supplied with Mg-containing fertiliser once every 3 to 4 years as per table below. If a crop deficiency appears, spray with an Mg chelate or there recommended inorganic formulation.
|Mg Recommendations – Tillage crops|
(0 to 25 mg/L)
(26 to 50 mg/L)
|Cereals||60 to 75 kg/ha||30 kg/ha|
A significant proportion of Irish soils will show a response to applied sulphur (S). There is no reliable soil test for S, but responses are most likely on light textured soils with low organic matter content.
The following rates of S are advised for cereals crops grown on soils responsive to S:
|Cereals||15 kg/ha per year|