In recent years, phosphorus (P) and potassium (K) application rates to grassland have tended to decrease, while nitrogen (N) application rates have remained relatively high as it is seen as the main driver of grass growth. However, other nutrients are equally essential to grass production, and a sufficient supply in the soil is vital in order to get full benefit from N applications.
Applying lime to increase the pH to around 6.3 should be the first priority for grassland.
The P fertiliser recommendations for grassland are designed to ensure optimum grassland productivity while minimising excess P in soil and potential for P losses to water. Having soil test P levels in Index 3 (5-8mg/l) is considered the optimum soil test index for this.
Assuming the soil P levels are at Index 3, the P fertilisation rate should be based on replacing the P removed in product, be that milk or meat. It should be noted that the amount of P removed will vary depending on the farming system. The total P fertiliser requirements for Index 3 soils for a grazing enterprise are shown in table 1 (above). The chemical fertiliser P rate will be the balance of total requirement less P supplied in concentrate feeds or organic manures. Rates are shown in kg/ha (units/ac in brackets).
Concentrate feeds contain considerable quantities of P. Depending on the feed, one tonne of concentrate feed typically contains 5kg of P. However, some feeds such as citrus pulp can be considerably lower than this. Thus, at a stocking rate of two cows/ha, with 500kg meal fed per cow, the concentrates will contribute 5kg/ha of P. Application rates for grazing should be reduced to account for this contribution of P. Some farms may require no P fertiliser because of the amounts of P in concentrate feeds used on the farm.
When soil P levels are very low (Index 1 and Index 2; below 5mg/l soil test P levels), there is a problem. At this stage, farmers have to take action on two fronts. In addition to replacing the P removed as per Index 3 soils, extra P must also be added to build up the soil P reserves. The rate of build-up can be slow. It is recommended that an extra 10kg/ha of P (equivalent to 0.5 bags/ac of 16pc super phosphate) should be applied above maintenance rates for Index 2 soils, and an extra 20kg/ha P (equivalent to one bag/ac of 16pc super phosphate) should be added for Index 1 soils, in order to build up the soil P levels to target Index 3 levels. This strategy should be continued until the soil test levels are increased to Index 3 levels. This can be monitored with regular soil testing, and normally takes a period of around five years to achieve.
Responses to fertiliser applications to Index 4 soils are rare. Soils with P levels in Index 4 will be productive without P fertiliser applications until the soil test P reverts to Index 3 levels, at which time P applications to replace usage should recommence.
Advice for K fertiliser rates for grazing follows similar guidelines to those for P. The optimum soil K is Index 3, but these soils will also need K fertiliser rates to replace offtakes. Rates shown in table 2 (above) are total K fertiliser requirements for grassland. The chemical fertiliser K rate will be the balance of total requirement less K supplied in organic manures. Rates are shown in kg/ha (units/ac in brackets).
On many grassland farms, silage crops will have the largest demand for fertiliser P and, especially, K. Table 3 (below) shows P and K requirements for silage crops, whether grown with or without slurry application.
Slurry is often applied to the silage area in order to return the P and K nutrients harvested in the silage crop, eaten over winter and excreted into slurry tanks, back to the fields and soils they came from. While this is good practice in theory, it is worth considering the P and K requirements of the silage area before making final decisions on slurry application rates.
Slurry is a valuable resource on grassland farms, particularly where P fertiliser might be restricted within the nitrates regulations.
In this case, slurry may be the only form of P available in the farm. While it is an excellent source of P and K, P and K contents can be highly variable (see table 4, below). For this reason, an effort should be made to approximate the dilution and the dry matter content to estimate the nutrient content. On-farm measurement tools such as the slurry hydrometer can be a useful aid in this.
When choosing a fertiliser product for grassland, consider the ratio of P and K required based on the stocking rate and system. Grazing systems typically require P and K in the ratio of between 1:2 and 1:4. By contrast, silage has a P and K requirement closer to 1:6. Therefore, the P:K ratio in the fertiliser should reflect the requirements of the field. This ratio will change depending on the soil index, concentrate feed usage, and whether slurry is applied.
It may also be appropriate to use a straight K, P, N or combination product where soil indices allow.
K can be applied all year round. There is no restriction on K application in autumn, provided it is applied as straight K. N and P applications are not permitted in autumn after September 15. High application rates should be avoided in spring to reduce the risk of grass tetany. It is advised not to apply more than 90kg/ha (72 units/ac) of K in spring. Apply the reminder in the autumn where high rates are required.
Where annual P application rates are low, little and often application in low rates during the spring and summer may be the best approach. This is because P content in grass tends to be lowest at these times.
Low P products such as 27-2.5.5 and 24-2.5-10 are suitable for this purpose.
Stan Lalor is a Teagasc researcher at Johnstown Castle, Wexford