PH level and lime requirements
Crops differ in their sensitivity to pH. In addition, the optimum use of fertilisers containing nitrogen and phosphorus is obtained when the soil pH is between 6.2 and 7.2. The availability of some trace elements, especially manganese and boron is decreased when pH is above seven. A deficiency of manganese is quite common in cereals which follow sugar beet in the rotation due to the over use of lime for the beet.
Although potatoes and oats can grow very well below pH 6.0 it is necessary to maintain the soil at a pH suitable for the rotation. The pH levels to aim for in mineral soils are as follows:
pH 6.3 Grass
pH 6.5 Cereals
pH 7.0 Beets, Beans, Peas
The target pH for grassland in the UK is 6.0 for mineral soils. One of the reasons for the slightly higher Irish figure is that clover is deemed to be very important in Irish conditions and it is reckoned that clover persistency require a higher pH than grass (MAFF 1994). In general terms a high pH at the establishment phase is important in three respects.
A. Both grasses and clovers require a higher pH at the establishment phase
B. Of all the elements, phosphorus is most important for seedling growth, and it is most readily available at around pH 6.5.
C. In minimum cultivation techniques there can be considerable quantities of organic matter on the surface (decaying grass etc.). These can exude organic acids in the decomposition process that are harmful to the seedlings. Lime neutralises these acids.
Having established a target pH in the soil for crops, there is a necessity to maintain it at that pH. This is an area that merits some consideration. There are different approaches that can be adopted. The current methodology is to raise the pH to 6.8 for grassland. The reason for this is that if the lime status is brought to that level, lime need not be applied for a further 8-12 years i.e. it will take up to 8-12 years for the pH to drop to below 6.0. A simpler approach to liming would be to do a soil test every 4-5 years and apply lime when required. It would be adequate to aim for a pH of 6.5 if this approach was adopted. Surface acidity (top 5 cm) often occurs in grassland due to our high rainfall and the use of nitrogenous fertiliser. This reduces the availability of fertiliser phosphorus. For this reason it is better to have frequent small application of lime than one large application at irregular intervals. M. Murphy at Johnstown Castle has suggested that a yearly application of lime and sulphur could be used to meet the sulphur requirements and minimise surface acidity. The lime requirement to bring the soil to 6.5 is based on the premise that optimum production is required. In today’s agriculture, optimum production is not always required. For instance in REPS no lime need be applied allowed if nitrogen usage is less than 20 kg/ha. Similarly, if less than optimum stocking rates are required, the amount of lime spread is at the discretion of the advisor. There are some areas where general lime recommendations do not apply i.e. in peats, on soils in high Mo areas and an acid soils of marginal/high Mo content, where more liming for grass/clover growth may increase Mo levels to “toxic” levels.
(1) Peats:- In the very high rainfall areas (1500 mm or higher) on the hills in the West of Ireland the leaching of calcium by rainfall becomes very intense. Iron and aluminium are washed down from the top soil and deposit again up to 500 mm below the surface. They form a layer or ‘iron pan’ which is impenetrable to water causing the whole soil to become water logged. Plant remains fail to decay under these conditions and peats develop. These peats are very acid but because they contain very little mineral matter and very low levels of aluminium or manganese a pH of approximately 5.5 is considered acceptable for most crops compared to 6.5 on mineral soils. However, when these peats are limed and drained the organic matter at the surface is broken down more quickly by soil microbes and the surface of the peat can shrink and disappear over a few decades.
Basin peats can be strongly acid, with pH as low as 3.5 at the surface but quite high in pH (6.0-7.0) near the base of the peat layer. Peat harvesting operations may complicate this pattern as the top peat maybe returned to the floor after most of the peat has been removed. The floor of midland bogs may have high molybdenum and a naturally high pH, while the peats may have low copper, so ploughing up sub-soil with peat can lead to acute molybdenum excess.
(2) High Molybdenum soils. When the level of molybdenum in herbage exceeds 1.5 mg/kg it may interfere with copper metabolism in the animal consuming it and cause illthrift and more specific disorders. If soil molybdenum is high, liming will usually increase Mo uptake by grass or clover, so such soils must be treated very carefully. A general recommendation is not to increase the soil pH above 6.0. To achieve this, the lime recommendation suggested by the lime test should be reduced by 7.5 t/ha. This is particularly important for old pasture where even relatively light dressings may increase the pH in the top 25 mm of soil and thereby increase molybdenum uptake. The presence or absence of clover may be more important than soil pH, as clover may contain significantly more molybdenum than that found in grasses. High rates of N fertiliser will tend to discourage clover and therefore reduce the risk of problems caused by high molybdenum. Fig. 1 shows areas of the country when high Mo content of pastures is most common. Not all fields or farms in those areas have high Mo. Similarly not all fields or farms in the remainder of the country are free from this Mo problem.