Choosing and using lime in the orchard
Note number: AG0091
The main purpose of applying lime in the orchard is to neutralise acidity and to improve soil structure. Lime also supplies plants with the important element, calcium
Why use lime?
Soils in southern and north-eastern Victoria and in the Goulburn Valley are naturally acidic, in contrast to the alkaline soils of the Mallee. Most fertilisers that contain nitrogen will slowly acidify soils and it is important to correct for this.
As soils become more acidic, hydrogen ion replaces valuable mineral elements, such as calcium and potassium; other elements, such as phosphorus, may become less available to plants. Minor elements, such as manganese and to a lesser extent aluminium, become more soluble in acidic soils and are taken up in excessive quantities by some fruit trees. Delicious apples and Josephine pears are particularly susceptible to manganese toxicity.
If large quantities of lime, or calcium salts such as gypsum, are mixed with the soil they improve its structure by forming more stable soil crumbs, thus making it easier for roots and water to penetrate.
When to use lime
Because of its low solubility lime should be cultivated into the soil so that it can reach the root zone of fruit trees. Ideally lime should be incorporated into the soil before planting. It is also more economical to apply fairly large quantities every five years or so once trees are established, if soil pH tests show that liming is needed. Generally, orchard soils need liming only where the top 200 mm of soil is more acidic (has a lower pH) than pH 5.6.
Which form of lime to use
Lime is available to the orchardists in three forms:
- Calcium carbonate –This is available as ground limestone, agricultural lime and shell lime. It is the cheapest but least reactive form of lime. The premium grades are more finely ground.
- Calcium hydroxide – This is available as slaked or hydrated lime (for example, "Limil"). It is more reactive than calcium carbonate, but rather expensive for agricultural use. It converts to the carbonate form when used for the absorption of carbon dioxide in CA stores.
- Calcium oxide – This is sold as burnt lime or quicklime, and is the most reactive form of lime. It heats and swells on absorbing moisture, and so must be stored dry.
Mixtures of the above are also available; for example, as cement kiln dust called "Green Pasture Lime".
The various forms of lime can be compared on the basis of their neutralising value; for example, 560 kg of burnt lime equals 800 kg of hydrated lime or 1000 kg of agricultural ground limestone. The neutralising value, unit cost and cartage cost all need to be considered in determining the best buy.
All forms of lime will eventually convert to calcium carbonate in the soil. This material becomes soluble very slowly and may be more suitable for use on heavy soils, such as clays and loams, because it will penetrate the soil more.
How much lime to use
The amount of lime that should be applied depends on both the acidity of the soil and its texture; a heavy soil will need more lime than a sandy soil.
What to watch out for
Don't let quicklime, or a mixture containing quicklime, contact the trunks of newly-planted trees. It can burn tender bark and roots. It is unnecessary to increase soil pH above 6.5 because doing this may cause deficiencies of trace elements, such as manganese. Such deficiencies cause interveinal yellowing of older leaves. Don't let quicklime or slaked lime contact animal manures or nitrogen fertilisers, as this will result in the formation of ammonia gas, which is partly lost to the air and, when it is in the soil, is toxic to young trees. Remember, lime is mainly used in the orchard to preserve the long-term fertility of the soil and to prevent troubles that would arise if the soil were too acidic. It is a slow-acting material that should be mixed into the soil for best results.
Table 1. Suggested amounts of lime (tonnes per hectare)
|Soil texture||Soil pH
W. Thompson, Knoxfield