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Following the principles of sustainable crop production requires that, whatever the financial pressures, we take increased care to maintain the lime status of the soil — which is at the same time an important step towards preserving a satisfactory soil structure, maintaining acceptable porosity and achieving balanced plant nutrition. Taking the soil improvement carried out so far into account, the area in need of improvement in Hungary is some 2.8 million hectares out of a total of 4.2 million hectares of agricultural land. Of this, 2.2 million hectares is acidic soil.

According to Filep (1988), lime fertilisation never became widespread in Hungary. The calcium supply of plants is secured on calcareous and saturated soils. The amount of calcium removed annually by the crop is 10–30 kg/ha for cereals, 50–90 kg/ha for row crops and 150–200 kg/ha for legumes. Part of the calcium removed by the crop can be replaced with superphosphate or calcium ammonium nitrate. Heavy mineral fertilisation, however, acidifies our soils, which is what justifies liming them.

As with every intervention affecting the soil — nutrient replenishment, liming or any other melioration — the soil must be tested in a soil laboratory first.

The intervention can then be planned on the basis of the results. Liming can be used to improve acidic soils; it is not a suitable method for improving sodic soils.

Acidic soils contain a relatively large amount of H+ ions (protons), and this affects soil structure, chemical properties and biological activity alike.

• In an acidic medium the humic substances do not coagulate. The number of Ca bridges linking and stabilising organic and mineral components is small, so durable, crumbly structural elements cannot form. The soil structure is usually compacted, and its water and air regime is unfavourable.
• As acidity increases (when pH < 5.5), the amount of free aluminium (Al3+) and manganese (Mn2+) ions in the solution rises, and beyond a certain value this is toxic to plants. A high concentration of Al3+ also limits the uptake of phosphorus, calcium, magnesium and iron.
• Biological activity also declines in acidic soil. Mineralisation is slower, nitrification is suppressed and a slight accumulation of ammonium can be observed.

To increase fertility, however, every acidic soil needs its harmful acidity eliminated and agrotechnical methods matched to the soil’s properties. (Source: Filep, Stefanovits, Füleky)

The effect of lime on the mobility of nutrients in the soil can generally be called positive in terms of nutrient utilisation by plants. The amount of some substances decreases when the soil acidifies, while others increase. By thoroughly changing the pH of the medium, liming has a very large influence on the mobility of the nutrients present in the soil.

• As a result of liming, the number of micro-organisms grows, and with it the amount of nitrogen available to plants.
• Excessive soil acidity is harmful, one important reason being the reduced availability of phosphorus. Research has established that liming increases the mobility of the phosphates present in the soil. The favourable effect of lime on the amount of plant-available phosphorus lasts for many years, which is explained by the improved conditions for microbiological processes: the organic matter of the soil is broken down and phosphorus is converted from organic bonds into mineral form. It is assumed that phosphoric acid becomes more mobile after liming because calcium displaces iron and aluminium from the sparingly soluble phosphates, and more readily plant-available calcium phosphates form as a result.
• Potassium deficiency is common in acidic soils. Adding lime to such soils increases the amount of available potassium, because calcium displaces potassium from its adsorbed state. Potassium uptake by plants does not increase when the soil is limed, however, which is explained by potassium–calcium antagonism.
• Magnesium deficiency is frequent on acidic soils of light texture. The obvious remedy is to work dolomite flour into the soil.
• Plants cannot survive without manganese either, but acidic soils generally hold a large manganese surplus, which is harmful to them. Excess manganese inhibits the development of the plant’s generative organs and often leads to the death of the whole plant. The more acidic the soil, the higher the amount of mobile forms of manganese. Too large a dose of lime, on the other hand, leads to plants showing symptoms of manganese deficiency.
• Iron is one of the most important plant nutrients. Iron deficiency can cause chlorosis and a significant drop in yield. Most iron occurs in soils in the form of oxides. In acidic, poorly aerated soils, however, ferrous compounds form, which are harmful to the plant. Liming is an effective remedy against the accumulation of ferrous compounds.
• Soils contain very significant amounts of aluminium, most of it inaccessible to plants. Acidic soils, however, hold large amounts of mobile aluminium, and mobile aluminium compounds are toxic to plants. The most radical means of protecting against the harmful effect of aluminium is liming the soil, which immobilises the toxic aluminium compounds.
• Boron deficiency occurs only in soils of alkaline pH. Over-liming therefore reduces the amount of available boron, which leads to deficiency symptoms in plants.

Avdonyin (1972) demonstrated that as soils acidify the number of bacteria decreases while the number of fungi increases significantly. Most bacteria do not develop below a pH of 4.5–5, while the bacteria that oxidise sulphur are characterised by high acid tolerance. Excessive soil acidification is particularly unfavourable for the development of micro-organisms such as Azotobacter and root-nodule bacteria (rhizobia), which enrich the soil with atmospheric nitrogen. Liming was especially favourable for important micro-organisms such as the nitrifiers, clostridium species and cellulose decomposers, which play an important role in increasing soil fertility.

Micro-organisms in relation to the pH of the medium:

Micro-organisms in relation to the pH of the medium:

Source: Avdonyin, N. S. (1972): Increasing the fertility of acidic soils

The tests described below are carried out by soil laboratories.

Based on the work of Kádár (1998), when the pH of the soil solution is below 7 we speak of actual acidity, characterised by pH(H2O). Soil pH is measured in a 1:2.5 aqueous suspension. If an acidic soil is shaken out with 1 M KCl, a lower value is obtained, because the cation of the neutral salt exchanges K+ for H+ on the surface of the colloids. This exchangeable acidity is of course greater than the actual acidity. In strongly acidic soils not only adsorbed H+ but also Al3+ is exchanged and enters the solution, and Al3+ is toxic to plants. Cation exchange reactions also take place with mineral fertilisers, so choosing the right fertiliser form matters. If the soil is treated with alkaline-hydrolysing Na acetate (a strong base and a weak acid), a stronger H+ exchange occurs. This is called hydrolytic acidity and is denoted y1. y1 exchanges all the H+ ions of the soil, whereas KCl exchanges only the readily exchangeable ions. In neutral or slightly acidic soils only hydrolytic acidity can be measured meaningfully; exchange acidity is negligible.

Several theoretical and empirical methods can be used to determine the lime dose needed to improve an acidic soil.

In practice the lime dose has for decades been calculated from the hydrolytic acidity (y1) and the binding number (KA).

The formula below gives the CaCO3 dose needed to lime the top 20 cm layer of one hectare.

CaCO3 t/ha = y1 × (KA/100) × 1.74 = tonnes of CaCO3 active content per hectare

The calculated amount refers to pure CaCO3, so when setting the dose of amendment actually applied, the CaCO3 content of the material used must also be taken into account.