Soil Management

Soil pH

There is a progressive acidification of soils in Poland, which is due to their post-glacial mineralogical composition. Currently, around 60% of arable soils urgently require liming, and almost 20% of soils are deficient in magnesium and a number of micronutrients (including boron, zinc, manganese and copper).

Soil reaction (pH) is a determining factor for many biological and physico-chemical processes in the soil. Soil pH is determined mainly by the mineralogical composition of the soil (acidic or alkaline nature of the parent rock), the transformation and content of organic matter and the climatic conditions that determine leaching. pH values of 5.5 to 7.2 are considered to be the optimum range for the biological processes involved in the metabolism of most plant species and soil microorganisms. At pH values below 4.5, soluble forms of trivalent aluminium appear in the soil solution, damaging the root hairs and impairing water and nutrient uptake. In acidic soils, the pH is a limiting factor for the yield of most crops, and the amount of yield loss depends on the sensitivity of the individual plant species, often exceeding 30%.

When the soil pH is too low, the uptake of fertiliser elements by plants decreases, and unused nutrients are leached out and into groundwater (nitrogen, potassium) or volatilised (phosphorus) to form forms that are not available to plants. At pH values below 4.5, it is virtually impossible to achieve satisfactory yields. It is important to remember that different crops differ in their sensitivity to soil acidification. Acidity-sensitive crops, which include wheat, sugar beet, will respond with a significant decrease in yield. Crops such as rye, oats and potatoes are more tolerant of soil acidity, but they also yield optimally on neutral or slightly acidic soils (pH above 5.5).

On acidic soils, liming treatment is necessary. The dosage required to completely neutralise the acidity is calculated on the basis of soil acidity analysis. The dose of lime is calculated by multiplying the hydrolytic acidity value by a factor of 0.85 to determine the converted dose of CaO in tonnes per hectare.

If oxide lime is used, an application rate equal to that calculated from such a conversion should be applied. If carbonate lime is used, the required fertiliser dose will be more than doubled, as the calcium oxide content of available carbonate fertilisers is in the region of 45%.

It can also be assumed that it is not necessary to lime soils whose acidity corresponds to less than 1 tonne of CaO per hectare, but the application of lime will not be wrong as it promotes the formation of soil aggregates and improves soil properties.

The form of other mineral fertilisers used, particularly nitrogen fertilisers, is also important for maintaining the correct soil pH. Fertilisers differ in their soil acidification potential. Fertilisers containing nitrogen in the form of a strong acid ammonium salt (e.g. ammonium sulphate) will acidify the soil to a greater extent (sulphate ion) than nitrate fertilisers (e.g. calcium nitrate) containing a strong base ion.

Acidic soils should be limed in order to improve the conditions for growing crops and to reduce the degradation of these soils. Adjusting the pH towards neutral stimulates the growth of microorganisms that form soil aggregates and fix atmospheric nitrogen. Correct pH helps to create and maintain proper soil structure, increase soil organic matter content and nitrogen abundance.

Calcium carbonate (limestone, chalk) or calcium-magnesium (dolomite) fertilisers are most commonly used for liming soils; oxide lime is used somewhat less frequently. Industrial by-products containing large quantities of calcium carbonate or calcium oxide, e.g. waste from the paper industry, can also be used, provided that they do not contain excessive amounts of impurities (e.g. waste on the list of permitted R10 recovery methods).

Carbonate lime can be applied to all soils regardless of granulometric composition. Lime in carbonate form slowly deacidifies the soil. The rate of deacidification with carbonate fertilisers depends on the age of the rock and the fineness of the rock. The finer the rock, the quicker the liming effect will be. However, the full effect of the carbonate lime application should not be expected before the second year after application.

Oxide lime acts much faster, so it should be applied to heavier soils with greater buffering capacity to avoid rapid changes in pH that are detrimental to plants and micro-organisms.

The amount of lime required to neutralise the soil depends on the degree of acidity of the soil and also its buffer properties related to its sorption capacity. In simple terms, the heavier the soil, the higher the dose of lime required to deacidify it.