A. Klougart, O. Bagge Olsen
The pH of the compost is as we already have heard, a very important factor for plant growth in containers. Lime is normally added in 3–6 kg/ m3 to achieve a pH of approximately 6.0, but very few growers realize the scale is logarithmic and by pH 5.0 the substrate contains 10 times as much acidity as by pH 6.0.

The amount of lime to be added depends largely on:

  1. initial pH of the peat,
  2. the stage of decomposition.

We have found that within the pH range of 3.5–7.0 30 g fine ground limestone (97% CaCO3) has to be added to 1 kg dry matter to change pH one unit (f. i. from pH 4.0 to 5.0). If pH and dry matter content is known the calculation is simple.

The peat types with high dry matter content also very decomposited, need large quantities. For instance a type with 180 kg dry matter/m3 and pH of 3.5 need 180 x 30 x 3 = 16,200 g limestone to obtain pH 6.5.

The limestone will neutralize the hydrogen ion of the humic acid and make Ca-humates and crumb structure. Peat has a very low content of Ca-ion and one should expect that the liming procedure should give the soil water many free calcium ions or easy extracable ions on the colloids. This is surprisingly not always the case. Apparently unsoluble calcium-humates has been produced, as very low values are found by water extraction, especially of the humified types.

Calcium is one of the major plant nutrients and should be available in a certain proposion to potassium (presumably 2 K+ : 1 Ca++). In normal mineral soil, where the base saturation on the colloids may by 80% Ca++ there is no problem, but in peat substrates with a fertilizing level of 30–60 mg K/100 ml we have often found only 5–15 mg Ca/100 ml. When all humic acid has been saturated with Ca++ and other cations the release of free calcium ions will take place. As gypsum is more soluble than limestone, it is recommended to add 1–2 kg gypsum/m3 with the limestone but the optimal quantity based on analysis we do not know today.

Experiments with tomatoes for Ca-deficiency (blossom end rot) showed that clay addition to peat reduced calcium deficiency considerably. In clay-peat mixtures the pH should not go below 6.0 (Al-toxicity!)

The problems of the acidophilous plants will often appears. These plants have a great uptake of micronutrients as iron and manganese, and can not get sufficient supply as these metalions by high pH are insoluble compounds. If we can supply these plants with sufficient iron and manganese (and Cu, Zn or Co), we can grow acidophilous plants even by

Klougart, A. and Bagge Olsen, O. (1969). NUTRITION OF CONTAINER PLANTS. Acta Hortic. 15, 34-36
DOI: 10.17660/ActaHortic.1969.15.7