Water-soluble and metabolically absorbed form of magnesium is of the greatest importance for plant nutrition. Black earths are richest in magnesium. Sod-podzolic and peat-bog soils (especially high and transitional peat bogs) contain less of it.
Based on a generalization of the named material, one can propose the following grouping of sod-podzolic soils according to the availability of exchangeable magnesium for agricultural lands depending on the granulometric composition of the soil (Table 3).
Table 3. The gradation of sod-podzolic soils according to the content
metabolic magnesium
| Security level | The soil | |||
| sandy and loamy | loamy and heavy loamy | |||
| mg MgO per 1 kg of soil | mg / equiv Mg per 100 g of soil | mg MgO per 1 kg of soil | mg / equiv Mg per 100 g of soil | |
| Very low | less than 40 | less than 0,2 | less then 80 | less then 0,4 |
| Low | 41-80 | 0,2-0,4 | 81-120 | 0,4-0,6 |
| Middle | 81-120 | 0,4-0,6 | 121-160 | 0,6-0,8 |
| Elevated | 121-160 | 0,6-0,8 | 161-200 | 0,8-1,0 |
| Tall | 161-200 | 0,8-1,0 | 201-240 | 1,0-1,2 |
| Very tall | more then 200 | more then 1,0 | more then 240 | more then 1,2 |
Approximate indicators of the optimal content of metabolic magnesium in soils under various crops are given in table 4. As a rule, on soils with a higher content of metabolic magnesium, the effectiveness of doses of mineral fertilizers increases by 15-25%.
The need for the use of magnesium-containing fertilizers in hayfields and pastures may arise despite the fact that plants do not experience a lack of magnesium to form a crop. In these cases, the need for the application of magnesium with fertilizers is determined by the need to obtain a crop of a certain quality. In herbs, the magnesium content should be at least 0.2% MgO, and the K: (Ca + Mg) ratio should be no more than 2.2 (calculated on air-dry matter). When using high doses of NPK, the magnesium content in herbs may be lower than the recommended zootechnical standards, especially in spring. Violation of zootechnical requirements for feed quality leads to the disease of “pasture tetany” of feed, especially milking, which most need magnesium.