Since the purchase of the Dolomite mine at Golden Bay by the Solly family in 2007, sales of agricultural dolomite have steadily increased and a quantum lift in sales appears imminent as it becomes increasingly apparent that magnesium is a ongoing requirement on many soils.
Dolomite is essentially a magnesium fertiliser i.e. dolomite is only applied where there is a requirement for magnesium.
Golden Bay Dolomite typically contains 11.5% magnesium and 24% calcium, both in the carbonate form. The deposit at Golden Bay is unique in this country although there are magnesium silicate (serpentine) deposits in both the North and South Islands.
Magnesium silicate is very much harder than magnesium carbonate and therefore the release period is longer. A field trial conducted in the Manawatu over a 2 ½ year period compared the agronomic effectiveness of a number of magnesium products that included both dolomite and serpentine rock.
17 months after application 90 – 100% of the magnesium in dolomite had dissolved while 45% of the magnesium in serpentine had dissolved.
Typically dolomite is applied to pastoral land at the rate of 200 – 250kg/ha. In our twenty year experience with dolomite, applying heavier rates does not necessarily provide better performance. Well-structured high organic matter soils may be able to store magnesium and calcium, however the economics of applying capital inputs when smaller quantities applied frequently provide the same results, need to be carefully considered.
Although technically the release rate of magnesium and calcium from dolomite may be slower than from magnesium oxide the improvement in animal health on properties to which dolomite is applied may be obvious within 10 – 14 days after application.
The method of grinding dolomite provides a small percentage of very fine particles. Because of the high surface to volume ratio of the fines, breakdown in moist acidic conditions on the soil surface can be rapid. Typically 62% is less than 150 microns.
To argue that a product should be applied because it is the cheapest regardless of its effectiveness is misleading. Product is only suitable for use if it provides the desired outcome, regardless of the cost of input, however the financial benefit(s) must outweigh the cost of supply and application.
The health of plants and animals are dependant on the soil health, with healthy plants and animals unable to be sustained on degraded soils. Under most conditions, particularly intensive livestock grazing or where there is regular cultivation, the responses from more oxygen entering the soil outweigh the responses from other inputs.
Dolomite is a proven soil conditioner and a single application is sufficient to start the process of improving physical soil structures, and because of its unique ability to deliver the benefits of improved soil and animal health it has become the standard by which other magnesium fertilisers are judged.