Magnesium fertilisers

Chittendon, E.T., Stanton, D.J., Watson, J. and Hodgson, L. 1964. Serpentine and dunite as magnesium fertiliser. New Zealand Journal of Agricultural Research 10 160-171. 

Calcined magnesite (80% MgO). This is the most concentrated magnesium fertiliser, but it is only slowly available in the soil. To help to maintain the magnesium status of a light sandy soil which is naturally low in magnesium, 300 kg/ha should be applied, say, every four years in a predominantly arable cropping situation. 

At index 2, 40 kg/ha of P and 30 kg/ha of K are required in the seedbed assmning a 3.5t/ha yield. 11kg of extra P and 12kg of extra K will be needed for every torme of yield above 3.5. However, allowance should be made for nutrients applied as organic manures. No nitrogen fertiliser will be required, but a magnesium fertiliser will be required at soil index 0 and 1. 

Recent developments to the hydrothermal process include improvements in yield and reaction rate and in overcoming the difficulty associated with the coproduct salt. One method of overcoming the co-product problem is to use magnesium nitrate instead of chloride, with the ammonium nitrate being utiHsed for fertiliser production [102-104]. At least one plant based on this concept is now in commercial production. While a considerable advance on the initial chloride process, the nitrate route does require close integration with a fertiliser process and thus lacks flexibility. An alternative approach being developed is to recycle the ammonium salt co-product (nitrate or chloride) and use it to leach magnesium oxide, a potentially inexpensive raw material [103]. 

Some of the ammonia produced by the Haber process is used to produce nitric acid. If ammonia is then reacted with the nitric acid, we have the basic reaction for the production of many artificial fertilisers. The use of artificial fertilisers is essential if farmers are to produce sufficient crops to feed the ever-increasing world population. Crops remove nutrients from the soil as they grow these include nitrogen, phosphorus and potassium. Artificial fertilisers are added to the soil to replace these nutrients and others, such as calcium, magnesium, sodium, sulfur, copper and iron. Examples of nitrogenous fertilisers (those which contain nitrogen) are shown in Table 11.7. 

By heating phosphate rock with serpentine or olivine to about 1550°C, fused magnesium phosphate fertiliser is obtained. Other commercial products include phosphatic soil conditioners based on phosphatic chalks with a 5-10% P2O5 content. 

Lime and fertiliser requirements. Some soil types have different abilities both to hold and release nntrients. Most clay minerals have a negative charge which attracts positively charged ions such as potassium, magnesium and calcium. This is called the cation exchange capacity. The presence of these charged sites affects the availability of nutrients. Sand and silt are relatively inert. Different soil textures with the same level of acidity have different recormnendations for the application of lime. This is due to differences in the cation exchange capacity of the soil. 

A decision has to be made as to how much and what type of fertilisers should be used for eaeh crop. For phosphoms, potassium and magnesium the amourrt applied should be based on an analysis of a soil sample. Soil analysis will show ... 

Available nutrients. This is the level of phosphate, potash and magnesium and it is indicated by index ratings 0-9. A deficiency level is indicated by 0 and an excessively high level by 9 (never reached imder field conditions). An index of 2 and 3 is satisfactory for farm crops. As indices drop below 2 more fertiliser is recormnended so that the index will go up over a period of time, and as they get above 2 or 3 the recommendations decrease until, at index 4 and over, no applications of phosphorus, potassiirm or magnesium are required. 

Kieserite (26% MgO). This fertiliser is quick acting and is particularly useful on severely magnesium-deficient soils where a magnesium-responsive crop such as sugar beet is to be grown. It also contains 50% SO3. 

Use FYM or slurry hme with magnesium limestone Epsom salts fertiliser containing kieserite... 

---