101 Questions answered
Adjust Fertiliser Rates and Manage Toxicity in Calcareous Soils
For phosphorus and trace elements – yes – reconsider your fertiliser strategy.
Because phosphorus is so easily tied up in calcareous soils, higher rates are often necessary. Make sure the right phosphorus test is used to calculate phosphorus rates. The DGT-P test works better in calcareous soils than Colwell P.
Higher fertiliser rates are also likely needed for manganese, iron, copper and zinc. Tissue tests are more reliable than soil tests to gauge trace element status in the crop. Use soil tests to monitor long-term trace element trends.
In extremely calcareous soils with >50% lime, fertiliser toxicity can be a problem. These soils often have higher background salinity levels. Fertilisers are salts and as they dissolve, they increase the salinity of the soil water. This makes it harder for seeds to imbibe the water they need to germinate. For seedlings, if salinity rises too high, the seedlings cannot take up water. Different crops have different sensitivities to soil salinity; seed safety with fertiliser applications is important.
The other issue is toxicity from ammonia gas. In alkaline soils, fertilisers containing ammonium can release ammonia gas which is highly toxic to seeds and roots. If the fertiliser is placed too close to the seed, the gas can damage the seed or roots.
