With fertiliser prices higher than ever, many farmers may be tempted to take or even extend so-called phosphate and potash “holidays” this autumn. But without soil analyses to justify not applying these base nutrients they could jeopardise 2009 harvest yields, warns Frontier Agriculture’s technical manager.
“Their immediate reaction could be to cut back or even omit applications to keep fertiliser purchase costs within budget,” says Mike Slater. “But this year especially, the requirements of each field or crop block must be reviewed to determine which policy to follow – to continue as usual, rely on soil reserves more or, in a few very poor areas, to increase the amount of P and K applied.”
Growers without soil analysis results from the past three or four years should get their land tested as soon as possible after harvest, urges Mr Slater.
“The analysis costs will be recouped many-fold, and without up-to-date analysis wrong decisions could easily be made.”
Soil analysis indicates the amount of readily available phosphate and potash – assuming the field’s soil is truly represented by the sample sent to the laboratory, he explains.
“Where a field may have two or more soil types, sample each area separately.
“Note the stone content because significant quantities will dilute the amount of available nutrients.
“We assume that a soil with an index of 2 for both phosphate and potash is satisfactory and for many growers this is true. But poorly structured soil reduces root growth and access to nutrients. So on thin soils, especially where there is a high stone content, indices above 2 are needed.”
Fields at index 3 and above should contain enough nutrients to support all combinable crops, he says. “Economies can be made in these cases, but beware of doing so in fields with a poor soil structure.”
The main problems occur where indices are below 2, although at index 1 on deep well-structured soils, crops should still be able to access large quantities of nutrients and only a small application of phosphate to help establishment may be required, says Mr Slater.
“Freshly applied water soluble phosphate is available to crops for about six weeks before it starts to be absorbed by the soil. However in more shallow soils, even at index 1, there are unlikely to be sufficient nutrients available for optimum uptake and yield potential.
“Soils with indices of 0 are very unlikely to provide enough, so all the crops’ requirements should be provided by some form of fertiliser, at the very least replacing the phosphate and potash removed in the previous crop.
“A 9t/ha crop of wheat with no straw taken off will remove 70kg/ha and 50kg/ha of phosphate and potash respectively.
“Remember that peak potash demand is more than double off-take, and on very low index soils extra potash beyond that removed should be applied.”