More soil fertility myths

Fert Options
with Robin Boom
Agronomic Advisory Services

Continuing from last month’s column, here are some more myths that are commonly espoused by people pedalling products and ideas to farmers.

Myth 5: The chloride in Muriate of Potash is harmful to the soil.

High amounts of chlorine will sterilise soil media, but chloride and chlorine are different animals, one being a solid and the other being liquid or gas. Plants actually need small amounts of chloride, and pasture dry matter is typically around one per cent chloride, with more chloride taken up than phosphorus. Many critics of muriate of potash (potassium chloride) are advocates of sea salt, which has a higher percentage of chloride. And the most common element in the sea apart from hydrogen and oxygen is chloride, yet the sea is teaming with micro-organisms in an environment many times richer in chloride than what we get when applying potassium chloride as a fertiliser source. Some ‘biological’ farmers and growers even get sea water and spray it onto their soils and crops, believing it provides beneficial properties for their soil, plants and animals.

Myth 6: All of the nutrients plants and animals require are naturally there in the soil.

While it may be true that all 16 elements plants and animals require to grow are naturally found in all soils, most soils will have at least some of these in such small quantities that their shortage will be limiting potential productivity, and making plants and animals more vulnerable to ill-thrift and disease. This is why soil and tissue testing is so important as the extractants used in the laboratory reveal the amounts of critical elements in the soil growing media, and whether these levels are sufficient for production. Many lawns that have been mown for decades, and which haven’t received any fertiliser over this time, will still have all elements plants need to grow present in the soil. However such lawns are only likely to be growing 3-5 tonnes of dry matter compared to fertilised pasture on the same soil which could be growing 12-15 tonnes of dry matter.

Myth 7: All you need is good soil biology.

It is true that soil organisms – earthworms, nematodes, bacteria, fungi, protozoa and other microbes – are an important part of our agricultural and horticultural production systems. In more intensive systems their numbers can be compromised from over-cultivation or the overuse of harsh chemical pesticides, fertilisers, herbicides and fungicides. However, it is soil chemistry which has a far greater impact on production as can be seen in hydroponics where plants can be grown in sterile water infused with soluble chemicals, with no earthworms or microbes are present. Having a highly fertile and sustainable soil is dependent on its physical, chemical and biological characteristics and how we manage and nurture these will determine its future productive capacity.

Myth 8: Artificial fertilisers are bad for the soil long-term.

Two fertilisers often given a bad rap are superphosphate and urea, which also happen to be the two biggest sellers in terms of volume sold in New Zealand. Although urea provides no long-term benefit for the soil, causing harm in terms of increased acidification and nutrient loss, as well as being an environmental contaminant, it is the cheapest source of artificial nitrogen that farmers and growers can use to boost production when nitrogen deficiency occurs. Although not necessary in pastoral situations where clovers should provide the necessary nitrogen, in cropping and horticulture applying nitrogen at certain times will have a significant impact on crop production. Superphosphate on the other hand does provide long-term benefit to the soil as a source of phosphorus, calcium and sulphur when these elements are found to be limiting. At the old Ballantrae Research Station in the Manawatu, where annual applications of superphosphate were applied over 35 years compared to where nothing was applied, the amount of pasture grown and number of earthworms found were much higher on the superphosphate treatments compared to the zero-fertiliser treatments, where woody weeds were encroaching and taking over. Phosphorus is a contaminant in our waterways, and although superphosphate is 85-90 per cent water-soluble, there are alternative sources such as serpentine super and the new Surephos fertiliser from Ballance that have less than one-third of water-soluble phosphorus, as well as dicalcium phosphate and RPR fertilisers which can contain no water-soluble phosphorus and can be considered in areas where excess phosphorus run-off or leaching is of environmental concern.

Robin Boom, CPAg, member of the Institute of Professional Soil Scientists. Phone: 0274448764.


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