Teagasc research scientist Dr. Karen Daly has confirmed that soil nutrients can be found in different forms – some are plant available, others are not.
Speaking on the Tillage Edge podcast she explained that it is the plant available fraction of the nutrient, whether it be phosphorous (P), potassium (K) magnesium (Mg) or sulphur (S), that is of specific interest.
“The available fraction is what the plant can see. All of the soil tests currently available are chemical in nature,” Dr. Daly said.
“All are trying to extract out that very discrete fraction of the soil’s nutrient content that is directly available to plants.
“In order to assess the total amount of a nutrient, the first step is to totally digest the soil. The reality is that all nutrients, e.g., P, are available in both inorganic and organic forms,” she added.
“The same principle holds for potassium, calcium, sulphur and magnesium.”
According to Daly, many fractions of a particular plant nutrient are not very water soluble nor are they readily plant available.
“Total P, e.g., is a function of many different fractions. There is the organic fraction, the inorganic fraction, the stable fraction and the pant available fraction,” she said.
“But for agronomic soil P testing, it is the plant available fraction that is of direct interest.”
So how accurate are soil test results? According to Daly, no test is going to be universal across all soil types.
“There will always be a couple of wild card soils, for which a test will not be very proficient,” she continued.
“Morgan’s P works really well on neutral and slightly acidic soils. However, for soils with a pH above eight, it isn’t all that reliable.
“Morgan’s P is also less reliable for peaty soil; in other words those containing 20 or more of organic matter.
“In these instances there will be a lot of interference from the organic fraction,” Daly explained.
The Mehlich test is also recognised by the Department of Agriculture, Food and and Marine (DAFM).
The strength here lies in its ability to determine the availability of up to 10 metals in soils. Significantly, this list includes aluminium.
“Aluminium has the potential to bind phosphorous, particularly under acidic conditions,” commented Daly.
“It can be useful to get a sense of this possible interaction taking place within a specific soil type.
“At the present time we are trying to work out a relationship between Morgan’s P and Mehlich, in order to convert one from the other.”