One of the functions of a healthy soil is the recycling of nutrient for plant uptake, and as soil becomes more efficient at holding onto nutrients the requirement for costly fertiliser input reduces.
The ability of soils to retain applied nutrient is based on a number of factors with good physical soil structures perhaps the most important, and physical soil structures can be measured accurately using Graham Shepherds Visual Soil Assessment developed here in New Zealand.
When the physical structure is ideal, or somewhere close, beneficial soil organisms are able to breathe, carbon is sequestered and humus is rapidly developed.
Humus is what is left when dead roots, dung and other litter on the soil surface is digested by soil organisms that breathe air. It’s stable and able to store large quantities of moisture and nutrient.
Humus is sometimes referred to as the ‘glue’ in the soil and, because without humus soils cease to exist, it is regarded by many as more valuable than gold.
Regardless, soils containing the most humus have the greatest growing potential and it is unwise to do anything that destroys it or limits its development.
There are two factors that are universally accepted as contributors to the degradation of soil and therefore limit the development of humus.
They are excessive downward pressure by animal’s feet and fertiliser nitrogen. A fair question is how much of each is excessive? By going to extremes the answer can start to be obtained, however as always it will depend on a number of factors.
Cows creating mud behind wires in wet weather destroy humus, or at the very least limit its development. Air is squeezed from the soil and valuable top soil exposed with a subsequent loss of carbon and pasture production while natural repair takes place.
How much nitrogen is too much? Ask a dozen people and a dozen different answers are likely depending on the knowledge and experience of those asked, and whether or not they feel they have an immediate need for it.
What we do know from twelve months of results from Nitrate Nitrogen leaching work carried out near Edgecumbe by Rotorua Lakes and Land Trust is that there is measurably less Nitrate N being leaked under carefully managed permanent rye and white clover pasture using less than 25kgN/ha annually than under soils where pasture growth is driven by regular urea applications.
The farm applying less than 25kgN/ha has grown in excess of 18 tonne of DM/ha in each of the last two seasons and over 1,465kg of milk solids per hectare has been produced in each of those seasons.
This is at least twenty per cent more pasture than a typical N driven programme, with almost all of the extra feed produced from mid-October until mid-May.
Based on Ministry for the Environment information the Nitrate N levels contained in leachate from this property are no more than natural background levels.
Where Nitrate N levels are low it is likely that the loss of other nutrients is also low.
There is no down side to a healthy well structured, biologically active soil. More feed of higher quality is produced during the main growing part of the season. More of all nutrients is retained in the soil and made available for plant growth, with less being lost to groundwater.
Creating and maintaining a soil capable of producing in excess of 18 tonne of DM every year requires an understanding of plant nutrient requirements as well as astute and thoughtful daily management.
Two key nutrient inputs for high performance are calcium and magnesium, with dolomite from Golden Bay being the magnesium product of choice when the focus is on maintaining excellent physical soil structures, and nurturing animals capable of outstanding performance with very low ill-health costs.