The structure of soil has a strong influence on the quantity of pasture grown from now until rain arrives in autumn.
After a wet winter, in areas where treading damage has been unavoidable, the soil may have become a little compacted with the bulk of permanent pasture roots concentrated in the top 10–12cm.
Where this is the case pasture growth over summer and early autumn will be less than optimum and come in fits and starts depending on the frequency and quantity of rain.
After 20mm of warm rain plant growth may be very rapid only to slow rapidly with the onset of two or three days of warm windy weather. This puts pressure on pasture management with some operators opting for an open-gate policy on the basis that it may as well be grazed before it turns brown and disappears.
Not all farmers have the same philosophy as well-structured soils allow pasture plants to access moisture from half a metre, or even deeper, resulting in more even growth. Well-structured soils also have the apparent ability to develop a crust reducing the loss of moisture to the atmosphere while still allowing moisture to be utilised from a lower depth.
A recent field trip to neighbouring dairying properties provided a graphic display of the degree to which soils with different management inputs vary in structure, especially with respect to the amount of granular aggregate and resultant rooting depth of clovers and grasses.
On the property with excellent structure, the outstanding soil characteristic was the amount of aggregate, particularly fine crumb. The quantity of fine feeding root was also impressive. There was little sign of root congregating near the surface with an almost even spread of root mass to the bottom of a 25cm spade. The roots went to a much greater depth, all quite fine and white indicating healthy development over winter.
As the rule of thumb is ‘weight below the ground in root equals weight above the ground in leaf’, the property with the better textured soil will grow measurably more total feed throughout the remainder of the growing season.
It was obvious that the property where soil structures were tighter and root development was closer to the surface, that heavy rain over the last few months had collected in hollows. Soil in that condition has less moisture holding capacity further limiting potential for summer growth.
One of the differences between the two visited properties has been the use of dolomite. The property with the well-structured soil has, for close to twenty years, had dolomite applied as the annual input of magnesium, and a portion of the 200kg/ha of calcium.
Dolomite, based on long-term field work, is the most effective magnesium fertiliser available and suitable for application to all soils where, based on soil test data and known usage, there is a need for magnesium.
On ‘heavy’ soils with a high clay or silt content, dolomite helps the formation of initially large aggregates increasing macro pore space, speeding the rate at which excess moisture is able to drain. On ‘lighter’ soils containing more sand dolomite also helps build aggregate by holding groups of particles together increasing moisture holding capacity.
An annual application of dolomite at the recommended rate provides at least twelve months supply of magnesium and due to the fineness of grinding the response time under good growing conditions is rapid. Performance is unmatched, it’s local,