What on earth do vets know about soils?
... Well, certainly not as much as we know about animals, and we only know a bit about them! Which is why we recently had Dr Alec Mackay from AgResearch present two of our Monitor Farm groups. Alec leads the Soils and Land Use group at AgResearch, gave us a very clear reminder of the basis of our soils' productive capacity, and answered lots of questions - maybe some that you have too - so we thought it worth summarising part of his presentation here.
Nitrogen
- In general, the main nutrient driving pasture growth is nitrogen (N) - supplied by clover in the sward
- However clover-derived N sets a ceiling on total pasture grown; at AgResearch's Ballantrae hill country research unit, where extra N was added year-round to make it non-limiting, pasture production went from about 12.5T to nearly 20T per annum
- Pasture growth responses to individual applications of N are not dependant on good phosphorus levels
Phosphorous
- The High Fertility/Low Fertility Farmlets study at Ballantrae (1972-2004) demonstrated the value of phosphorus (P) to our legume-based farming systems:
o Low Fert - 125kg super/ha/yr: Olsen P 10; 10su/Ha; pasture production 10T/yr
o Hi Fert - 625kg super/ha/yr for 5 years then 375kg/ha/yr since (+lime as
required): Olsen P 55; 16su/Ha, pasture production 14T/yr
o Obviously the Hi Fert system described here would not be pursued on a
conventional hill country farm; however, in the Hi Fert system it was measured
that: - Pasture production started earlier in spring by several weeks and continued longer into the late autumn
- There was a significant increase in the percentage of white clover and ryegrass
- All slopes and aspects were grazed more evenly
- Less need for breeding stock to ‘tidy up' pastures
- There were more than double the number of earthworms in the High Fert soils: more litter to feed on, acidity controlled by liming and sheep-only system (no treading damage from heavy cattle)
- As a comparison, another farmlet was set up where fertiliser was completely withheld; this farmlet has become unmanageable as stock will not graze the slopes and these are reverting to woody weeds
The above points demonstrate the value in maintaining a strong focus on good P levels where a clover-based pasture system is in place
What affects P availability in the soil?
- Increased soil microbial activity does not lead to increased P levels
- Most applied P remains in the topsoil layer; it does not leach
- 75% of maintenance P requirements are needed not to replace P exported off the farm as product, but to ‘replace' P redistributed by grazing animals via dung and urine!
- Soils that ‘retain' P do so because of the inorganic reactions within the soil; there is no evidence to support any way of ‘releasing' this tightly-held P; Alec:"If you want a legume system you have to maintain or increase P levels in the soil"
- Recent work shows soil which has been compacted via treading/heavy cropping has a higher optimum Olsen P - around 40 for intensively farmed sites on ash soils studied so far
Sulphur
- Also required by clover
- Needs to be applied annually
- Applied sulphur is not available to the plant until it is mineralised in the organic fraction of the soil
- Once sulphur is mineralised it can also leach from the soil
- In years where P is not applied, there is not likely to be value in applying sulphur only in the hope of driving some clover growth - need to apply both
Alec also presented more information on soil testing, other measures of soil fertility, the use of ‘humates' on soils under pasture, and soil biology.
To read more, see the newsletter from the B+LNZ Manawatu Finishing Farms visit to AgResearch, available on the ‘Monitor Farms' section of the Totally Vets website.
