Cost of soil erosion revealed

Cost of soil erosion revealed

Cropping
Lucerne with little ground cover slow to recover (12 days post-rain) after storm rains, it is evident most of the water ran off.

Lucerne with little ground cover slow to recover (12 days post-rain) after storm rains, it is evident most of the water ran off.

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Crops or pastures require soils to capture often erratic and occasionally heavy rain for maximising production.

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Crops or pastures require soils to capture often erratic and occasionally heavy rain for maximising production. And good capture of rainfall is so often dependent on adequate ground cover.

Take for example recent measures of rainfall capture in pasture mixes at NSW Department of Primary Industries Tamworth Agricultural Institute. Rainfall between January 9 and February 20, was 237 millimetres. Soil conditions were dry following three years of drought and perennial pastures had dried down the soil profile.

Senior research scientist, Soil and Water R&D Unit Dr Sean Murphy, measured pastures high in lucerne captured only 15-17 per cent of that rainfall or 36-41mm of soil water. Lack of ground cover, leading to surface runoff and evaporation, mainly explains the poor capture of rainfall.

In contrast, pastures high in Premier digit grass, with moderate ground cover, were able to capture 53-61pc of the often-heavy rain events or 126-144mm of soil water. That's more than three to four times the rainfall capture and potentially represents 2.5-3.0 tonne a hectare in future dry matter production.

This photo of tropical grass taken on the same day as the lucerne above, shows far better capture of storm rains.

This photo of tropical grass taken on the same day as the lucerne above, shows far better capture of storm rains.

Over the six-weeks, pastures high in digit grass grew ~4t DM/ha, while those high in lucerne grew ~0.8-2t DM/ha. Premier digit, with higher ground cover, captured more rainfall in the soil water bank to sustain growth over the following weeks.

Even when well grazed down, a perennial like Premier digit has a mass of fibrous roots at ground level and commonly, if not totally flogged by grazing, has useful ground cover residues. The plant crowns of digit grass spread with pasture age, increasing ground cover and so improve rainfall capture.

Dr Murphy's data is not suggesting Premier digit is better than lucerne, or vice versa, but it is a great illustration of the importance of adequate ground cover for soil capture of rainfall.

These results mirror a lot of what has been experienced in many parts of the state this past summer, where often poor paddock capture of rainfall has occurred, with subsequent slow growth.

Cropping research has shown stubble retention via zero or no-till has been far more effective at capturing rainfall than cultivation, where residues often disappear quickly, especially over an average or greater rainfall summer.

Zero till with stubble retention on average stores around 35mm more soil water, with extra grain yield of around 1.0t/ha (cereal), provided crops are grown in rotation and fallow weeds are controlled.

Of greater significance is soil protection from ground cover retention. This past drought has seen some paddocks with no or very little ground cover repeatedly subject to often massive wind erosion events. Research has also measured water erosion can cause soil losses of over 100t/ha per event on sloping country with no ground cover.

Soil loss of 100t/ha can result in 100-200kg/ha loss of total nitrogen. Phosphorus losses can be as high as 150kg/ha per 100t/ha soil with around 7.5kg of it in the immediately available form. These losses contrast with relatively slow natural soil formation of as little as 1.5t/ha year.

Estimated starting ground cover, rainfall capture and herbage production data for a six-week cutting cycle from Jan 9 - Feb 20 at Tamworth Agricultural Institute.

Estimated starting ground cover, rainfall capture and herbage production data for a six-week cutting cycle from Jan 9 - Feb 20 at Tamworth Agricultural Institute.

Droughts, especially of long duration, tempt us to use that "bit of ground cover for feed". As an example, the last 0.5t/ha of available dual-purpose crop, pasture, or crop residue, may be worth $150/ha or so, but the cost in erosion and lost soil can commonly far excess that feed value.

An aspect of future drought management is the ability to assess ground cover and animal consumption rates. An example is 100, 300kg steers consuming 10kg of dry matter, per beast/day, or 1.0 t/day. They may trample a similar amount.

A 30ha paddock with 3.0t/ha dry matter, not growing, would reduce to 0.5t/ha in around 35 days. Such calculations provide warnings to either move to other feed, sell, supplementary feed (lengthen the time of sufficient cover) or place in a sacrifice area with full feeding.

Prograze courses run by NSW DPI or LLS have been a great way to upgrade skills in forage assessment and feed budgeting.

Next week: African lovegrass management when eradication is impossible.

  • Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
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