Ground Cover North : Ground Cover 049 April-May 2004 - North
FEATURE 23 APRIL 2004 THE NO-TILL DEBATE By BRAD COLLIS Fallowing has remained an integral part of cropping in the Mallee, long after the practice has disappeared from other grain growing areas, but changing established wisdoms takes a lot of science and a lot of convincing. Frequent ploughing to control weeds has, for example, been a logical way to conserve precious soil moisture – fewer growing plants, less soil water being used. Also, traditional cultivation and a long fallow is, at frst glance, more economic than minimum tillage. However, when the timeliness of operation is considered, the reduced compaction of the soil, the increased groundcover that is maintained, the reduction in erosion, the increased biological activity and the associated increased soil structure, then minimum tillage fallows are a good option. The obvious downside is that land under fallow, especially dry land, can blow away, like it did in March 2003. About one million tonnes of topsoil were stripped from the Mallee and turned into a spectacular dust storm that swept over south-eastern Australia – relocating millions of dollars worth of nitrogen and phosphorous paid for by farmers. However, the Mallee Sustainable Farming Project (MSFP) now has several years of research and crop trial results with which to demonstrate that fallowing is not required on all soil types, because it often does not achieve many of the benefts that have long been attributed to it. It is also starting to show that the higher costs associated with no-till systems is more than offset by improved yields and that the sandy soils should be treated differently to the loamy soils, that is, those soils with more than 10 percent clay content. It has also shown that fallowing sand or loamy sand has, in fact, little soil moisture beneft simply because sandy soils have an inherent inability to hold moisture, particularly during summer when evaporation can remove a large proportion of stored moisture. Figure 1 shows the amount of plant-available water in the top one metre of a sandy ‘mallee’ soil with 47mm of storage capacity and a loamy ‘belah’ soil with 135mm of storage capacity. Fallowing to store moisture will therefore have more beneft on the loam than the sand because there is a greater ability to store moisture for later crop use. These results indicate that loamy soils may be more suited to fallowing, but the trial results are only preliminary and are not conclusive on this issue. The research has clearly measured the relationship between the number of cultivations and wind erosion risk. On average, one to three cultivations, plus sowing, will put most paddocks at risk of erosion (Figure 2). The research has also dispelled the notion of mechanical long fallow being needed for nitrogen production and storage on sandy soils. Ploughing-in stubble was thought to be important for mineralising nitrogen by providing soil microbes, directly, with an essential carbon source. “Ploughing-in the stubble certainly creates nitrogen early in the fallow in sandy soils, but most of it is leached below the root zone by the time you sow the crop in average to above average rainfall years,” says Dr John Leys, a senior soil research scientist with the Department of Infrastructure, Planning and Natural Resources in NSW, and who is working with the MSFP. Because sandy soils hold less water, they also leach water and nutrients faster below the root zone. Figure 3 shows the nitrogen storage at sowing of a sandy soil that has been long fallowed with 98kg/ha and the same soil that was short fallowed with 137kg/ha. The short fallow stored more nitrogen because there was less leaching of the nitrogen during the summer. “You reduce the risk of nitrogen leaching and get much better nitrogen utilisation by the crop through the slow release that occurs by keeping the stubble on the ground. And that ground cover not only helps to retain more nitrogen, but reduces moisture evaporation from the soil.” These are some of the factors that growers in the Mallee region, which stretches from northern Victoria and south-west NSW into SA, are now being encouraged to consider. However, growers remain divided, with a wariness based on the cost of change and the perceived risks of any new system. Bruce Salau, who crops 1600 hectares north-east of Mildura, is one of many who feel there are still strong arguments for staying with established practices – proven weed control and cost. “I can see I would save a lot of time with no-till, and that it’s better for erosion control, but does it offer the same economy?” he asks. “Out here, for example, the dust factor is a problem for Roundup®. It acts as a barrier against the chemical working effciently and the paddy melons seem to grow a foot a day if you don’t stop them. “Also cultivation costs about $1 a hectare. Chemicals cost $2-$3 a hectare and to me they still have some long-term unknown consequences.” Bruce says he will continue to take an interest in the work of the MSFP because he does want to develop a more complete farming system: “But I need to see that no-till works before I risk the cost of switching to new methods and equipment.” GRDC RESEARCH CODE MFS 1, program 4 For more information: Dr John Leys, DIPNR, 02 6742 9505, firstname.lastname@example.org 0 20 40 60 80 100 25 20 15 10 5 0 0 20 40 60 80 100 05101520253035 0 1 2 3 4 5 6 0 102030 40 50 60 DEPTH m DEPTH m 1999 No. of Cultivations 2000 Available nitrogen kg/ha PAW mm EROSION RATE [g/m/s] Figure 2 Relationship between the number of cultivations and the erosion risk on the MSFP focus paddocks for two years. Note blade-ploughed field had 5 cultivations but low erosion. Figure 1 Plant-available water (PAW) profiles for 'belah' and 'mallee' soil. Figure 3 Available nitrogen profiles for two fallow lengths at the same Mallee site. Loamy 'belah' soil 135mm to 1m depth Blade plough Sandy 'mallee' soil 47mm to 1m depth Long Fallow = 98kg/ha Short Fallow = 137kg/ha To fallow or not to fallow Spreading the message: soil researcher Dr John Leys. Waiting to see if no- till works: grower Bruce Salau. CROPPING GUIDELINES The outcomes from MSFP research on core trial sites, plus focus farmer paddocks, have been used to draw up new guidelines for cropping in this region. These include: n As the number of cultivations used in the fallow increases, the risk of erosion increases (Figure 2). However, if groundcover and aggregation levels are low to start with, just one cultivation and sowing will put a paddock at risk of erosion. n Cultivation disrupts the stabilisation of aggregates formed through the actions of micro- organisms. Aggregates of larger size (more than 1mm) are less likely to be eroded. n On average, one to three cultivations plus sowing will put a paddock at risk of erosion, although the number of cultivations can be increased if blade ploughs or chisel ploughs with wide sweeps are used. n Summer weed control using herbicides and blade ploughs reduces erosion risk. n Fallowing in dry years stores minimal soil moisture. If there is no signifcant rainfall, there is little value in fallowing. n Sandy soils have low-moisture storage capacity. There is little value in fallowing before January on these soils. n Crops direct drilled into standing stubble retain soil moisture longer than those using cultivation systems. n Long fallowing means forgoing a possible crop or pasture and this needs to be considered when evaluating systems.
Ground Cover 050 June-July 2004 - North
Ground Cover 048 February-March 2004 - North