Mike Bell and Phil Moody look at the importance of soil"s chemical fertility to its long-term health
Department of Primary
Industries and Fisheries principal
agronomist and project
leader Mike Bell.]
The current interest in soil health in Australian agricultural systems has tended to focus solidly on soil biota and the interactions between biota and management practices (for example tillage and crop rotation). However it is important not to overlook the fact that soil chemical fertility (both the rate at which chemical properties change and the management practices employed to counter these changes) is just as important to long-term soil health.
Nutrient balances, fertiliser application strategies, rates of acidification and subsequent amelioration and soil organic matter management are key components of soil chemical fertility. Development and implementation of management practices to address chemical fertility issues will have long-term implications for sustainable land management.
The northern grains region is dominated by alkaline heavy clay (Vertosol) soils, although there are smaller but important areas of Kandosols in the west and Red Ferrosols in the east and far north.
The high seasonal rainfall variability in the region (while countered somewhat by good water-holding capacity in some deeper Vertosols), combined with often (originally) high starting soil fertility, has presented particular problems for the economically sustainable maintenance of chemical fertility in these systems.
The following are examples for rainfed cropping systems on the Darling Downs and the inland Burnett regions of southern Queensland.
The inland Burnett and nearby Darling Downs provide interesting contrasts in cropping systems and subsequent challenges to maintaining sustainable nutrient balances.
The neutral-alkaline pH, high background fertility and cereal grain-dominated cropping systems of the Darling Downs have resulted in an overwhelming focus on nitrogen fertility, although in more recent years responses to phosphorus and zinc inputs have become important in some areas.
Farm records of fertiliser inputs and grain removal over the past 40 to 45 years have allowed tentative nutrient budgets to be undertaken for this system, with the example of sorghum shown in Figure 1.
Figure 1. Case study of nutrient balance for sorghum crops on the Darling Downs each decade from 1960
Figure 2. Case study of nutrient balance for peanut/maize cropping in the inland Burnett from 1984 to 2000
Sorghum yields have increased dramatically from an average of one tonne per hectare in the decade of the 1960s to more than 6.5 tonnes a hectare since 2000. During this period nitrogen inputs have risen from nil to more than 130kg/ha, phosphorus inputs have risen from nil to about 7kg/ha but have stalled over the past 15 years and zinc is now a common component of phosphorus fertiliser blends.
However, despite these large increases in inputs, the nutrient balance has only recently approached neutrality for nitrogen and zinc (not shown), while becoming increasingly negative for other major nutrients.
In contrast, growers cropping the acidic and lowerfertility Red Ferrosols near Kingaroy have a better nutrient program in the typical peanut-maize rotation, although cations (molecules carrying a positive charge) are unbalanced with lime applications to counter soil acidification resulting in positive calcium balances, while inadequate potassium or non-existent magnesium inputs leaving other cations with a negative balance. The net nitrogen input from the peanut crop helps supply nitrogen to a following maize crop, although a conservative approach to nitrogen application in the maize crop means overall nitrogen inputs still need to rise.
The widespread adoption of conservation tillage systems on the Darling Downs, while improving fallow moisture efficiency and contributing to more reliable crop production, has imposed limitations on nutrient application strategies. In these systems nitrogen is commonly applied prior to planting (as much as three months earlier in some cases) and phosphorus and zinc applied with the seed at planting. The early commitment to nitrogen inputs increases the risk of leaching losses, although denitrification risk is minimised by incorporating the nitrogen in bands. If additional nitrogen is needed in good years, options are generally limited to in-crop broadcasting with associated risks of gaseous losses.
Basal fertiliser (including a small amount of additional nitrogen) is often applied with the seed at planting, although the amounts are limited by the negative impact of some fertilisers on crop establishment.
The wide row spacings of crops like sorghum (onemetre rows, or wider in skip-row plantings) exacerbate this problem. Side banding fertiliser away from the row, either at planting or later in crop growth, is not common due to the additional tyne/disc requirements (or additional fertilising operation), and the risk that sidebanded fertiliser will be marooned in dry surface soil without follow-up rain.
The Red Ferrosols of the inland Burnett are still largely cropped with conventional tillage (mainly due to a need for loose soil to maximise peanut recovery at harvest), although zonal tillage systems are being increasingly adopted in recent years. This adds a deal of flexibility to nutrient management options.
The riskier cropping system, with increased reliance on in-crop rain due to lower soil water storage and rapid internal drainage during wet periods, has resulted in only low-risk fertilisers (like potassium) and soil amendments (like lime) being applied well in advance of sowing. Basal fertilisers are applied in bands below and to the side of the row at planting and side-banding additional nitrogen is commonly used in the maize crop in good years.
Both cropping systems face the problem of stratification of less mobile nutrients like phosphorus and potassium (and zinc) and ameliorants like lime in often-dry topsoil layers. While surface layers are enriched, depletion of nutrient reserves occurs at depth due to irregular rainfall and a great reliance on deep roots to access water and nutrients. The continued use of tillage helps to address this problem in the Red Ferrosols, although the move to zonal tillage could have negative implications. There are real issues emerging in some Vertosols under direct drill, with problems further confounded by taking soil samples for nutrient testing from the enriched 0 to 10cm layer. The nutritional status of deeper layers may be more relevant to crops living largely on stored soil moisture.
The continuing decline in soil organic matter, even under direct-drill systems, is perhaps the greatest concern for soil biological activity and soil health in cropping soils of the northern region, with the adoption of periodic pasture leys and direct drill cropping the only practical way of reversing this trend. Therefore, while the conventional tillage afforded more flexible nutrient application strategies in Red Ferrosol systems, the resulting decline in soil carbon stores will have an overall negative impact on soil biology.
However, the high legume crop frequency and subsequent reduced reliance on nitrogen fertiliser inputs in Ferrosol systems will offer some benefits to soil biology.
For more information: Mike Bell, 07 4160 0730, email@example.com
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