Managing subsoil constraints - crop and varietal tolerance, rotations, fallow management, crop responses

Introduction

Changing management for subsoil constrained paddocks relates to the severity of the constraint. Relatively minor changes (eg. altering varieties) may be appropriate on soils with minor constraints, whereas substantial changes, such as altering rotations and fallow lengths and possibly using ameliorants may be worthwhile on severely constrained soils. SIP08, a multi-agency project linked with several farming systems projects (Eastern, Western and Central Qld Farming Systems) is looking at a range of possible solutions on sites ranging from unconstrained to severely constrained throughout northwest NSW, central and southern Qld. The approaches being investigated can be grouped according to the following categories.

Crop species /variety adaptation to subsoil constrained soils

There are three broad factors that contribute to crop or varietal adaptation to subsoil constrained soils;

1. The amount of water that a crop needs, the rate at which it demands this water and the crop's water-use efficiency.

Crops/varieties that need less water, that have low and even rates of demand for water, and those that make more efficient use of water are likely to be favoured on constrained sites. Table 1 shows the difference in these features between 5 winter crop species grown in 2004 on unconstrained sites.

Table 1: Rates of water use between planting (mid May) and flowering-early grain fill (late August) for five winter crops in low-unconstrained sites around Goondiwindi, 2004. Water-use efficiency = yield/(soil water use + (rainfall × 0.66))

Results from a species and varieties trial on an unconstrained soil in northwest NSW in 2004 (Table 2) also showed species differences in soil water use, but no differences between the varieties chosen for the study even though there were some differences in observed rooting depth and grain yields. The growing season rainfall favoured longer season higher yielding varieties and the ranking of barley varieties showed this. The low yield for Wallaroi despite similar biomass as Bellaroi may be the result of frost.

Table 2: Species and variety responses at an unconstrained site in northwest NSW. Variety differences within a species are shown by the letter after the result; those with the same letter following were not significantly different.

Notes: The site had 172 mm in crop rainfall. Biomass and yield figures were from 2.7m 2 hand cuts. Root depths were observed by breaking apart intact soil cores. Soil water use is the difference between soil water at harvest and sowing. Soil test data from the site are given in Appendix 1. Grain yields from the pulses and oilseeds were not yet available at the time of writing.

2. The ability of the crop roots to extract water from hostile subsoils

Bread wheat, barley, canola and mustard all have a similar capacity to extract water from moderate-highly constrained soils (Tables 3 and 4). In contrast, durum wheat, chickpeas and field peas extracted significantly less water than wheat at three sites in southern Queensland and one site in northwest NSW in 2004. The trial in northwest NSW, in the same paddock as the one reported in Table 2, also showed significant differences in root depth and soil water use between varieties of bread wheat, barley, durum wheat, canola and mustard (Table 4). Differences in varietal tolerance to subsoil constraints may be less noticed where the constraint is extreme. Changing species or farming system may be more beneficial.

Table 3: The mean soil water use of five winter crops on three moderate-highly constrained soils in Southern Queensland in 2004. l.s.d = least significant difference; p < 0.05.

Notes: Crop yields (t/ha) via handcuts-highly constrained sites: wheat 0.95-1.25, barley 1.25-1.3, durum 0.8-1.1, chickpea 0.6, canola 0.4-0.5. Chickpea could not have been machine harvested due to height of the bush at either site. Crop yields (t/ha) via combine-moderately constrained site: wheat 2.1, barley 1.9, durum 1.4, chickpea 0.95, canola 0.55

Table 4: Species and variety responses at a constrained site in northwest NSW. Variety differences within a species are shown by the letter after the result; those with the same letter following were not significantly different.

Notes: The site had 172 mm in crop rainfall. Biomass and yield figures were from 2.7m 2 hand cuts. Root depths were observed by breaking apart intact soil cores. Soil water use is the difference between soil water at harvest and sowing. Soil test data from the site are given in Appendix 1. Grain yields from the pulses and oilseeds were not yet available at the time of writing. The constrained site had 21 mm more stored soil water between 30-110 cm at sowing than the unconstrained site owing to poor growth by the previous chickpea crop.

3.   Crop susceptibility to ion toxicities (eg. chloride, sodium).

Sodium toxicity, and K + :Na + imbalance, is particularly important in durum which, unlike bread wheat, has reduced capacity for sodium exclusion (Table 5). High levels of sodium and very low K + :Na + ratios relative to wheat have been observed in durum growing in saline-sodic soils in southern Queensland.

Ion toxicities are also very important in legumes, many of which have poor capacity for excluding salt from the shoot or compartmentalising salt in the vacuole. Symptoms of chloride toxicity, and toxic chloride concentrations have been observed in the leaf tissue of this crop growing on saline-sodic soils in southern Queensland (Table 5). As a consequence cereal crops growing on highly saline-sodic soils will produced diminished yields, but chickpeas can yield nothing.

Table 5: Leaf tissue (youngest mature blade) concentrations of chloride, sodium and the potassium:sodium ratio of five crops grown in the soil described in Appendix 2. Strong comparisons among species aren't warranted due to the likely different age of the leaf tissue at sampling-nevertheless the trends are similar to those established in the scientific literature.

Notes: Crop yields (t/ha) via handcuts: wheat 1.25, barley 1.25, durum 1.1, chickpea 0.6, canola 0.5. Chickpea could not have been machine harvested due to height of the bush.

Rotations

Exclude crops from paddocks, strips, or management zones that show marked intolerance of subsoil constraints. For example, while chickpeas provide an important rotation benefit, they are susceptible to subsoil salinity. In constrained areas, they could be replaced with a more tolerant legume where available, such as field pea, or an alternative rotation crop. Without these alternatives, sorghum should be used as a summer break to run down crown rot levels, rotate herbicide groups etc.

Fallow management

Alterations to fallow management are most appropriate in moderate to highly constrained soils. Basically, the smaller amount of soil water able used because of subsoil constraints, the quicker the soil will refill, providing infiltration is unimpeded. The profile will be full more frequently necessitating an increased cropping frequency. Moreover, the extra expense in maintaining long fallows is unlikely to be returned in extra yield from the following crop, with deep drainage also more likely. The increased crop frequency required on highly constrained soils is likely to have consequences on the use of residual herbicides in the cropping rotation.

Soils ain't soils.

It's tempting to think that subsoil constraints are the only problem here but really it's poor gross margins or return on capital that's the problem. Subsoil constraints are only one of many possible causes of this problem. Poor return on capital can be avoided by ensuring that the price paid for land is in line with its true productive capacity. Having a deep soil is no great benefit to cropping if some subsoil constraint is preventing your crops from using that depth.

Appendix 1: Site soil test data from the paired unconstrained and constrained species and variety trials in northwest NSW 2004.

Appendix 2: Soil characteristics of a saline-sodic soil on which a species trial was carried out in southern Queensland.

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