Effect of cropping on the physical fertility of krasnozems

Abstract

The physical properties of krasnozems and euchrozems in Australia have received little attention due to the widespread perception that they are 'good' soils. We present data from three locations which are representative of these soil types in the South Burnett region of southern Queensland. At each location, paired sites were chosen where continuously cropped and uncropped land existed side by side. The effect of continuous cropping on physical fertility, relative to that of soil in virgin condition, is assessed and related to published data from other locations where available. Continuous cropping has reduced infiltration rates from 80 mm h-1 or more to values as low as 25 mm h-1. Originally low bulk densities ranging from 0.6 Mg m-3 at the surface to 1.3 Mg m-3 at 70 cm depth have been increased to corresponding values of 1.0 and 1.4 Mg m-3. Local maxima of 1.3 Mg m-3 occur between 15 and 30 cm depth. Penetration resistance has been increased markedly at low water contents as a result of increased cohesion, with an uncropped soil having values <2.5 MPa over the available water content range comapred with values of >6 MPa in the continuously cropped soil. Organic carbon in the surface soil has been reduced from 4-5% to 1% and aggregate stability under rain has been reduced, the % aggregates <0.125 mm diameter increasing from 10% to 20% or more. Soil water storage has been reduced together with crop water extraction, with these effects attributed to a loss of water storage pores in the top 30-50 cm of the profile and restricted root development. The problems of rehabilitating these soils are briefly discussed. Continuous cropping under dryland conditions has greatly reduced the physical fertility of krasnozem and euchrozem soils and made them more susceptible to variation in seasonal conditions. Available data suggest degradation is also occurring on krasnozems used for intensive vegetable production. Methods of rehabilitating physical fertility of degraded krasnozems are required, as are farming systems which are more sustainable in the long term.

Rapid internal drainage rates in Ferrosols

“…The 0.25 m layers held significantly more water near saturation and significantly less water towards PWP than the soil from the 1.50 m layer, with the 0.75 m layer intermediate. The theoretical available water content (AWC) for these Red Ferrosols has been defined as water held between suctions of 100 cm H 2 O and 15 × 10 3 cm H 2 O (Bridge and Bell 1994). AWC declined with depth, ranging from 0.14 m 3 /m 3 at 0.25 m, to 0.10 and 0.06 m 3 /m 3 at 0.75 and 1.50 m, respectively.…”

“…Infiltration was also measured using modified disc permeameters (Perroux and White 1988), with supply potentials of −4, −3, −2, and −1 cm H 2 O applied through a single contact sand pad (Bridge and Bell 1994). Measurements were made at 0.10-0.15 m depth in order to avoid large surface porosity caused by recent tillage or soil fauna.…”

“…Four replicate undisturbed soil cores, each 0.10 m diameter by 0.05 m long, were collected from 3 distinct layers in the soil profiles under direct drill and conventional till at the Goodger site, described by Bridge and Bell (1994) and Bell and Bridge (2001). These layers were at 0.25 m (immediately below the tilled zone), 0.75 m (at the bottom of the zone in which effects of vehicle traffic could be detected), and at 1.50 m (the bottom of the effective root-zone of most crop species).…”

“…One common characteristic of these soils after longterm cropping is a loss of the original very high rainfall infiltration capacity-a key factor in successful rain-fed cropping (Bridge and Bell 1994). This has been attributed to 2 factors: firstly, a decline in labile organic carbon resulting in surface crusting, and secondly, subsoil compaction that reduces the hydraulic conductivity down the profile (Bell et al 1997(Bell et al , 1998.…”

“…Unfortunately, subsequent experimental crops were unable to capitalise on the improved infiltration rates because of the relatively low available water capacity, around 0.1 cm 3 /cm 3 (Bridge and Bell 1994;Bell et al 2001). Soil moisture monitoring in the abovementioned experimental crops confirmed that after large rainfall events, infiltrated water moved to depths of at least 0.90 m much more quickly in a pasture ley/zero till treatment than continuous crop/conventional till.…”