Nutrient Management Research Database

Rising costs and air quality regulations have created interest in California’s San Joaquin Valley (SJV) in production systems that reduce tillage operations and soil disturbance. From 1999 to 2009, we evaluated conventional (CT) and reduced tillage (RT)
systems for a cotton (Gossypium hirsutum L.)/tomato (Solanum lycopersicon Mill.) rotation with (CC) and without (NO) cover crops in a Panoche clay loam soil (fine-loamy, mixed, superactive, thermic Typic Haplocambid) in Five Points, CA, in terms of
yield, soil C, and the NRCS soil conditioning index (SCI).  The RT reduced tractor operations by 50% for tomato and 40% for cotton. Cover cropping produced 38.7 t ha–1 of biomass. Tomato yields were 9.5% higher in RT vs. CT systems and 5.7% higher in NO vs. CC systems.  The CT cotton yields were 10.0% higher than RT yields and 4.8% higher in NO systems, but yield patterns were not consistent from 2005 to 2009. Soil C content was uniform (0–30-cm depth) in 1999 (19.72 t ha–1) and increased in all systems in 2007 (t ha–1): RTCC 29.11, CTCC 26.36, RTNO, 24.09, and CTNO 22.65. Soil C content of RT and CT systems did not differ, but was greater in CC than in NO systems. In the 0- to 15-cm depth, RT increased soil C, indicating stratification, and also increased C in the occluded light and mineral fractions.  The SCI was positive for RT treatments, predicting a soil C increase, and negative for CT systems, predicting a soil C decline, but measured soil C content increased in all systems. Results show that RT maintains or increases yields relative to CT, and CC stores more soil C than NO.

This field comparison was conducted over 10 years (1999-2009) in tomato and corn rotations both under conservation and standard tillage and with and without cover crops at the UC West Side Research Extension Center in Five Points, CA.

• Management treatments were: Conventional tillage without cover crop (CTNO), conventional tillage with cover crop (CTCC), reduced tillage without cover crop (RTNO), and reduced tillage with cover crop (RTCC).  Each treatment was replicated 4 times in a randomized complete block design.
• In RT treatments, tractor traffic was restricted to the furrows, no tillage was done following tomatoes and before the cotton crop, just 2 passes were done following cotton, and crop residues were left on the soil surface
• A cover crop mixture of Juan triticale, Merced ryegrain and common vetch was planted at a rate of 89.2 kg ha^-1 in advance of winter rains.  The cover crops were chopped in mid-March using a Rolling Stalk Chopper, disked in in the conventional treatments and sprayed with a glyphosate solution after chopping in the reduced tillage treatments.
• Tomato variety SJV, 8892, was transplanted in the first week of April each year, and RoundUp Ready transgenic upland cotton variety Riata and Flex Pima varieties were grown.
• Soil C and N, bulk density, particulate soil C fractions (free light, occluded and mineral), the NRCS Soil Conditioning Index and the soil tillage intensity index (STIR) and estimates of fuel use were assessed.  

• Tractor trips were reduced by approximately 50% for tomato and 40% for cotton in RT treatments compared to CT.
• Tomato yields:
  • For 4 yrs of the study RT led to increased yields when compared to CT, while in other years yields were similar.
  • Cover crops (CC) led to decreased yields when compared to plots with no cover crops (NO) in 3 years, while in other years no differences between CC and NO were observed.
• Cotton yields:
  • Yields in cotton were not consistently different between treatments
• Soil carbon and bulk density:
  • RT treatments lead to an increase in bulk density in the 15-30cm depth.
  • Soil carbon (0-30cm) increases for all treatments, with the larges increases occurring in treatments with cover crops.
• Soil Conditioning Index (SCI) and and Soil Tillage Intensity Index (STIR)
  • SCI was positive in RT treatments and negative for the two CT systems, suggesting that soil organic matter was predicted to increase.
  • STIR values were lower for RT, pointing towards potentially lower C losses from soil to the atmosphere and higher water infiltration rates.