Composting, Tillage, and Rotation Effects On Soil Fertility and Physical Properties of Cultivated Soils.

Reduced crop yields and potato tuber quality are suggested to be related to intensive cultivation in a two year potato/grain rotation system in southeast Idaho.  Intensive cultivation has reduced fertility and increased soil bulk density because of decreased soil organic matter (SOM).  Application of soil amendments (e.g., compost and manure) can improve yield potential by increasing soil fertility and improving physical properties.  The objective of this study is to determine the effects of crop rotation tillage, and composting on SOM, nitrogen (N), phosphorus (P), yield, and tuber quality.  The rotations consisted of four year wheat, wheat, alfalfa, and potatoes rotation and a two year wheat and potato rotation.   Treatment rotations were conventional tillage, composted with tillage, minimum tillage (tillage only before and after potatoes), and a control (two year rotation with intensive cultivation).  Compost source was steer feedlot manure (11.2 g kg^-1 N and 7.0 g kg ^-1 P) and was fall applied at the rate of 11.2 Mg ha^-1.  Research plots were located on a Blackfoot silt loam and a Pocatello variant silt loam near Rexburg, Idaho.  Bulk density and soil fertility samples were taken in the spring and after harvest and compared to native soils from the original year which included undisturbed soils from the same soil type as the research plots.  Native soils had a bulk density of 1.14 g/cm³ which was lower than the plot bulk density of 1.31 g cm^-3.   Within the plots, the composted treatment had a lower bulk density (1.28 g cm^-3) then the conventional tillage and minimum tillage (1.33 g cm^-3 for both). No statistical differences were observed for crop yield and quality among treatments.  Soil tests showed that SOM in the research plots 19.3g kg^-1 and 43.3g kg^-1 for the native soils. Comparison of organic C between 2007 and 2008 showed an increase in SOM for all treatments with largest increase in the minimum tillage treatment (3.2 g kg^-1 increase).  The increase in SOM for all treatments may be a result of previous crops (vegetable garden) and the SOM is adjusting to a new equilibrium.  Preplant NO₃^- samples from 2007 to 2009 were similar (average 32 kg N ha^-1) for the conventional tillage, minimum tillage, and compost treatments, but preplant NO₃^‑ decreased by 20 kg N ha^‑1 for the two year rotation (control treatment). These results suggest that a four year rotation is more sustainable than the current two year rotation regardless of tillage or composting practice.  Although, it may take more time for the minimum tillage and composting treatments to show difference as suggest by previous research.