Nonerodible Aggregates and Concentration of Fats, Waxes, and Oils in Soils as Related to Wheat Straw Mulch

Nonerodible (>0.84 mm) soil aggregates and concentration of fats, waxes and oils in the surface 5-cm depth were determined as related to (i) rates of initial straw mulch, (ii) date of initial fallow tillage, (iii) removal of straw, and (iv) nitrogen fertilization. Nonerodible soil aggregates were also determined as affected by tillage implements commonly used in fallow with residue removed by burning and with residue present. Increasing the amount of wheat (Triticum aestivum L.) straw mulch from 1,680 to 3,360 kg/ha on the soil surface at the start of fallow increased nonerodible aggregation by 5% and fats, waxes, and oils in the soil by 0.019 mg/g of soil. Delaying initial fallow tillage after harvest until spring increased nonerodible aggregates by 3% and fats, waxes, and oils in the soil by 0.052 mg/g of soil. Without N, mulched soil contained 7% more nonerodible aggregates and 0.010 mg more fats, waxes, and oils/g of soil than did bare soil. All implements tested, except the oneway disk, increased initial nonerodible aggregation by 2 to 7% when wheat stubble had been removed prior to the tillage operation. The oneway disk decreased nonerodible aggregation 4%. During subsequent tillage, all implements decreased nonerodible aggregation except the chisel. Successive operation with the oneway disk decreased aggregation the most at 9%/operation. Nonerodible soil aggregates at time of winter wheat seeding (Sept.) and the following spring were highest with a fallow tillage sequence of tandem disk, blade twice, and rodweed with flat chisel points twice. This sequence destroyed 67% of the initial residue. Nonerodible aggregation was lowest when the tandem disk was used three times, followed by rodweeder with flat chisel points twice. These operations destroyed 90% of the original residue. Overwinter decrease in nonerodible aggregation was smallest when only the blade and rodweeder with flat chisel points were used.