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 reduced with little incorporation (Walter et at. 1987). Soluble nutrients leaching from crop residue may
also increase soil fertility (Barisas et al. 1978; Smith et al. 1974). Losses of dissolved nutrients are a
function to the total runoff volume. Unless increased nutrient concentrations in conservation tillage are
not offset be decreases in runoff volume, dissolved nutrients flux will increase. Conservation tillage and
nutrient management combined should reduce dissolved nutrient losses to a level approaching
conventional tillage.
Terraces. Three types of terraces, practice 600, are considered for their effects on nutrient
losses: gradient with full flow outlets, and level storage with or without an underground outlet (SCS
1988). The primary mechanism for the control of nutrients is deposition of sediment-attached nutrients.
Terraces can have a detrimental effect if they convey runoff directly to surface waters or if nitrate
leaching becomes a problem.
Midrange decreases in surface runoff are 20 to 50% for terraces with underground outlets.
Increases in deep percolation are more variable and can range from 5 to well over several hundred
percent (SCS 1988).
Although terraces require extensive soil disturbance and are expensive to build, they reduce
transport of eroded soil and sediment-attached nutrients from fields. Gradient terraces increase
infiltration and percolation and decrease water runoff and they transport greater amounts of sediment-
attached nutrients compared to storage terraces. Level terraces can trap up to 95% of sediment-
attached nutrients and from 30 to 90% of dissolved nutrients (SCS 1988).
Decreases in dissolved nitrogen concentrations from terraces compared to corn grown on a
contour were 14-50% and sediment-attached nitrogen concentration losses were decreased 7-13%
(Baker 1985). Level terraces can reduce total nitrogen loadings by 85% (NWQEP 1982b) . Terraces
are recommended as nitrogen controls where no potential ground water problems exist.
Total phosphorus losses can be reduced 67% by terraces as compared to contour farming
(NWQEP 1982b) . Dissolved phosphorus losses can decrease by about one-third on a concentration
basis (Baker, 1985) and from 40-60% on a mass basis (SCS 1985). Sediment-attached phosphorus
concentration losses were reduced up to one-third (Baker 1985) and nearly
100% (SCS 1988).
Irrigation System Furrow Improvements. Nutrient losses from irrigation return flows
continue to be a serious problem. BMPs to reduce furrow erosion also reduce export of sediment-
attached nutrients. Unless meteorological factors dominate, farmers have greater control of system
hydrology (e.g. irrigation scheduling and rates) and return flow quality compared to farmers in
nonirrigated humid areas.
4.2-11
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