Site specific management for nonpoint environmental protection involves a number of management practices. This chapter discusses how the application of good management on a site specific basis can optimize crop production while maintaining soil and water quality, and addresses its potential to protect against environmental degradation when compared with conventional uniform field management. Conservation tillage is considered the most promising management practice for keeping soil erosion at acceptable levels on rolling landscapes. Most landscapes have natural variations in the phosphorus content of the soil due to variations in parent material, weathering, and movement of water and sediment across the surface. Movement of pesticides through soil is dependent upon the solubility of the chemical, the resistance of the chemical to breakdown by microorganisms, the sorptivity of the chemical onto soil particles, the hydraulic properties of the soil, and the water regime. Vegetative barriers are effective in trapping sediment in toe slope positions along streams or drainageways.

Within-field nutrient variability causes some areas of a field to be more or less responsive to fertilization. The best soil sampling and fertilization strategies are those that best estimate and apply economic optimum fertilizer rates across a field. Although current site-specific management practices could achieve this goal, questions remain concerning the cost-effectiveness of alternative sampling strategies. This study compared various soil sampling schemes in eight fields using soil test P, K, pH, and organic matter. The schemes were grid sampling, sampling by digitized and detailed soil survey map units, sampling by elevation, and a targeted sampling based on various layers of information. The soil-test variability patterns varied markedly across the fields. The schemes varied greatly in reducing the within-unit variability and in the recommended fertilizer rate, but no scheme was superior across all fields and nutrients. The efficacy of all sampling schemes was lower for P and K, probably because of the larger impact of fertilization on small-scale variability. Over all fields, only schemes based on field averages or digitized soil survey maps resulted in significantly lower correctly fertilized areas than other schemes. For P, the most highly variable nutrient, the grid and targeted schemes usually were similarly effective, although the latter sometimes required fewer sampling units. Consideration of costs, field fertilization history, and likely response to fertilization is needed to select among various similarly effective sampling strategies.

Plant surface to be protected against fungal diseases differs on small-scale in he- terogeneous cereal crops. Measurements of several years in winter cereals showed a linear relationship between the parameter plant height and plant surface when ears are fully emerged. Small-scale information about the actual plant height within the cereal stand can be obtained by indirect measurement with the "pen- dulum-meter", a mechanical sensor to detect biomass. There was a high linear correlation between the deviation angle of the "pendulum-meter" and measure- ments of plant height along transects within heterogeneous cereal stands. To ob- tain plant surface per unit area to adjust application rates knowledge about stem density is necessary. But correlation between the deviation angle of the "pendu- lum-meter" and measurements of stem density at the transect sampling points was weak.