Nutrient Management Research Database

Subsurface drip irrigation (SDI), because it can precisely deliver nutrients close to plant roots, could lead to carefully determined applications of fertilizer to meet crop needs and less risk of nitrate (NO3 −) leaching to groundwater. Appropriate fertilizer applications, however, depend on an accurate assessment of the spatial distribution of the main plant macronutrients (N, P and K) in the soil profile before planting. To develop nutrient sampling guidelines, we determined the spatial distributions of preplant nitrate (NO3 −), bicarbonate extractable phosphorus (Olsen-P) and exchangeable potassium (K) in the top 20 inches (50 centimeters) of subsurface drip irrigated processing tomato fields in three of the main growing regions in the Central Valley of California. Nutrient distribution varied with depth (P and K), distance from the center of the bed (NO3 −) and growing region (NO3 − and K). No depletion of NO3 −, Olsen-P or K in the root feeding areas close to the drip tape was detected. Preplant NO3 − ranged considerably, from 45 to 438 pounds N per acre (50 to 491 kilograms/hectare), the higher levels in fields with consecutive crops of tomatoes. A sampling protocol that growers could use, developed from analysis of the distribution results, provided reliable estimates of preplant NO3 − as well as P and K in all surveyed fields.

Residual soil nitrate levels offer a source of N even before fertilizer is applied. However, it is important to have an accurate picture of the distribution of soil nitrate in the field before crafting a fertilization plan.

This study investigated the required sampling to accurately measure residual soil nitrate, also measuring soil Olsen-P and potassium levels.

Sixteen sites were included in the study, split between Yolo, San Joaquin, and Fresno Counties.

Nitrate Results

Preplant soil nitrate levels ranged from 45-438 lbs N/ac, and were higher in San Joaquin and Fresno than in Yolo. The residual nitrate levels were related to the number of years the field had been continuously cropped in tomatoes.

The whole plant N uptake was 247 lbs/ac on average, while the residual N plus fertilization ranged between been 209 to 758 lbs N/ac. This suggests that many fields could sustain yields with reduced N fertilization.

Phosphorus Results

No clear differences were observed between P levels across sampling regions.

Majority of fields had P values lower than 15 ppm within the threshold windows (12-20 ppm) where yield increases are likely to result from fertilization. 

Potassium

Potassium levels were also consistent across regions.

Most fields K levels were well above 130-150 ppm threshold of expected yield increase with fertilization. However, K fertilization can increase fruit quality even when K is not limiting for crop yield.

Sampling Protocol

This study allowed for the formulation of a testing protocol that allows for accurate measurement of residual N in the top 20 inches of soil, while minimizing the number of samples.

For fields with 80 inch beds (all cores taken to 20 inch depth)

• Two cores taken at 15 and 30 inches from the bed center, or two cores taken at 20 and 25 inches from the bed center.
• In at least 4 different locations within each field.

For fields with 60 inch beds (all cores taken to 20 inch depth)

• Three cores taken at 5, 10, and 20 inches or 5, 20 and 25 inches from the bed center.
• In at least 3 different locations within each field.