Bok choy and Napa crops show a larger response to nitrogen (N) fertilizer than lettuce responding up to 250 lbs N/acre at low soil nitrate-N levels (Figure 3-6-1). The total amount of in-plant accumulation of N is typically less than 225 lbs N/acre, and the amounts removed at harvest are typically less than 160 lbs N/acre (Table 3-6-1). 

The N uptake through thinning and cultivation is a modest 10 lbs/acre or less. Thus, early season N application may be required only if pre-plant soil test nitrate-N levels are less than 10 ppm and pre-plant monoammonium phosphate (MAP) application rates are under 200 lbs/acre (see section 2.1). Between 10-40 lbs of N/acre can be applied through pre-plant broadcast applications or by sprinkler fertigation for the period spanning germination and emergence.

The main seasonal N fertilizer application occurs immediately after thinning and integrated into cultivation operations. Current guidelines call for side-dress N fertilization when soil nitrate-N test levels fall under 30 ppm. The required application rate is the difference between the minimum required threshold of 30 ppm and the soil nitrate-N test level. Approximately 4 lbs of N/acre needs to be added to raise the soil test nitrate-N by1 ppm. Late season N as a furrow water run might be applied if soil test nitrate-N levels are less than 20 ppm. 

Bok choy and Napa typically show response to phosphorous (P) fertilizer up to an Olsen P soils test levels of 30 ppm (Table 3-7-2). Band or strip application of P allows for reducing P rates 25-35% compared to broadcast fertilization.

Bok choy and Napa have not shown yield responses to potassium (K) fertilizers in Arizona. However, soil K levels should be occasionally monitored to ensure AA soil test K levels remain above the threshold 150 ppm (Section 2.3). Plant tissue testing can also be used to monitor K nutritional status, and levels that constantly fall below 3.5% might indicate deficiency (Section 4.3). 

The only soil micronutrient that may need to be occasionally applied as fertilizer for the production of bok choy and Napa in Arizona is zinc (Zn). The fertilizer management guidelines for Zn, which were developed based on DTPA soil tests, are summarized in Section 2.5 and are applicable to all vegetable crops grown in the region. Plant tissue analysis for Zn levels can also be used to assess plant nutritional status (Section 4.3). For bok choy and Napa crops, leaf Zn concentrations of 20 ppm or greater indicate sufficiency.