## 4 Discussion  
The N, P, and K fertilizer needs of crops can vary widely, and these variations in the responses of crops to N, P, and K fertilizers have been attributed to the differences in soil nutrient supply, crop N, P, and K use efficiency, and crop N, P, K demand [16; 21]. The accurate recommendation of N, P, and K fertilizers has important environmental implications by reducing residual N, P, and K, which are likely to move from the soil profile into ground or surface waters [16; 22]. The recommendation of fertilizers must balance the pollution risk of overuse to our ecological environment with the reduction output risk of under-application to the profitability of the farmers. As traditional experience-based fertilizer, N, P, and K recommendations are derived from the mass balance approach. The limited ability of experience-based N, P, and K recommendations in yield goal alone is weaker than N, P, and K recommendation systems based on models developed to accurately predict fertilization [22; 23]. Thus, an economically optimum N, P, and K rate can be obtained from formula fertilization via soil testing, creating a balance between yield goal and N, P, and K input, because it is based on fertilization model and soil tests, rather than depending on experience. Formula fertilization via soil testing aims to set down the scheme of fertilizers on the basis of the formula fertilization model of different crops and different soil types [23; 24; 25]. The method of formula fertilization mainly includes the nutrient balanced method, soil nutrient abundance or deficiency indices method and fertilizer response function method. The fertilizer response function method based on the fertilization field experiment objectively reflects the effects of a fertilizer under specific conditions with excellent feedback. The assessment of the N, P, and K fertilization response functions of crops is an important indicator for studying the effects of fertilizers [26]. In a previous study, it was revealed that cauliflower and broccoli responded well to optimum fertilizer levels in terms of both yield and quality [17; 22]. However, a high rate of N fertilization can produce excessive vegetative growth that delays maturity and harvest and reduces cotton lint yield and N uptake [17; 27]. Equally, the demand of the same production goal of the total nutrient amount from soil and fertilizer in any recommended field and response function trial field needed to be met. The model of recommended formula fertilization was established according to the above-mentioned assumptions. An appropriate fertilization scheme with an optimum fertilizer rate and an optimum N, P, and K proportion can be directly recommended, and the fertilization recommendations were able to be obtained [11; 22; 24]. [PERSON] et al. demonstrated that broccoli yields were highly relevant to the application of N fertilizers, and at high N content, yield was barely changed. [PERSON] and [PERSON] established correlation models between the N fertilization level and attribution for yield and the quality of broccoli curds, respectively. The yield of broccoli had a 'low-high-low' binominal curve with the increase of nitrogen level. They considered that an adequate N availability must be ensured in broccoli. Our conclusion was different from findings by [PERSON] et al., but it was consistent with [PERSON] and [PERSON]'s outcome [28; 29; 30; 31]. [PERSON] et al. found that the yield of broccoli significantly increased with the application of N and P fertilizers; however, there was no significant difference among different K fertilizer treat ments [32]. Our conclusion on the contribution of K fertilizer to broccoli yield is different, as we found that the efficiency of fertilization decreased with the increase of K fertilizer application. In the present study, the field experiments showed that adequate N, P, and K nutrients were important to the improvement of yield in broccoli.