Assessing crop N status of fertigated vegetable crops using plant and soil monitoring techniques

Evaluation of crop N status will assist optimal N management of intensive vegetable production. Simple procedures for monitoring crop N status such as petiole sap [NO ₃^--N], leaf N content and soil solution [NO ₃^-] were evaluated with indeterminate tomato and muskmelon. Their sensitivity to assess crop N status throughout each crop was evaluated using linear regression analysis against nitrogen nutrition index (NNI) and crop N content. NNI is the ratio between the actual and the critical crop N contents (critical N content is the minimum N content necessary to achieve maximum growth), and is an established indicator of crop N status. Nutrient solutions with four different N concentrations (treatments N1-N4) were applied throughout each crop. Average applied N concentrations were 1, 5, 13 and 22 mmol L^-1 in tomato, and 2, 7, 13 and 21 mmol L^-1 in muskmelon. Respective rates of N were 23, 147, 421 and 672 kg N ha^-1 in tomato, and 28, 124, 245 and 380 kg N ha^-1 in muskmelon. For each N treatment in each crop, petiole sap [NO ₃^--N] was relatively constant throughout the crop. During both crops, there were very significant (P < 0.001) linear relationships between both petiole sap [NO ₃^--N] and leaf N content with NNI and with crop N content. In indeterminate tomato, petiole sap [NO ₃^--N] was very strongly linearly related to NNI (R² = 0.88-0.95, P < 0.001) with very similar slope and intercept values on all dates. Very similar relationships were obtained from published data of processing tomato. A single linear regression (R² = 0.77, P < 0.001) described the relationship between sap [NO ₃^--N] and NNI for both indeterminate and processing tomato, each grown under very different conditions. A single sap [NO ₃^--N] sufficiency value of 1050 mg N L^-1 was subsequently derived for optimal crop N nutrition (at NNI = 1) of tomato grown under different conditions. In muskmelon, petiole sap [NO ₃^--N] was strongly linearly related to NNI (R² = 0.75 - 0.88, P < 0.001) with very similar slope and intercept values for much of the crop (44-72 DAT, days after transplanting). A single linear relationship between sap [NO ₃^--N] and NNI (R² = 0.77, P < 0.001) was derived for this period, but sap sufficiency values could not be derived for muskmelon as NNI values were >1. Relationships between petiole sap [NO ₃^--N] with crop N content, and leaf N content with both NNI and crop N content had variable slopes and intercept values during the indeterminate tomato and the muskmelon crops. Soil solution [NO ₃^-] in the root zone was not a sensitive indicator of crop N status. Of the three systems examined for monitoring crop/soil N status, petiole sap [NO ₃^--N] is suggested to be the most useful because of its sensitivity to crop N status and because it can be rapidly analysed on the farm.