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Several experiments on multispectral radiometer showed its suitability in driving nitrogen fertigation in tomato crop. Nir-Green light ratio describes crop nitrogen status well, highlighting element deficiency or excess, which is a great help to farmers in choosing timing and intensity of fertilizer application. The scientific literature reports several studies about nitrogen management only, but not phosphorus and potassium. Because of the advantage obtained with N, it would be desirable to also adapt it to phosphorus and potassium management. For this purpose, a preliminary trial was carried out on the omato cultivar Brigade grown in pots in a greenhouse. Four nutrient solution were supplied. Three were lacking in N, P, or K—the last had all elements needed for a balanced growth. Radiometer readings were taken once a week during the crop cycle, around noon. First results were encouraging. After some data elaboration, it appeared evident that, in some cases, it was possible to set the fertigation treatments apart by only having a look at the single wavelengths measured by the instrument. Through the Nir/green index, used in N management, phosphorus deficiency was identified as well. Potassium trend line was completely different from those of nitrogen and phosphorus, and very similar to that of the control. The utilization of the radiometer in handling potassium fertigation in tomato appeared somewhat difficult. Its application might be desirable, instead, for phosphorus fertigation in addition to nitrogen. The 560 and 710 nm wavelengths might be the especially more useful for this purpose, although a simple index or a combination of some simple indices able to identify phosphorous deficiency/excess and to screen them from those induced by nitrogen are needed.

Technology provides new tools for agriculture to be able to optimize fertilization. Optical instruments are becoming valid tools for farmers in making decisions about fertilization, even though they need to be calibrated for specific crops. Chlorophyll meters and multispectral radiometers have been tested on rice, corn, and wheat and afterwards on vegetables, in timing fertilization. Today, threshold lines that are able to detect crop N status in tomato crops are available. These thresholds, obtained in experiments carried out at Padova University, were validated in three open-field experiments. The first experiment was carried out in 2004 at the University experimental farm on tomato cv. Perfect Peel. The second and third experiments were conducted in a commercial farm at Codigoro (Ferrara) in 2004–2005. Tomato cultivars used were `UGX 822' and `Precocix' in 2004, in 2005 `Jet' was also used. In all trials, a “standard fertilization” management was compared with fertigation guided using SPAD and/or Cropscan. Optical tools were used to manage fertigation adopting both “threshold method” and “reference plot method”. In general “guided fertigation” resulted in less nitrogen application (N supply reduced between 18% and 45%), especially when “threshold method” was adopted. Yields were comparable to “standard fertilization” treatments, showing a better efficiency of “guided fertigation”. In some cases, guiding fertigation by means of optical instruments allowed higher fruit fresh weight, although dry matter content and °Brix were not influenced. Guided fertigation reduced also the number of damaged fruit and the percentage of nonmarketable product.