Not all spatially variable properties of soil and plants are really essential for the in-field site-specific plant production. Only those quantities with high variability and great influence on economic returns should be considered seriously, like the availability of the nutrients or the population of weed and pest. Practicable solutions for on-line sensing of nutrients and state of plants or soil have not been introduced yet, proposals are under discussion. To monitor the state of the soil regarding tillage or fertilizing, in general, one has to evaluate a set of composite quantities. It seems that sensing for organic matter content by optical methods or for nitrate by ion-selective electrodes might be sufficient for a first order evaluation and can be introduced into the practice in future. Measurements like tractive power or pressure in horizontally moved penetrometers are applicable for on-the-go monitoring of dynamic soil properties. Machine vision may be used for the evaluation of the quality of tillage and plants. The application of biocides depends on the state of plants, which means imaging of shape and colour, a question principally solved, but really cost-effective systems are not available yet.