Nutrient ratios in plant tissue have often been used as diagnostic tools with varying degrees of success depending on whether or not they have been misunderstood or misinterpreted. The meaning of a nutrient ratio is explored so that the relationship between yield and a given ratio can be clearly defined and understood. When a nutrient ratio has an optimal value, any yield is possible, the actual level being determined by the factors contributing to yield. When a ratio is too low, a response to the element in the numerator will be obtained if, indeed, it is limiting. If the element in the denominator is in fact excessive, a yield response may or may not be recorded depending on the level of other yield factors. When the ratio is too high, the reverse is true. Thus consideration of a nutrient ratio in isolation does not permit one to identify which of the two nutrients being considered is limiting yield. However, as the number of nutrient ratios considered simultaneously increases, identification of the nutrient(s) limiting yield becomes easier and more reliable.