**2.1 Growth-limiting factors**

Field trials to document practices are conducted under the assumption that all factors but the ones being varied are equal or at optimum levels. Liebscher's law of the optimum stated that "a production factor which is in minimum supply contributes more to production, the closer other production factors are to their optimum" [8]. The law of the maximum aimed to optimize controllable factors given the impossibility to modify factors that are not controllable in the present state of knowledge and technology [21]. A provisionary list of growth-impacting factors is provided in **Table 1**.


**Table 1.**

*Partial list of noncontrollable and partially controllable growth-limiting factors [21, 22].*

Nutrient interactions impact crop yield through synergism, antagonism, dilution, excess, toxicity or crosstalks. Nutrient interactions are addressed as pairwise ratios [23]. Nutrient crosstalks occur where change in sulfur availability alter tissue compositions of micronutrients [24]. An extreme case of nutrient excess is toxicity where vital processes are affected. In field experiments, synergism is also viewed as positive interaction occurring where plant response is greater by combining two nutrients than from individual effects [25]. A list of nutrient interactions is presented in **Table 2**.

Face to the formidable task to optimize tens of growth-limiting factors and myriads of factor interactions, most of them being unknown, each case under study could rather be viewed as unique combinations of factors. For successful cases in the neighborhood, most factors are equal except those impacting the performance of defective specimens, facilitating side-by-side comparisons.


#### **Table 2.**

*Nutrient interactions in soils and plant tissues [23–29].*

*Machine Learning, Compositional and Fractal Models to Diagnose Soil Quality and Plant… DOI: http://dx.doi.org/10.5772/intechopen.98896*
