Facilitative and Competitive Nutrient Interactions with Changing Crop Spacing and Density in a Broccoli and Fava Intercropping System.

Intercropping, the growth of two or more crops simultaneously in the same field, is an agricultural management practice common in many parts of the world. Leguminous plants, such as fava beans (Vicia faba) are often used in intercropping systems, to increase soil nitrogen (N) content through rhizodeposition and phosphorous (P) solubility through root exudation of acids, thereby benefitting the companion crop. However, intercropping may also increase competition between crops. Therefore, exploring spacing between crops and legume density may reveal optimization thresholds. We performed a field study with an Alfisol in a Mediterranean climate on an intercropping system of fava bean and broccoli (Brassica oleracea var. italica) in order to (i) quantify the effect of spacing and density on broccoli yield and nutrient content and to (ii) link those effects to nutrient uptake and soil characteristics (e.g. pH, total N, and extractable P). We tested different spacing (10 and 15 cm) between fava and broccoli and varied fava density (2, 4, 6 fava). A broccoli monoculture was used as the control. Plant and soil samples were collected at maturity (59 days after transplanting). Sampling included broccoli heads, broccoli leaves, and rhizospheric soil in order to determine broccoli yield, measure broccoli nutrient concentrations, and identify potential changes in soil characteristics respectively. Results were analyzed using pair-wise t-tests to compare treatments to the control. Only the treatment with 4 fava spaced 10 cm from broccoli led to a significant increase in yield compared to the control (p < 0.04). A density of 2 fava did not provide enough facilitation and a density of 6 fava outcompeted the broccoli, therefore the intermediate fava treatment was the most optimal. Increased understanding of crop nutrient interactions can better inform future agricultural management to utilize biological processes in place of fertilizers, which are expensive economically and environmentally.