Development of infection by vesicular-arbuscular mycorrhiza (VAM) was studied in some field-grown crops. An infection plateau was reached within the first month after seedling emergence of spring barley, oats and peas. During the rest of the growth period the proportion of root length infected by VAM remained at about 50% in the cereals and 75% in the peas. In the spring, infection levels were low in winter wheat, winter rye and winter barley, and development of infection was slower in them than in the spring-sown crops.
VAM infection was also studied in relation to soil depth and root density. The proportion of root length infected decreased markedly below 40 cm soil depth. Root density varied greatly between crops, whereas the absolute length of infected roots was similar in all crops. This indicates that susceptibility to infection was independent of host species.
The results are discussed in relation to final phosphorus uptake and dry-matter production, and it is proposed that a relatively low soil-phosphorus availability was counterbalanced, to some extent, by a well-developed VAM infection.