Studies in soil fertility with special reference to organic manures. II. Plant growth and nutrition in the field.

Abstract

This paper reports a study in crop physiology in which an analysis of plant growth response and nutrient intake under irrigation in the field is used to interpret the effects of rice hulls as an organic manure. An exploratory experiment showed that relatively speaking the main effects of treatment took place quite early in the growth of the crop. For this reason the main experiment was virtually restricted to these early growth stages. The test plant in both experiments was the tomato. It was found that rice hulls can influence the growth of the crop in at least three ways. Seedling emergence may be delayed, early growth may be retarded by temporary shortage of available nitrogen, and growth may be stimulated by increase in available phosphorus. All three effects are modified by the addition of sulphate of ammonia, but in all cases the result is an increase in yield with rice-hull treatment. Mean seedling emergence for the six experimental treatments had a range of 2.8 days, and it is shown that the mean weights of 18-day-old seedlings could have been accounted for by these differences alone. The pattern of response at 18 days remains for at least five weeks and is not without its effect on the final yields. In spite of their low nitrogen content, rice hulls alone had only a transitory adverse effect on the nitrogen status of the soil. It is suggested that this may not have had any direct effect on the rate of dry-matter production, but that there was a temporary deflection of assimilates to favour root growth at the expense of shoot growth. Such an effect would, of course, have an adverse effect on fruit yield. Plant analyses suggest very considerable increases in available soil phosphorus due to rice-hull treatment. Much of this phosphorus appears to have been fixed during the course of the experiment, but Truog values obtained after the experiment indicated that significant effects of treatment were still present. The increases in Available phosphorus were less when sulphate of ammonia was added with the rice hulls.