Determining Potassium (K+) Release, Crop Availability and Uptake from Three Red Algal (Rhodophyta) Species.

Abstract

As a result of increasing fertilizer costs associated with rising oil prices, many growers in the Pacific region have become interested in locally available resources that can be used as low cost inputs to improve crop health and productivity. In recent years, use of algae and their extracts have gained in popularity due to their potential use in sustainable farming and may be used as an alternate to synthetic fertilizers. In Hawaii efforts are underway to control the most commonly found invasive algae species; Eucheuma denticulatum, Gracilaria salicornia, and Kappaphycus alvarezii. These are dominant invasive, non-indigenous species on Hawaiian reefs. The average potassium (K+) dry matter content in these species is around 14 -20%. Although these seaweeds were used in the past for crop production, not much is understood about application rates, yield performance, species efficacy for crop nutrition, nutrient release pattern and mechanisms of release. To address these gaps, greenhouse and laboratory experiments were conducted to: 1) determine the efficiency of the cardy meter as a rapid, low-cost tool for evaluating the tissue K status of pak choi from fresh sap, 2) describe effect of three invasive algae species on growth (yield) and K nutrition (tissue K+) of pak choi at different K fertilizer rates, 3) to evaluate algae biomass as a replacement for K+ synthetic fertilizers, 4) compare the two buffers to minimize ionic interference using a K+ selective electrode, and 5) to understand the K+ nutrient release pattern and mechanics of these three species. Our data show that the invasive algae species increased the yield and growth of pak choi and the response was greater when algae was applied to provide K+ at the rate of 224-284 kg.ha-1. Results of the algae comparison with synthetic K+ showed no significant differences between them for yield; e.g. Plants grown with K. alvarezii, KCl and KNO3 had an average dry weight of 7.5 g when K+ was provided at 280 kg.ha-1. Only K+ rates, not K+ source were significant for yields and tissue K concentrations in all trials. Although the values between rapid electrode and ICP measured tissue K+

values were well correlated, ICP was identified as the preferred method of K+ quantification in tissue. However, in soil solution, ion strength adjustment buffers (ISAB) were able to significantly mask the other ionic interferences when measuring K+ and with a selective electrode. No notable difference was observed among the two buffers, NaClO4 and NaCl (slope near 1) with a strong correlation (r =0.99). This is the first report of such a comparison in the literature. Based on the results from the leachate study, we conclude that the total amount of K+ released from algae was lower than synthetic K fertilizers applied at equivalent rates of K+. Results from the both the whole plant and polysaccharides leachate studies show that there is a difference in K release pattern among the algae with the cumulative recovery of K+ from both whole plant G. salicornia and extracted agar were significantly lower (10-12%) than whole plant K. alvarezii and extracted carrageenan at both K application rates. These differences in release among algal species may be explained in part by differences in cell wall composition, chemical properties including sulfate groups differences and physical properties such as higher gel strength of Gracilaria agar. To our knowledge, this work is the first detailed report of these red algae species as significant sources of K+. This is also the first report in the literature of the less expensive NaCl buffer being an effective substitute for NaClO4, allowing for cost effective, high-throughput analysis of soil solution K+. We propose that carrageenan containing species may be more efficient sources of K+ than those containing agar, but future studies are needed to elucidate the mechanisms for these differences. Our study may also provide the established protocols for further investigation of the functional properties of naturally extracted agar and carrageenan from these species and at different locations.