Reinhardt H. Howeler

Practical aspects of mycorrhizal technology in some tropical crops and pastures

Greenhouse and field experiments were conducted on the effect of VA mycorrhiza (VAM) on the growth of cassava, various tropical grass and legume species, as well as beans, coffee and tea. A large number of VAM fungal species were evaluated for effectivity in increasing cassava growth and P uptake in acid low-P soils. The effectivity of VAM species and isolates was highly variable and dependent on soil pH and fertilizer applications, as well as on soil temperature and humidity. Two species, Glomus manihotis and Entrophospora colombiana were found to be most effective for a range of crops and pastures, at low pH and at a wide range of N, P, and K levels.

Accumulation and distribution of dry matter and nutrients during a 12-month growth cycle of cassava

Abstract Two cassava ( Manihot esculenta Crantz.) cultivars were planted in fertilized and non-fertilized plots to determine the accumulation and distribution of dry matter and nutrients in the plant and the effect on soil fertility. At monthly intervals eight plants were harvested and separated into four parts, the upper, middle and lower thirds of the aerial growth, and the roots, while each aerial part was divided into leafblades, petioles, and stem. These samples were analyzed for dry matter (DM) and nutrient content. Soil samples were also taken at monthly intervals and analyzed. Results indicated that in cassava DM accumulation was slow during the first 2 months, increased rapidly during the next 4 months and slowed down during the final 6 months as DM production was partly offset by leaf fall. At harvest (12 months) DM was present mainly in roots, followed by stem, leaves and petioles. Nutrient concentrations varied between plant parts and with the age of both the tissue and the plant. The concentration of most nutrients was highest between 2 and 3 months and then declined. Each nutrient had its own particular concentration profile within the plant. Thus, N, P and S concentrations tended to be high in the leaves, while those of K, Ca and Mg were high in petioles and stem; the Mn concentration was particularly high in petioles, whereas Fe concentration was low in petioles but high in roots. B and Cu were evently distributed within the plant. Most nutrients accumulated initially in leaves and stem, but were translocated to roots in the latter part of the growth cycle. Only Ca, Mg and Mn accumulated more in stems than roots. Removal of accumulated nutrients in the root harvest can seriously deplete nutrient reserves in the soil, particularly that of K, and to a lesser extent for N. Soil pH, exchangeable Al, Ca, Mg as well as available P were not greatly affected by plant growth, while exchangeable K decreased markedly during the crop cycle.

NUTRIENT UPTAKE AND SOIL EROSION LOSSES IN CASSAVA AND SIX OTHER CROPS IN A PSAMMENT IN EASTERN THAILAND

Abstract Total nutrient uptake and nutrients removed in harvested plant parts were determined for cassava grown for either root or forage production, maize, sorghum, peanut, mungbean, pineapple and sugarcane. All crops were grown in replicated plots on 7% slope on a sandy loam soil in Sri Racha, Thailand, during a 4 1/2-year period. Erosion losses associated with each crop were also determined by weighing at monthly intervals the soil sediments that had collected in plastic covered channels at the bottom of each plot. Cassava for root production had the lowest total uptake of major nutrients of all crops except mungbean. The amounts of N and P removed in the harvested plant parts were also much lower than those removed by other crops, while the amount of K removed by cassava was similar to other crops but much lower than pineapple or cassava grown for forage. This latter crop had a very high nutrient uptake and removal, especially that of N, K, Ca and Mg. On an annual basis, soil losses due to erosion were highest in cassava grown for roots, followed by cassava for forage, sugarcane, mungbean, sorghum, peanut, maize and pineapple. Thus, when cassava is grown for root production on slopes, it is likely to cause more erosion than most other crops due to its wide spacing and slow initial canopy development. Cassava farmers should therefore be encouraged to use special management practices that reduce erosion. However, it is unlikely that cassava causes soil degradation by depleting the soil nutrient supply, as N and P removal in the harvested part of the plant was actually lower than, and K removal was similar to that of other crops tested.

Identifying plants adaptable to low pH conditions

Many screening methodologies have been developed for identifying acid-soil tolerant genotypes of the major foodcrops and pasture species, both in the temperate regions and in the tropics. These include rapid screening tests in nutrient solutions and in soils in the greenhouse, as well as field screening techniques. The methodologies are described and their relative advantages and disadvantages discussed. Different formulas and graphical methods are presented for classifying germplasm into different categories of Al-tolerance, depending on specific selection objectives. Finally, recent progress in identifying Al-tolerant germplasm of wheat, barley, maize, sorghum, rice, beans, cowpea, soybeans, peanut, sweet potato and cassava is discussed.

Response of cassava to VA mycorrhizal inoculation and phosphorus application in greenhouse and field experiments

Cassava (Manihot esculenta Crantz) was grown in the greenhouse and in the field at different levels of phosphorus applied, with or without inoculation with VA mycorrhiza in sterilized or unsterilized soil. When grown in a sterilized soil to which eight levels of P had been applied the non-inoculated plants required the application of 3200 kg P ha−1 to reach near-maximum yield of plant dry matter (DM) at 3 months. Inoculated plants, however, showed only a minor response to applied P. Mycorrhizal inoculation in the P check increased top growth over 80 fold and total P uptake over 100 fold.Relating dry matter produced to the available P concentration in the soil (Bray II), a critical level of 15 ppm P was obtained for mycorrhizal and 190 ppm P for non-mycorrhizal plants. This indicates that the determination of critical levels of P in the soil is highly dependent on the degree of mycorrhizal infection of the root system. In a second greenhouse trial with two sterilized and non-sterilized soils it was found that in both sterilized soils, inoculation was most effective at intermediate levels of applied P resulting in a 15–30 fold increase in DM at 100 kg P ha−1. In the unsterilized soil inoculation had no significant effect in the quilichao soil, but increased DM over 3 fold in the Carimagua soil, indicating that the latter had a native mycorrhizal population less effective than the former.When cassava was grown in the field in plots with 11 levels of P applied, uninoculated plants grown in sterilized soil remained extremely P deficient for 4–5 months after which they recuperated through mycorrhizal infection from unsterilized borders or subsoil. Still, after 11 months inoculation had increased root yields by 40%. In the non-sterilized soil inoculation had no significant effect as the introduced strain was equally as effective as the native mycorrhizal population.These trials indicate that cassava is extremely dependent on an effective mycorrhizal association for normal growth in low-P soils, but that in most natural soils this association is rapidly established and inoculation of cassava in the field can only be effective in soils with a low quantity and quality of native mycorrhiza. In that case, plants should be inoculated with highly effective strains.

Short-and long-term fertility trials in Colombia to determine the nutrient requirements of cassava

One-year simple NPK trials were conducted in 22 locations in four regions of Colombia to determine the response of cassava (Manihot esculenta Crantz) to N, P and K and to relate this response to the available P and K contents of the soil, as well as to the N, P, and K concentrations in youngest fully expanded leaf (YFEL) blades sampled at 3–5 months after planting. It was found that cassava responded mainly to P applications in the low-P soils of the Eastern Plains and of Cauca Department, to K applications only in the Eastern Plains and to N applications principally in the sandy, low-OM soils of the Atlantic Coast. By relating the relative response to P and K to the available P and exchangeable K content of the soil, respectively, critical levels of 4 mg P/kg and 0.17 me K/100 g (both extracted with Bray II) were determined. Similarly, critical levels of 5.6% N, 0.41% P and 1.42% K were determined in YFEL-blades.A long-term fertilizer trial with 35 treatments was conducted for 8 consecutive years in the highly acid low-fertility soil at CIAT-Quilichao, to determine the effect of cassava production and fertilization on soil fertility. Although the first year response was mainly to P and N, already in the second year K became the most limiting nutrient and the importance of K fertilization increased over the years. This trial, and similar long-term fertility trials conducted in Colombia and in various parts of Asia, have all shown the importance of adequate annual applications of K for maintaining high yields of continuously grown cassava.

Influence of species of VA mycorrhizal fungi on cassava yield response to phosphorus fertilization

SummaryA three different sites with acid soils in Colombia field trials with cassava were monitored for frequency of VA mycorrhiza. Increasing levels of phosphorus (P) from 0 to 200 kg P/ha had been applied. The fields differed in the composition of species of VA mycorrhizal fungi. At all sites infections of the roots by the total mycorrhizal population decreased with increasing P fertilization, but at two sites the relative frequency and activity of one species,Glomus manihotis, increased with increasing P applications. This species was only present at two sites, and only in these sites a cassava yield response to up to 200 kg P was found. The differential activity of fungal species was confirmed in greenhouse trials, whereEntrophospora colombiana was found to be most effective at 50 kg P andG. manihotis, at 200 kg P.

Potentials and limitations of mycorrhizal inoculation illustrated by experiments with field-grown cassava

SummaryField inoculation trials with cassava (Manihot esculenta Crantz) were conducted in Quilichao (typic Dystropept soil) and Carimagua (Haplustox soil). In Quilichao, with a large and effective native VA-mycorrhizal (VAM) population, inoculation withGlomus manihotis did not increase cassava yields significantly, neither when different sources and levels of inoculum material were used, nor with different cassava cultivars, or after stabilizing soil temperature through mulching. Field inoculation did result in a decrease of the coefficient of variation with respect to yield. The high dependency of cassava on an effective VAM association was indicated by a marked decrease in yield after eradication of native VAM by soil sterilization. In Carimagua, with a lower native VAM population, mycorrhizal inoculation withG. manihotis increased yields significantly at intermediate levels of 100 kg/ha of applied P, using either inoculum of cassava orPanicum maximum roots or inoculum of a soil-root mixture of maize or tropical kudzu. Higher or lower levels of P decreased the effect of inoculation on yield. There were no significant differences among P sources, ranging from highly soluble triple superphosphate to low solubility rock phosphates. Inoculation with different VAM isolates had a variable effect on cassava yields, and showed that there may be an interaction between P fertilizer level and isolate efficiency. It is concluded that there may be a potential to increase yields or decrease the fertilizer P requirements of cassava through field inoculation with effective VAM isolates, in the vast areas of acid infertile Oxisols and Ultisols with low native VAM fungal populations, represented by Carimagua.

Micronutrient deficiencies and toxicities of cassava plants grown in nutrient solutions. I. Critical tissue concentrations

Abstract The programmed nutrient addition technique was used in a series of 5 experiments to determine the response in growth and micronutrient content of cassava (Manihot esaulenta Crantz) cv. M Aus 10, to 8 supply levels of boron, copper, iron, manganese and zinc respectively. The experiments were of 9 weeks duration and utilized 22 litre pots of nutrient solution. The supply levels for each micronutrient covered the range from severe deficiency to toxicity. Critical tissue concentrations for deficiencies determined by relating total dry matter production to the nutrient concentration in the youngest fully expanded leaf blades were (μg/g): boron 35, copper 6, manganese 50, and zinc 30. Likewise, critical concentrations for toxicities in the same index tissue were (μg/g): boron 100, copper 15, manganese 250, and zinc 120. In the iron experiment, the data were too variable to allow precise determination of critical concentrations for deficiency and toxicity. Critical micronutrient concentrations in the pe...

Application of the flowing soulution culture techniques to studies involving mycorrhizas

SummaryA technique to study mycorrhizal effects on growth and P-uptake of cassava (Manihot escultenta, Crantz) grown in flowing solution culture is described. Phosphorus concentrations were carefully maintained constant at 0.1, 1, 10 and 100 μM by daily analyses and adjustment of the nutrient solutions. Inoculation with mycorrhizal roots hada positiveeffect on P content of plant tissue and/or plant growth only at the two lowest P-concentrations in soulution. These concentrations are two to three orders of magnitude lower than those normally used in conventional nutrient solution cultures.