Background
Agricultural production is often limited by low phosphorus (P) availability. In developing countries, which have limited access to P fertiliser, there is a need to develop plants that are more efficient at low soil P. In fertilised and intensive systems, P-efficient plants are required to minimise inefficient use of P-inputs and to reduce potential for loss of P to the environment.

Plant and microbial strategies to improve the phosphorus efficiency of agriculture

Gypsum is a relatively common mineral that is widely available in agricultural areas and has a number of specialized agronomic uses, principally as a Ca source on legumes and as a soil conditioner on sodic soils. Recent research has suggest that the utility of gypsum may extend to a greater range of soils and crops than previously acknowledged. The purpose of this review is to document both current practices and potential new uses of gypsum in agriculture, in order to establish the state of the art in current gypsum use, and to point out needed research in this important area.

Use of Gypsum on Soils: A Review

A review of studies conducted in a range of plant species indicated the importance of an adequate supply of P during early crop growth and outlined plant adaptations for accessing early season P. Potential implications of the requirement for early season P in the development of management practices to optimize P supply for crop production were also discussed. Phosphorus plays a critical role in energy reactions in the plant. Deficits can influence essentially all energy requiring processes in plant metabolism. Phosphorus stress early in the growing season can restrict crop growth, which can carry through to reduce final crop yield. Deficiencies during early growth generally have a greater negative influence on crop productivity than P restrictions imposed later in growth. Plants respond to P deficiencies by adaptations that increase the likelihood of producing some viable seed. The adaptations increase the ability of the plant to access and accumulate P and include modification of rhizosphere pH, diversio...

The importance of early season phosphorus nutrition

Suboptimal phosphorus availability is a primary constraint for terrestrial plant growth and crop productivity. Root hairs are subcellular extensions from the root epidermis that play an important role in the uptake of immobile nutrients such as phosphorus by increasing soil exploration. The objective of this study was to identify quantitative trait loci for root hair length and plasticity in response to phosphorus stress in maize. Using a cigar roll culture system in a controlled environment, root traits including root hair length, tap root length, root thickness, and root biomass were evaluated in 169 recombinant inbred lines derived from a cross between B73 and Mo17. These parents have contrasting adaptation to low phosphorus availability in the field. The parents segregated for the length of individual root hairs under low phosphorus. Average root hair length (RHL) of RI lines ranged from 0.6 to 3.5 mm with an average of 2.0 mm under fertile conditions, and RHL was increased from 0% to 185% under phosphorus stress. Using composite interval mapping with a LOD threshold of 3.27, one QTL was associated with RHL plasticity, three QTL with RHL under high fertility, and one QTL with root hair length under low phosphorus. These QTL accounted for 12.7%, 31.9%, and 9.6% of phenotypic variation, respectively. No QTL were detected for taproot thickness and root biomass. Six QTL were associated with 53.1% of the total variation for seed phosphorus in the population. Root biomass plasticity was significantly correlated with RHL induced by low phosphorus, taproot length plasticity, and seed phosphorus reserves. Our results suggest that genetic variation in root hair length and plasticity may be an appropriate target for marker aided selection to improve the phosphorus efficiency of maize.

Mapping of QTL controlling root hair length in maize (Zea mays L.) under phosphorus deficiency

The early growth of wheat is slow compared with that of barley and triticale. This is expected to limit the yield of wheat in environments where greater seedling vigour is advantageous. To overcome the slow growth of wheat, genetic sources of seedling vigour are required for use in breeding programs, and/or ways to increase seedling growth by manipulating seed characteristics. This study reports (i) new sources of and 3 heights at cutting (cutting was done when the grass reached 0.5, 1, and 1.5 m above the ground). The N fertiliser treatment did not yield any significant difference in DMD, ND, or IVDMD. Height at cutting had a significant (P < 0.05) effect on rumen DMD and ND and their degradability characteristics for all incubation times. There was a reduction in DMD and ND and their degradability characteristics as plant height increased at cutting. Similarly, as height at cutting increased there was a decline in IVDMD. There was a positive linear correlation between IVDMD and both DMD and ND at 48, 72, 96, and 120 h (r = 0.917, 0.923, 0.921, and 0.850 for DMD; r = 0.795, 0.814, 0.787, and 0.787 for ND). Hence, further study on intake and performance of animals is suggested to develop Napier-based diets for smallholders.

Seedling vigour in wheat - sources of variation for genetic and agronomic improvement

Wheat seeds of uniform weight selected from 3 batches of seed with phosphorus (P) concentrations of 0.14, 0.17 and 0.19% were sown in soil with a range of applied P treatments. Seedling emergence was more rapid the higher the seed P. By 25 days after sowing all plants grown at the highest concentration of applied P were similar in size, but with lower applied P, plants from seed with higher P concentrations had an advantage. A range of weight classes was selected from 2 batches of seed raised at low or high concentrations of P. There were 11 groups ranging in mean seed weight from 28 to 58 mg and in seed P from 0.13 to 0.37%. Seedlings from the high P batch had larger first leaves, a higher dry weight, and longer roots than those from the low P batch when grown in a highly P-deficient sand culture system. Within each batch, heavier seeds produced larger leaves, heavier plants and longer roots. The yield differences between batches were largely accounted for if the seed P content (¦g P/seed) was considered rather than either percentage P or seed weight alone. Differences in seed nitrogen concentration or content did not consistently account for differences in seedling growth.

Effect of seed weight, and seed phosphorus and nitrogen concentrations on the early growth of wheat seedlings.

The pasture response to four particle sizes of gypsum fertilizer was measured on a sulphur deficient basaltic soil over a three-year period. At a low rate of application, pasture response was inversely related to particle size ; at a higher rate, only the coarsest fraction differed from the other treatments. The reduction in plant sulphur content and uptake with this fraction suggests that particles >5 mm may be approaching the upper limits of usefulness on this soil. In the first year, the negative interaction between particle size and level of gypsum applied showed that particle size may replace level of application in its effect on response. More efficient use of the sulphur in the fertilizer resulted from application of the finer fractions. Separation of particle size effects into those due to rate of solution of sulphur from the compound, and those due to placement through physical distribution over the soil surface, showed that rate of solution of sulphur was the more important variant. Recognition of its influence could lead to prescribing the correct particle size of gypsum for every sulphur deficient soil. The main effect of the applied sulphur was on the clover component of the pasture. The results suggest that a sulphur content of 0.07 per cent in mature plant tops may be a critical value indicating an adequate sulphur supply, and that 5 lb sulphur an acre in the plant tops may be all that is required for maximum clover production.

The influence of gypsum particle size on pasture response on a sulphur deficient soil

Gypsum and elemental sulphur were compared as sulphur fertilizers at three times of application on pasture growing on a sulphur deficient basaltic soil. There was no substitute for early sulphur applications. Sulphur was required early in the growth cycle of the pasture plants for maximum yields. Gypsum was the more productive fertilizer of the two forms used, and showed to particular advantage under the adverse conditions of late application, or dry seasonal conditions. It had greater flexibility in the time when it could be used because it was the more readily available form. Elemental sulphur had a greater residual value than gypsum because the initial utilization of sulphur from this form was less than from gypsum. However, the total recovery of sulphur was no greater from elemental sulphur than from gypsum. Evidence is presented which suggests that this greater residual value of the elemental form is of little benefit on this non-leaching soil.

The influence of time application of gypsum and elemental sulphur on the pasture response to sulphur

Calcium sulphate was applied initially at rates equivalent to 0, 8.4, 16.8, 33.6 or 50.4 kg sulphur ha-1 to an existing subterranean clover (Trifolium subterraneum) -phalaris (Phalaris tuberosa) sward. At two of these rates, 8.4 and 33.6 kg sulphur ha-1, four maintenance levels (0, 8.4, 16.8 and 33.6 kg) were applied in each of the succeeding three years. In the production of dry matter, most efficient use of the fertilizer was made when the requirement of the pasture (33.6 kg S ha-1) was fully met in the first year, and subsequent annual dressings were made at the rate of 8.4 kg S ha-1 a year. Plant response was related to the level of sulphur available, and the success of particular programmes depended on the rapidity with which the potential yield of the pasture was realized under them. There was evidence, with the change from clover to grass dominance, that grass made more efficient use of residual sulphur in producing dry matter than did clover. Low maintenance rates were effective because they enhanced the residual value of the initial application.

A comparison of fertilizer programmes for the development and maintenance of sown pasture on a sulphur deficient basaltic soil

Sulphur, as calcium sulphate, was applied to a subterranean clover (Trifolium subterraneum)- phalaris (Phalaris tuberosa) pasture on a non-leaching basaltic soil. Treatments were such that both current and residual effects on yield and sulphur uptake could be measured over a period of four years. In favourable seasons, most of the sulphur applied was used by the plants in the year of application. Where it was unused, as a result of poor seasonal conditions, it remained available for use in subsequent seasons. When the full soil sulphur requirement was met in the year of application, residual value fell substantially after one season's active growth and was negligible after two such seasons. Because season and botanical composition influenced residual value, no simple formula discounting it with time is practicable.

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Papers

Effect of seed weight, and seed phosphorus and nitrogen concentrations on the early growth of wheat seedlings.

The influence of gypsum particle size on pasture response on a sulphur deficient soil

The influence of time application of gypsum and elemental sulphur on the pasture response to sulphur

A comparison of fertilizer programmes for the development and maintenance of sown pasture on a sulphur deficient basaltic soil

Current and residual effects of calcium sulphonate applications on a sulphur deficient basaltic soil