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Low available soil phosphorus (P) is associated with its immobility, which renders it unavailable for plant uptake. In addition, farmers normally apply inorganic fertilisers to legumes to activate soil-bound phosphorus using root exudates. Sufficient soil mineral nutrition is key to sustainable crop production, and hence food and nutritional security. The aim of this study was to quantify the acid and alkaline phosphatase activity as an indicator of P supply and availability under varying levels of nitrogen, phosphorus and potassium (NPK) fertilization and different cropping systems. An intercropping (cowpea and amaranth) and fertiliser (control, 25%, 50%, and 100% of the recommended NPK levels) field trial was laid out in a 2 × 4 factorial treatment structure in a completely randomized design (CRD) with four replications. There was higher acid and alkaline phosphatase activity in the rhizosphere of cowpea and amaranth grown as sole crops compared to those from intercropping. The cowpea and amaranth plants grown without fertiliser or 25% NPK had the highest rhizospheric phosphatase activity, while 100% NPK application exhibited the least. The markedly higher phosphatase activity from the low fertiliser application treatments indicates the possible stimulation of microbial activity to supplement P demands for the crops. The study revealed that the application of lower rates inorganic fertilisers in a legume intercrop stimulates the activity of the phosphatase enzymes, which can subsequently liberate soil-bound phosphorus. Plant tissue phosphorus concentration of cowpea and amaranth plants increased proportionately to the increase in fertiliser application up to 50% of the recommended NPK level. The land equivalent ratio (LER) was greater than 1, indicating that it is more beneficial to intercrop cowpea and amaranth as opposed to growing them as sole crops. Overall, the application of NPK fertilizer to amounts of up to 50%, based on the results of this study, appear to be better than 100% in terms of biomass accumulation and phosphate activity.

Water and nitrogen shortages in annual ryegrass (Lolium multiflorum) production can be improved by using proper irrigation and nitrogen scheduling methods. The aim of this research was to analyse the effect of water and nitrogen application on dry matter production and forage quality of annual ryegrass. Treatments consisted of three irrigation levels, scheduling application to field capacity once every two weeks (W1), once a week (W2) or twice a week (W3) and three nitrogen levels, top-dressing after each harvest at a rate of 0 (N1), 30 (N2) or 60 (N3) kg N h -1 cycle -1 . Yield increased as a function of the amount of water and nitrogen fertiliser applied. The dry matter (DM) content, in vitro organic matter digestibility (IVOMD), crude protein (CP) and metabolisable energy (ME) were higher in the treatments being irrigated once every two weeks. These results conclude that higher irrigation coupled with high nitrogen application significantly improved the dry matter yield, while water stress, to some extent, did improve the quality of the pasture by increasing the DM content, IVOMD, CP and ME values.

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Poor irrigation management in pastures can lead to yield and quality reduction as well as loss of income through extra pumping and leaching of nitrate fertiliser. A number of irrigation scheduling techniques of varying levels of sophistication have been developed over the years to address limited irrigation water availability and maximise productivity. Despite this, the adoption of irrigation scheduling tools by farmers remains low. The objective of this study was to assess the use of simple irrigation scheduling calendars based on average weather data to improve irrigation management in ryegrass. The calibrated Soil Water Balance (SWB) model was used to generate simple irrigation calendars and assess effectiveness for different scenarios by mechanistically simulating water dynamics and pasture growth. Scheduling irrigation using the calendars gave similar irrigation applications, water losses and yields compared to a more scientific real-time scheduling (in response to soil water depletion by the crop). While site-specific irrigation scheduling calendars can easily be generated by consultants and irrigators, even simpler monthly estimates of average daily water use can also be useful. Application of calendars by farmers is encouraged

a b s t r a c tNitrogen is often poorly managed in irrigated agro-ecosystems. Accumulation and leaching of N can occur due to excessive fertiliser N, high soil inorganic N carryover between seasons, rapid mineralisation in spring and poor irrigation scheduling. This can reduce forage yield, quality and N-use efficiency, and lead to pollution of soil and water resources. Experiments were conducted to test whether adaptive nitrogen and irrigation management approaches using ryegrass as a case study could (1) reduce N application without compromising yield, (2) maintain or improve forage quality, (3) improve water use efficiency, and (4) minimise potential for nitrate leaching, using the current local recommended fertiliser rates as a baseline. Adaptive management strategies based on the concentration of nitrate measured in a wetting front detector at different depths reduced fertiliser N application by 28-32% compared to the baseline recommendation, reduced residual soil N that is potentially leachable, and improved forage quality without reduction in forage yield. The essence of the adaptive approach is to set thresholds for action that are relatively easy to monitor, based on a simple conceptualisation of the system. The thresholds were defined for the depth that a strong wetting front could be passively detected under field conditions, and for the concentration of nitrate in the percolating water. These thresholds were chosen as simple integral measures of the water and N cycles. Results suggest that a good adaptive manager would improve the thresholds for action as more experience is gained.

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