Very stable as a mulching material. Other Materials There are many proprietary organic materials, based on plant or animal remains, such as fish waste, seaweed, poultry and other manures, but with nutrients added to improve nutritional value. Check the levels of nutrients they contain and use according to manufacturers recommendations and crop requirements. NITROGEN FERTILISERS Given satisfactory soil temperature and seasonal conditions, the amount of nitrogen available as ammonia and nitrate is the limiting factor in crop growth. Nitrogen encourages stem and leaf growth and to achieve high vegetative growth rate, adequate nitrogen must be available to meet plant needs. Provided that the soil is moist and warm (above 4i°FJ5°C]) a steady supply of nitrate from decomposing organic matter will be available for plant growth. During the winter months there is little bacterial activity and very little release of nitrate. As the soil warms in spring nitrate becomes increasingly available, reaching a peak about mid-June-by which time crops are growing rapidly and utilizing large quantities of nitrogen. Nitrate continues to be released during the autumn, and where crops are ripening or have been harvested will not be used and will be washed from the soil by winter rains. The most critical period of nitrogen availability is early spring, where spring crops are coming into strong growth during April and early May, when a shortage of soil-released nitrate will often result in a considerable check in growth, ultimately seriously affecting crop yield. This is the period when quick-actirtg nitrogenous fertilisers are needed, either as base dressings prior to sowing or planting, or as surface dressings around established plants. It is important to remember that the application of large amounts of nitrogen will encourage lush stem and leaf growth. Although this may be desired with leafy vegetable crops, balanced growth is required from crops or plants grown for their flowers, fruit or edible roots. While the application of quick-acting nitrogen fertilisers will encourage rapid growth, growth will be limited by the availability of other nutrients as in utilising larger amounts of nitrogen a plant's demands on available phosphorus or potassium will also increase. Where there is any doubt regarding levels of available phosphorous or potassium it is better to use a balanced base fertiliser dressing in spring. On heavily manured soils there is less need for nitrogen because of the larger amount of nitrogen released from the soil. Nitrogenous fertilisers: organic: Hoof and horn meal (13 per cent N) Shoddy (wool waste) (9-12 per cent N) Dried blood (14 per cent N) These are all of animal origin, rotting in the soil to produce ammonia, then nitrate. Although they are regarded as slow-acting the rate of bacterial break-down is related to soil temperature. Applied in spring outdoors there will be little soluble nitrogen available in the early part of the year and they are more useful for crops or plants which make their growth during mid to late summer.