Presently, we are having one of the coolest, wettest summers I can remember. It could be a good time opportunity to consider top dressing your pastures to boost pasture growth at a time of the year when it is traditionally pretty risky. By acting early you will be better set up for autumn and winter. Like any farming endeavour, it will require an outlay of time, labour and money, so ensure you are set up to reap the benefits, whether it is conserving fodder or increasing your stocking rate.
I’ve mentioned in earlier columns the importance of utilising what’s already there in your paddock by encouraging existing grass species. Top dressing with chemical or organic fertilisers is a fantastic way to boost pasture growth, but there are a few tricks to ensure you are playing your strongest hand. Most grasses become reproductive, (ie. go to seed – and hence become less nutritious) by day length. So even though growing conditions are unseasonably favourable, you have a whole heap of feed that is still relatively unpalatable. Topping your pasture with a slasher or mower prior to top dressing will ensure a flush of palatable, high quality regrowth.
If you are top dressing with inorganic fertilizers like granulated urea, you need to apply before a significant rainfall event to maximize the uptake of nitrogen by the individual plants in your paddock by allowing it to dissolve into the soil. Another option is a granulated sulphate of ammonia, commonly sold as Gran-Am. Many Australian soils have not had decent amounts of sulphur applied since the super phosphate subsidies were wound up decades ago. A one-off application of Gram-Am can do wonders.
High analysis nitrogen fertilizers need 15mm of rain for most soils to get it into the root zone. If there isn’t adequate moisture to wash it off the leaves and/or inadequate moisture in the soil it will dehydrate the plant. This is what causes burned pastures. Beneficial pasture plants like clovers and medics can be particularly susceptible to high soil temp/microbial activity. These may need to be reintroduced. On the plus side, it’s a double edged sword – acting as both a booster and a herbicide (most weed’n’feed type lawn herbicides are based on sulphate of ammonia, so they can be very effective in reducing the survival of weed seedlings).
I’d recommend hedging your bets by applying these in test strips (fenced off if possible, and in areas where you think there is already good fertility) and gauge for yourself how they respond. With the wet season, you could look at higher application rates of a bag to the hectare using the ‘N’ analysis of urea as the benchmark.
I have only limited exposure to organic fertilizers, but if all things are equal, you would be looking at significant applications of animal manure to realize the same amount of nitrogen. If considering organic growth boosters, encouraging N fixation from legumes in the pasture is probably more in line with the farm’s management style.
I think it is important to remember that top dressing isn’t really about building the nutrients in the soil. It is about providing nutrients instantly available to the plant for a rush of growth. In the warm moist conditions we are experiencing, the soil biota is at its peak activity and I have certainly seen excellent responses to different animal and plant based emulsions when applied as a top dressing. There are also products that introduce and/or stimulate the interaction of enzymes, bacteria and invertebrates in the root zone of pastures. As a rule, they also provide nutrients to the soil that are very accessable to the plants. The prevailing weather should provide near optimum conditions in the soil for these to work.
Put in a test strip and give it a go, I say.
There is literally another world in the root zone, which is still not fully understood by soil scientists.
Published in Tasmanian Country Newspaper.