Improving Fertiliser Efficiency on Peat
Quin Environmentals (NZ) Ltd is the scientific advisor to Group ONE.
Peat soils in the greater Waikato area comprise about 94,000 hectares. Their formation has taken place over the past 18,000 years, mainly as a consequence of the Waikato river changing its previous course towards the Firth of Thames, to passing through the Hamilton Basin. As the river slowly developed its current course, many poorly-drained areas became either directly flooded or waterlogged from rising groundwater, forming saturated, high organic matter content bogs or peats.
There are 5 different peats recognised, differentiated mainly by their depth and mineral content. Peats formed by waterlogging by groundwater have received relatively little nutrients, and in their ‘unimproved’ state are therefore very infertile, and highly acidic. Those formed by flooding with surface water and trapped rainwater are generally much more fertile, because of higher nutrient loadings, and less acidic. Undeveloped peat has a very low bulk density – typically only 0.05 – 0.15 tonnes dry weight per cubic meter, which makes peat soil very spongy and easily pugged.
The Development of Peat Soils
The ‘development’ of peats into grazed pasture involves cultivating the topsoil (and incorporating large quantities of lime), installing drainage, and applying capital fertiliser. It involves weighing short-term advantages against long-term disadvantages. With good management, especially with respect to drainage and water-table control, and fertiliser, peat soils can be very productive, capable of producing well over 1000 kg milksolids per ha. Poorly managed, excessively drained peats dry out rapidly and shrink, at anything up to 10cm per year, and suffer from a myriad of nutrient deficiencies.
Once this happens, the only solution is basically to start over by recultivating the dry, cracked layer into a new layer below it, and readjusting drainage depth. However, any new drainage of peat soils, or changes to existing drains, requires consent from Environment Waikato, because of flow-on effects on other land-users and the environment. Developed peat typically has a bulk density of 0.5 tonnes/cubic meter.
Even the best-managed peats are shrinking – that is, the ground level is dropping – about 2cm per year. This is partly a result of compaction by animals and machinery, and partly by the decomposition of soil organic carbon to carbon dioxide gas resulting from the original cultivation. Eventually, in maybe 200 years, there will be no ‘peat’ a such left, as we will be left with a more or less mineral soil made up of the residue of the original peat and whatever underlies it, typically alluvial material containing allophanic clays.
Improving Peat Soils
Basically, to ‘improve’ peat to the point where it will support a good permanent ryegrass/clover pasture, enormous quantities of lime – typically 20-30 tonnes/ha – need to be incorporated into the top 7.5-10cm over a 5-10 year period, to increase the pH to around 5, and then maintained at this level with about 1 tonne/ha of topdressed lime every 3 years. Large capital applications of phosphorus, sulphur and potassium are also needed. Various trace elements can also be needed, but their requirement can vary dramatically from one area to another. Molybdenum, for example, varies from highly deficient to borderline toxic, reflecting inputs in the original water source. However, selenium and copper are very frequently deficient in animal health terms.
Very large responses to fertiliser N are obtained during the peat development phase, due to the very high C to N ratio of the peat, but these responses decline rapidly over time, to efficiencies that tend to be lower than most mineral soils, due to the poor ability of peat soils to recycle N. Likewise phosphate and especially sulphate are relatively inefficient on peats, although the efficiency of P does improve over time, as mineral levels improve. Run-off from peat, especially during the development stage, has a strong yellow-brown discoloration due to the presence of organic compounds such as tannins resulting from the decomposition process.
As peat soils are developed, while they slowly improve their ability to retain P, they come to utilise K less efficiently. This is probably because progressively more of the cation exchange sites in the soil are being occupied by calcium from the lime, leaving less ability for K to be retained against leaching. For a whole variety of reasons, soil tests on peats are far less useful and reliable than on mineral soils – herbage testing is far superior.
Given all these nutrient retention and efficiency problems on peat soils, it is perhaps surprising that these soils have not seen widespread use of foliar or even fluid fertilisers. One of the main reasons for this is that, while foliar application is likely to be far more efficient in terms of additional growth per unit nutrient applied, 30-50% of any one application to pasture that is taken up through the leaves and turned into growth, is consumed in 2-4 weeks in the next grazing, and either removed in milk etc or returned to the soil in excreta, where it will suffer the same inefficiencies as solid fertiliser. The remaining 50-70% of the nutrient applied will either be in the residual (uneaten) foliage, translocated to the roots from where they will supply growth for the next grazing in a few weeks time, or be in leaves trampled into the soil and being slowly consumed by soil micro-organisms. It is unrealistic therefore to assume that any one foliar application will continue to fully maintain growth for more than 6-8 weeks. You need to keep reapplying foliar fertiliser regularly – theoretically, at least every second grazing. And this is the nub of the problem. Fertiliser application costs are significant – either directly if a farmer uses a contractor, or indirectly in terms of ‘opportunity cost’ of his time if the farmer does it himself, as well as financing, servicing and depreciation of equipment.
However, circumstances and production pressure are going to bring about a major change in how we use and apply fertiliser. Already, most dairy farmers apply granular urea at least 4 or 5 times per year. Science has already proven that applying urea as a thick fluid to which urease inhibitor has been added is a massive 2.4 times as efficient per kg N. What this means is that the massive economic benefit of applying N in this form easily carries with it the cost of co-applying the maintenance requirement of P, S, K, lime and trace elements. Moreover, putting them on in say 5 small applications per year with the N, instead of 2 heavy applications, results in further efficiencies and far lower losses to the environment.
Most nutrients, in some form or other, can be taken up in at least some amount through the leaves. Urea-N, most cations, di- and trivalent anions and trace elements (sulphate and chelated forms) are examples. In situations where nutrients are being applied quite frequently, application of fertiliser in fluidised form enables the plant to quickly replenish any deficiency through foliar uptake, while the remainder is washed through to the soil to maintain reserves for root uptake. In many cases, nutrient requirements can be reduced by half or more, compared to heavy twice-yearly applications of solid fertiliser, which on peat soils is very vulnerable to leaching and run-off.
We need to also keep in mind that it is not as simple as a straight nutrient ‘balance-sheet’ budgeting approach, especially on peat soils, which suffer huge losses in drainage. Plants whose nutrient content is kept at an optimum level through foliar application will have larger, more efficient root systems, meaning that more of the nutrients that would otherwise be lost in drainage and eter waterways are intercepted and utilised for growth.
I have been studying the issue of nutrient efficiency closely over the last few years, from the perspective of my 15 years as a MAF fertiliser research scientist (including 3 years as Chief Scientist for Soil Fertility at Ruakura from 1984-7), and 18 years developing and promoting new products in the fertiliser industry, as CEO of Summit-Quinphos. We need to be putting vastly more resources into investigating just what is the best strategy for getting the nutrient into the plant as efficiently as possible.
If we assume that about 1500 kg DM is consumed at each grazing, this will result in the production of about 1500 litres of milk, which will contain about 7 kg N, 1.5 kg P, 2.3 kg K, 1.2 kg Ca, 0.4 kg Mg, 0.15 kg Na and smaller amounts of many other elements. Greater quantities than these of most nutrients – especially N – are normally lost to the environment. Although foliar application is extremely efficient, we still need to be applying these quantities of nutrient at each grazing (or double the amount every second grazing), if we are going to rely solely on foliar feeding, just to cover removals in milk, plus at least something to cover unavoidable losses. However, the cost of the actual spreading operation generally makes this uneconomic. So what is the answer?
The Benefits of Fluidised Fertilisers
The answer I believe is to use ‘fluidised’ fertiliser. Fluidised fertilser, meaning a heavy, low water-content product, is much cheaper to produce than a pure solution, and much cheaper to apply per unit nutrient. Typically, up to 5-10 times as much nutrient can be applied per dollar. It gives the farmer the best of both worlds. The fluid itself can contain a mixture both of nutrients in pure solution, and particles of varying sizes.
The plants ability to take up and optimise nutrient levels through foliar uptake is met, through the incorporation of a soluble foliar fertiliser containing all its nutrients in the optimum form to maximise foliar uptake, plus on-leaf dissolution and uptake of small, urease inhibitor-treated urea. The rest of the mixture passes through the leaf canopy to the soil, where its extremely uniform coverage maximizes uptake through the roots, and minimises losses to the environment.