9 8 Plenty to consider with fertiliser decisions IT’S that time of year again when budgets for the next nut tree crop growing season are being adjusted as the current harvest comes to a close – an exciting time, but also one tempered with the caution of what will the new season bring? The main requirements of water, power, bees and fertiliser do not change – or, more correctly, they change with the yield expectation. We cannot grow a great yield if we don’t have enough hives for pollination and we must fertilise and water to our expected yields. The idea of reducing fertiliser inputs or compromising nutrient quality may cross our minds, however is this really a good idea? Will it save money today but have an impact on the harvest in one to two years’ time? Fertiliser costs have also come back in price, so is it worth compromising fertiliser quality? As we know, when we have a lower yield, we have less fertiliser inputs required and yet also the ability to improve tree health to positively impact the following year’s yield. On the contrary, if you have a big crop and don’t supply enough nutrients, this will cause a low crop for the following year and possibly two. This is a result of using-up storage from the big crop load, most likely impacting leaf hold and tree health, preventing further nutrient storage. This points to leaf health at harvest being an important factor as a carbohydrate storage indicator. Is it possible the wide range of leaf health observed at harvest this year may have a consequence on tree storage levels and thus on future crops? The trick is to know your yield, which can be very hard to estimate. Looking at tree vigour, leaf health and supplying normal nutrient to tree demand during the season is the safest approach. Maintaining leaf health and then, once you know your yield at harvest, measuring up post-harvest applications By Sheri Robinson Southern Agronomist, Haifa Australia is the safest way to minimise inputs. This approach is supported in Dr Patrick Brown’s recent presentations to the Australian Almond Conference, showing proof that we need to match our applications to tree demand, and we may be wasting some inputs and spending precious dollars on excessive fertiliser applied post-harvest. The other factor is the considered use of ammonium or urea-based fertilisers. If applied in excess of plant requirement, these are the most likely cause of concern in soil acidification of the irrigated zone. Ensure you know your irrigation, fertigation water and soil pH in the irrigated zone to help manage the effects of your fertiliser choices. thus the cause of acidification effects. Potassium nitrate, if supplied to the demand of the tree, is the most efficient supply of potassium and nitrate due to the cation and anion counterbalance and, due to this efficient uptake, causes no acidity effect. Foliar application In some instances, plant nutrient uptake needs to be supported by foliar applications. Haifa also has a product called Multi-K Bonus, which contains mostly potassium nitrate with some phosphorus. The wetting agent in this product assists it to remain on the leaf and re-absorb at times of rewetting (for example, overnight). Multi-K Bonus can be applied at higher rates due to the lower risk of phytotoxicity resulting from the use of the wetting agent. The product also is designed to adjust a spray tank mix whilst supplying nutrients in a spray application. The high compatibility of Multi-K Bonus offers another option to provide trees with nutrition, especially for early-season applications when nut development is occurring. Ammonium or urea convert to nitrate for plant uptake in hours, or days in hot soils, while it can take weeks in cool soil. This nitrogen conversion contributes one hydrogen ion to soil acidification, and yet if the nitrate is not utilised by the tree, the acidification doubles due to the loss of the nitrate anion and thus another H+ ion is left in the soil/plant balance. Research shows that potassium and calcium uptake does not occur when soil nitrate levels are low and ammonium levels are high. Therefore, the ratio in soil solutions needs to be factored for ideal nutrient uptake. The management of fertiliser in cool, wet soils early in the season and post-harvest is a potential risk of oversupply, and Irrigations Another often overlooked issue is: Do fertilisers used have an impact on the irrigation infrastructure? Some products with high bicarbonates may react with calcium, magnesium or other impurities in the irrigation water and form complexes that will drop-out of solution and have the potential to block drippers. In the case of acidic water or soil, potassium bicarbonates will react with the hydrogen ions and create water and CO2 and, thus, have an alkalizing effect to the consequence of CO2 loss to the environment. This reaction makes knowing your soil pH very important or otherwise risk losing availability of calcium, magnesium or micro elements due to the liming effect. There are also risks in mixing potassium carbonate as incompatibilities (ie, with UAN) can cause hazardous reactions, releasing CO2 and ammonia into the atmosphere. This reaction is the same as lime causing losses of nitrogen, as it reacts to form ammonia. Looking at the fertiliser options, we must also consider their availability to the plants, as well as our ability to get them into solution. If we look at the table (see next page), we see that potassium carbonate is the most soluble and this explains the high potassium rates in liquid fertilisers. Potassium chloride (KCl) is also a soluble source, with potassium nitrate (PN) coming a very close third and sulphate of potash (SOP) coming a distant last. What does this mean on the farm? Potassium nitrate, being twice to three times more soluble than SOP, lower in sodium and more efficient in uptake, without any contribution The wetting agent in Haifa Bonus assists it to remain on the leaf and re-absorb at times of rewetting (for example, overnight). Multi-K Bonus can be applied at higher rates due to the lower risk of phytotoxicity resulting from the use of the wetting agent. to chloride, is the best fertiliser option. The solubility of the product has a direct impact on how we can supply the product – by using PN we can get more K in the irrigation water. Not only can we get more of the product in solution, we can do it in a timely fashion, and we must include the cost of labour and safety in trying to dissolve products – is it fast or slow? The idea of using water soluble fertilisers is to use a solution, not a suspension, and also to consider the final EC applied to the soil at the time of fertigation. A solution will only drop out if the temperature changes, however a suspension will drop out once agitation slows or stops. In this way, we must not see the stated solubilities as a target, but the upper end guide. A smart approach is to back off 10 per cent to allow for temperature changes (ie, 31.6g in 100mL at 20⁰C, aim for 28.4g/100mL).
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