Precision agriculture for plant nutrition

Precision Agriculture is a term used to describe fine-tuning of land management with the use of the Global Positioning System (GPS) where growers can mark and measure crop harvest variances within a particular field and try and determine the cause of these variations.

By Robin Boom

The United States National Research Council defined Precision Agriculture as: the application of modern information technologies to provide, process and analyse multisource data of high spatial and temporal resolution for decision making and operations in the management of crop production.

In the past it has mainly been used in large, broad acre cropping farms in North America and Australia for grain and oil seed crops where a field can often be hundreds of hectares in size and the harvester records yield variations via GPS. Growers, agronomists and other rural professionals can then GPS map soil tests from the better and poorer areas and compare them. Sometimes the yield differences may have nothing to do with soil chemistry and may be related to soil physics, drainage or some other factor. But more often than not, the differences are fertility related. Historically the field would have had the same fertiliser inputs blanket applied at the same rate. All too often, all the fields on large farms are treated with the same fertiliser mix and rate. Precision agriculture refines the nutrient inputs to specific areas to address known deficiencies, and ensures that nutrients are not applied to those areas within a field that do not require them. For instance, there may be a certain section that requires lime, whereas other parts of a field may have good pH (acidity/ alkalinity) and calcium levels. Applying lime to these areas would not only be a waste of money and effort but could do more harm than good, as too high pH levels can impact on the soil’s ability to retain other elements such as magnesium and potassium; and micro-nutrients –

boron, iron, manganese, copper, cobalt and zinc – become less available from liming. Through intensive soil testing, parts of a field that are low or high in phosphorus can be identified. Excessive phosphorus applications are not only a waste of money but can cause run-off, leaching phosphorus into New Zealand’s waterways causing eutrophication and toxic algal blooms. I have always advocated for a much broader soil audit than those done by most fertiliser reps. Normally, these reps only test for six elements, yet plants need 16 elements and animals need 17 to grow and function properly. Micronutrients play important enzymatic roles in plant nutrition, assist with defence against diseases and act as catalysts for the absorption and uptake of macro-elements. Under some forms of intensive horticulture, copper sprays have been and still are liberally used. High copper levels have been found under some kiwifruit vines where copper has been the go-to solution for controlling Pseudomonas syringae pv actinidiae (PSA). Excessive copper can impact on the availability of other micro-nutrients such as iron and zinc and will also negatively affect phosphorus availability. It also impacts on soil biology, not only controlling undesirable fungi and bacteria, but also inhibiting the positive soil micro-organisms too.

Plants need 16 elements and animals need 17 to grow and function properly

For high value fruit and vegetable crops, where yield differences exist even within small areas, it is worth separately testing the high and low yielding areas to ascertain what is missing – or conversely what is phytotoxic to the plants. With the current high shipping costs of macro-nutrients being one of the drivers for fertiliser price increases, some of the essential micro-nutrients which are not required in large amounts are likely to remain relatively cheap compared to the macro-nutrients nitrogen, phosphorus and potassium. Elements such as boron, iron and manganese can make big yield differences if they are low in certain crops. In recent years, a number of farmers have done all-paddock testing on their properties and made considerable savings in fertiliser input costs as they target specific elements to be applied only on those paddocks that need them. Over a couple of years they have been able to even out these differences so the entire farm can be treated with the same fertiliser mix once again.

In recent years, a number of farmers have done all-paddock testing on their properties and made considerable savings in fertiliser input costs as they target specific elements to be applied only on those paddocks that need them

Price increases on locally manufactured superphosphatebased fertilisers have been less than on the imported, high analysis and compound fertilisers most commonly used in cropping and horticulture. Considerable savings can be made using locally manufactured phosphate fertilisers, urea and potassium chloride/sulphate for NPK elements, compared to the likes of DAP (diammonium phosphate), Nitrophoska and YaraMila. A problem with these compound fertilisers is that they contain a certain ratio of nitrogen, phosphorus and potassium which is usually broadcast everywhere at the same rate. Whereas Precision Agriculture, using GPS and more intensive soil testing data to target the application of specific nutrients only where they are needed, becomes a win-win for the both the bank balance and the environment.

Robin Boom is a member of the Institute of Professional Soil Scientists.

0800 116 229 biostart.co.nz

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