Fert Options with Robin Boom Agronomic Advisory Services

 Taking soil tests to ascertain nutrient levels for pasture and crop production is a very small but essential cost, which can pay back large dividends. Not only will a good soil test identify which elements the fertiliser budget should focus on applying and what quantities are required, but it will also identify which elements are not required. Applying unnecessary or excessive nutrients to the soil is not only a waste of money, but they can also be bad for the environment when these are leached into ground water or lost as run-off into streams. Too much of one element can also negatively affect overall plant or animal performance, making plants more susceptible to attack from pathogens, and affect the quality of crops grown and the shelf-life of certain fruits and vegetables.

When taking soil tests, one should identify any soil type, aspect or contour differences within the area being sampled. If the area is relatively uniform, with a known fertiliser history between paddocks, a random walk across the area taking 10-20 plugs should suffice. However, if there are ponding areas, or if there are distinct soil type or contour differences, these should be sampled separately – or not be included in the sample should they only be a small fraction of the area the sample is to represent. Including these anomalous areas in the bulk sample can skew the results and lead to incorrect diagnosis of what fertiliser nutrients are needed.

Testing areas to avoid

For soil testing pastures, areas around water troughs, gateways, shelterbelts, hedges or trees should be avoided because these are areas where livestock will have congregated and will be higher in fertility than other parts of the paddock due to more dung and urine being deposited there. If the paddock has been recently grazed with cattle in particular, you should wait for at least two-three weeks after grazing to identify urine patches and avoid including these in the sample because this will lift the nitrogen and potassium levels in particular. Sheep and deer urine patches are not as big and concentrated so will not skew the sample as much as cattle urine patches.

Soil samples should not be taken if effluent or fertiliser has been applied within the previous three months because this could contaminate sample. Nor should they be taken in drought conditions or if the soils have been at saturation point for an extended period of time. When growing a crop, sampling should be done when there is still plenty of time to take remedial action in being able to organise any base fertiliser before planting. Applying lime just before planting on an acid soil is unlikely to start having much effect in the first two-three months which could impact on crop establishment.

Test depth

For most crops the standard soil test depth is 15cm. But if the soil is being ploughed deeper, then sampling to the plough depth is a good idea. When interpreting soil test results from the lab, knowing soil testing depth is important. Most soil tests on pastures are only taken to a depth of 7.5cm here in NZ, although in Australia and South America the recommended depth on pasture is 10cm and in Europe and North America a 15cm depth is normal. For my pastoral clients I like to do 10cm samples because I think this better represents where the bulk of the root mass is on client’s farms, with 7.5cm being too shallow and 15cm being too deep for permanent pasture. Phosphorus applied to the soil surface on most soils only moves about 1cm, so a deeper soil test will result in a lower phosphorus reading. However potassium and sulphur may increase with depth because these leach through the soil, and aluminium toxicity and acidity are also likely to increase with increasing depth.

Elements needed

Although most people in the fertiliser industry only look at six elements in the soil, plants need 16 elements to grow and animals need 17. I think it is important to also look at the organic matter or carbon level in the soil, nitrogen, boron, copper, zinc, molybdenum and aluminium levels. Manganese and iron deficiencies are rarer in NZ unless soils have been over-limed. It is important your advisor understands all of these extra elements if they’re going to be tested for and what the optimum levels should be.

Most farmers and growers get fertiliser company reps to do their soil testing, but this can become problematic because the advice given will be slanted towards the particular flavour of products which that company sells. And I find there are often marked differences between the cost of nutrients between various fertiliser companies for similar products. Soil testing is the cheap part.

Robin Boom, CPAg, member of the Institute of Professional Soil Scientists. Ph: 0274448764.