Soil testing to save money and increase profits

The only way to get accurate, reliable information on soil nutrient levels (and fertiliser requirements) is regular soil testing.

“Soil testing helps you apply just the nutrients needed to optimise production, so you can save money by avoiding unnecessary applications, and optimise your profits,” says Ballance Agri-Nutrients Nutrient Dynamics Specialist Jim Risk.

The key to reliable results is consistent  soil testing over multiple years. Soil test results can be variable (see Table 1) so while one-off results provide some guidance, trends over time are generally better when deciding on fertiliser inputs.

“Using results over several years to determine trends in soil fertility helps overcome variability, and gives more reliable information for adjusting fertiliser inputs,” Jim explains.

The variability in test results is thought to be due to differences in conditions at the time of sampling, with soil temperature and moisture levels affecting test results. As soil is a biological system, nutrient levels can also vary between years and different locations.

In summer dry conditions, microbial activity continues but plant growth slows, so the release of plant available nutrients such as sulphate sulphur and phosphorus can exceed plant uptake. This can elevate levels in the soil, resulting in, for example, higher sulphate sulphur soil test results.

In soils that leach easily, significant rain events can lower sulphate sulphur. Potassium can also leach in soils with low cation exchange capacity such as coarse or sandy soils. Phosphorus is not affected as it does not readily leach in most soils.

In the spring and autumn flush, rapid nutrient uptake by plants can cause phosphorus, potassium and sulphate sulphur levels to be temporarily depleted. As soils warm, microbial activity can also temporarily reduce phosphorus levels.

“For pastoral farms, taking samples at the same time of year and location, and in similar conditions, while avoiding extreme wet or dry or conditions, is recommended.”

“The best time for testing is late autumn  or winter, when soils aren’t dry, soil temperatures are generally at their lowest, and soil biological activity is low. Similar moisture and temperature at sampling lessen the impact of climatic factors on soil test variability. Results will better reflect the nutrient levels available to actively growing pasture and crops.” 

Recent application of fertiliser and grazing can affect soil test results, so testing within three months of fertiliser application, and sampling from dung and urine patches should be avoided. Taking samples from the same soil testing transects (lines for collecting samples) over multiple years also helps to tackle variability in test results.

“Given the variability in soil fertility, it can seem logical that more intensive sampling like all paddock testing (where each individual paddock is tested) would better reflect the variability and enable more tailored fertiliser recommendations, but this isn’t always the case.”

A review of soil sampling methods concluded that variability in soil fertility, maximum potential yield and crop value need to be considered when choosing a soil sampling strategy¹.

“All paddock testing is useful if you don’t know your base fertility, for example when you buy a farm. It can help guide future sampling, and can be valuable for farmers who are already using precision technology.”

“But with good soil test history, well established blocks and stable management, traditional soil sampling of blocks or land management units is perfectly adequate for creating robust fertiliser plans.”

Talk to your local TSR or visit a Farm Source store to find out more about soil testing.

Table 1: Typical Variability in laboratory soil tests¹

Article supplied by Ballance.

References
1 - Edmeades, D. C., Cornforth, I. S., & Wheeler, D. M. (1985). NZ Fertiliser Journal.
2 - Knowles, O., & Dawson, A. (2018). Current soil sampling methods – A review. http://flrc.massey.ac.nz/publications.html