13 Feb 2011

How would you know - keeping a watch on soil nutrient levels

Soil test values vary from region to region as well as within region depending on management. A reliable soil test done on a representative soil sample will give the starting point for nutrient management decisions.

How would you know? - Keeping a watch on soil test values

Weather over this Australian summer has shown that we really are a land of drought and flooding rains. And these conditions have a big effect on the nutrient status – and therefore future productivity – of the soil.

But the effects are localized and it is not really possible to make generalizations about nutrient status with some areas having large nutrient removals following high yields, water logging leaching mobile nutrients and movement of soils around in flood waters.

Test results from a large number of soil samples taken in the first half of 2010 were reviewed. The samples came from Incitec Pivot business partners in particular regions and were sorted into a regional soil type to give a picture of the variation in soil test values for phosphorus and sulphur within a region. All the samples had low (<150) PBI values.

The values are given in Table 1 and these are interpreted in terms of critical values of Colwell P and KCl40 for wheat.
Location and Soil Type
Colwell P
KCl 40 Wheat
KCl 40 Canola
20 to 35
3 to 5
Harden Red & Brown Loams
Horsham Grey & Brown Clay Loams
Maryborough Red & Brown Clay Loams
Cummins Red & Brown Sandy Loams

At Horsham and Cummins – for example, half the samples presented had Colwell P values less than the critical value of 35 mg/kg - a level that represents a potential of 90% of maximum grain yield for wheat. Samples from the other two locations had a lower proportion of potentially responsive test values.

Soil test values for KCl40 sulphur showed that few samples have low test values (<3 mg/kg) for wheat. However, when interpreted in terms of canola which has a higher demand for S than wheat, about three quarters of the samples indicated that additional sulphur could show a response. A deep S soil test down to 60 cm can be a better indicator of sulphur supply than a 10 cm test, especially if there is significant sulphur down the profile.

A series of soil tests across the farm will also help identify paddocks that are more or less responsive. As the soil supply of a nutrient increases, crop yield increases until the soil becomes sufficient. Beyond this sufficient level, yield does not increase, so allocate nutrient to the more responsive areas which will give the largest economic return to investment. But do not forget the less responsive areas, as even with moderate test levels, most of the crop response occurs with the first few units of added nutrients.

While soil tests are not a guarantee of response to added nutrients, they are an effective indicator of the nutrient levels in soil. With the uncertainty seen across soil type and seasons, it is important to know these levels – it is not something you can guess. With 2011 giving growers opportunities with full soil profiles, getting nutrition right will be a key strategy to use that water effectively.

Tips for taking soil samples:

A soil test is only as good as the sample presented to the laboratory. A bag of soil submitted, in the lab becomes a tablespoon of soil that will represent an entire paddock!

In order to gain a representative sample it may be necessary to take at least 20 soil cores from across the paddock or within the management zone or area. Try to sample at about the same time each year, and make sure the samples are taken from areas that have not been fertilized in the current season.

Sample depth is critical as the values are interpreted for a particular layer – usually the top 10 cm – and this is where most of the P (in particular) is present. Taking shallower or deeper samples, even by one or two centimetres, will give relatively higher or lower test values respectively when compared to a 10 cm sample.

Because soil P is relatively immobile, where banding of P is practiced such as with precision placement of fertilizer, soil sampling requires special consideration, as random sampling would give a high test result if only a few bands were included in the sample. Where the locations of the bands or drill rows are known, research has suggested that a ratio of 1:8, 1:16 or 1:20 in-the-band cores to between-the-band cores should be considered for 30 cm, 60 cm and 75 cm band spacings respectively *. An alternative is to take a slice of soil across the rows to include banded and non-banded soil, although the reliability of this method has not been evaluated for predicting fertilizer responses relative to other sampling methods.

In paddocks with multiple soil types, try to sample from the dominant soil type. If you are going to manage these soils differently then sample them separately.

Mix the samples thoroughly in a clean bucket and keep them in a cool place.

Clearly label and pack the samples into the containers provided by the laboratory and dispatch as soon as possible.

Use a testing service that participates in Inter Laboratory Proficiency Programs such as the program undertaken by the Australian Soil and Plant Analysis Council.

*Kitchen NR, Havlin JLN, Westfall DG (1990) Soil sampling under no-till banded phosphorus. Soil Science Society of America Journal 54, 1661- 1665.

More about: Best Management Practices