Understanding basic soil analysis
NRM
Agriculture
For many years, growers and testing facilities have been analysing samples for pH, phosphate, potash and magnesium, but the results are not always utilised fully for effective crop nutrition planning. Soil analysis is a cost-effective management tool used to enable and identify more appropriate fertiliser programs to be followed for efficient crop production, optimising input and output.
To ensure that results are as representative as possible, samples must be taken with care and attention to detail. In fields above 4 hectares (ten acres), each different soil type should be sampled separately to avoid masking potential deficiencies. A ‘W’ pattern should be followed, with at least 16 subsamples being mixed and bulked up, from which the sample for testing should be taken. The W pattern for each 4 hectare zone of a field is the minimum requirement to generate reliable information to help growers make the most informed decisions on treatment. Most growers now use a precision soil sampling system, where samples are taken at an even lower resolution, typically one sample per soil zone, which could be per hectare or even per acre on high value crops. Precision soil sampling therefore results in more efficient use of fertiliser, reduced nutrient loss and protection of the surrounding natural resources. An agronomist, arable advisor or precision farming specialist will be able to advise you on the best resolution for your farm.
In general arable settings, sampling depth should be 15cm (6 inches), but with minimum or zero tillage depth should be 23-25cm to recognise the lack of soil mixing through the rotation. On long-term grassland, the sampling depth can be 7.5cm.
For a comparison with previous results, sample at a similar time each year. Avoid sampling within six months of a lime, P, K or Mg fertiliser or manure/slurry application. Results are reported using the ADAS index classification in England and Wales. Below target index 2, consideration should be given to building the soil status, while above 2 should indicate encouraging the indices to ease down, especially phosphate, to reduce the risk of losses to water courses.
Results are also provided as elemental nutrient concentrations in milligrams per litre (mg/l). Fertiliser recommendations are presented as the oxide of the nutrients. To reconcile results and nutrient requirements, the conversion factors used are:
Phosphorus x 2.291 = P2O5
Potassium x 1.205 = K2O
Mg x 1.658 = MgO
ADAS Indices
pH
Soil pH affects availability of all nutrients, with optimal levels around pH 6.2 to 6.5. Phosphate is less available at both high and low pH levels, while potash is available over a wider range, and magnesium is significantly less available in acid conditions. The carbonate in calcium carbonate, found in most liming products used to correct acidity, reacts with hydrogen ions to reduce acidity and raise pH levels, releasing calcium to be utilised as a crop nutrient. After liming there is an initial rise in pH and then a slow decline dependent on soil type, rotation and fertiliser use. High nitrogen use has the potential to acidify soils and, as most sulphur fertilisers are based on sulphate, these can more rapidly acidify soils. Soils with very high pH levels will particularly reduce phosphate, manganese and boron availability.
Phosphate
A target index of 2 is generally ideal for combinable crop rotations. However, if there is a high proportion of winter cereals and soils are well structured, index 1 will suffice. The extraction methods used in the laboratory are designed to reflect the action of crop roots. In England and Wales, the Olsen method is used to analyse of phosphate in neutral to alkaline pH, while in Scotland the Modified Morgan reagent is used as their soils tend to be more acidic. For poorer rooting crops, such as potatoes, a phosphate index of 3 is preferable.
Potash
The target index for potash is 2- for combinable crops, and 2+ for root crops due to their greater crop demand. At peak uptake, cereals demonstrate a high uptake of potash. Usually as crops senesce much of the potash is returned to the soil, whereas for root crops potash is stored in the tuber or root and removed at harvest. Thus, potash recommendations are designed to recognise crop offtake.
Magnesium
Crop demand is less than for phosphate and potash, but it is still essential that magnesium levels are maintained as magnesium is a key component of chlorophyll for photosynthesis. For cereals a target index of 1 should be maintained and, where sugar beet, brassicas and potatoes are part of the rotation, an index of 2 is desirable.
Maintaining soils at target indices will ensure there are adequate nutrients for optimal crop growth and yields without high losses to water courses. Where indices are above target, allowing them to decline to target will reduce input costs and the risk of losses. Keeping soils at target will also help to ensure higher nutrient use efficiency and profitability. Speak to your agronomist or soil sampling provider or contact NRM to arrange for samples to be analysed to ensure your crop nutrition plans are suitable to efficiently produce high crop yields. Talk to your FACTS qualified adviser for more guidance on fertiliser planning to increase nutrient use efficiency.
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