Measuring available nutrients
Measures of ‘available’ nutrient in soil are routinely undertaken in laboratory tests. These reflect a portion of the nutrient which may be available to a crop within its growing season.
- Extracted amounts from laboratory tests are necessarily operationally defined and mostly give a snapshot of the amount of available nutrient in the sample at that time.
- Measurement of different nutrients will be subject to different shortcomings, however if sampling and testing are consistent through time, values can be used to compare to each other and in conjunction with tissue tests, to improve understanding of nutrient availability in the system in question.
- Figure 1 shows the different concentrations of magnesium extracted from soils, depending on the extraction method / solvent and the soil texture. Notice that for each soil texture, different amounts were obtained by different methods. In practice, none of these will reflect the exact amount available to plant.
Figure 1. Comparison of extraction methods and soil textures for magnesium, from Staugaitis and Rutkauskienė (2010) in ‘Potassium application to potatoes 2014’
Availability of nutrients to plants
Other factors affecting the amount of nutrient which a plant can access during its life include:
- Root exploration – how well roots establish and move through the soil; can be affected by plant factors such as disease, and soil factors such as compaction;
- Soil moisture content – in order to be taken up by the roots, nutrient MUST be in solution. Therefore is soils are too dry, nutrient will not be accessible;
- Speed of uptake by plant – if plants take up large amounts of nutrient, a high concentration gradient will be established close to the root and thus diffusion will drive more nutrient into the root zone. The relative importance of this depends on the nutrient in question and the plant’s natural tendencies.
Changes with soil depth
Understanding how nutrient concentration can vary at different depths in the soil, and ensuring sampling reflects likely depth of root exploration, is vital.
- Figure 2 shows how soil potassium concentration can vary with depth. Consider the implications of sampling different parts of the profile in each case, for example:
- If top 15 cm was sampled in the Sharpsburg or Moody soils and the assumption was made that this reflected concentration throughout the rooting zone (to say 90 cm), this would greatly over estimate the total available nutrient.
- If only the 15 – 30 cm profile was sampled in the Hastings soil, the available nutrient would be greatly underestimated and resulting fertiliser applications could potentially cause problems with cation antagonism and magnesium deficiency (see Cation competition).
- It can be unhelpful to make assumptions about how nutrient concentrations will change with depth, and far more prudent to sample different sections of the profile to understand better how availability to plant is likely to vary. Factors affecting concentrations through the soil profile include:
- natural concentrations of nutrients in topsoil and subsoil layers;
- historical fertiliser practices;
- recent fertiliser applications;
- soil texture and chemistry and the effects these have on retention and movement of nutrients through soil.
Figure 2. Chemical characteristics of four soil profiles; potassium concentration is highlighted in red for each. Taken from Olsen et al. (1982) in ‘Potassium application to potatoes 2014’.