Visual soil and plant quality assessment
|Main authors:||Abdallah Alaoui and Gudrun Schwilch
|Source documents:||Alaoui, A. and Schwilch, G. (2016) Soil quality and agricultural management practices inventory at case study sites. iSQAPER Report 28 pp
Alaoui, A. (2018) Visual Soil Quality Assessment Manual v2: assessment of soil and plant quality for the season 2018. iSQAPER Report 49 pp
Visual assessment of soil and plant quality is an important part of evaluating the effects of different agricultural management practices. In this section of iSQAPERiS we describe the methods for visually assessing 14 different parameters linked to soil quality and 6 for assessing plant quality.
Note: For analysis of the impact of agricultural management practices on soil quality using visual soil assessment methods see Alaoui, A, Barão, L, Ferreira, CS, et al. Visual assessment of the impact of agricultural management practices on soil quality. Agronomy Journal. 2020; 112: 2608– 2623. https://doi.org/10.1002/agj2.20216
1. Selection of sampling plot
The aim is to compare the soil and plant quality
- in a field where promising agricultural management practices (AMP) have been implemented for at least 3 years (plot_AMP)
- with another field which does not use the AMP (plot_control), but which is within the same pedo-climatic zone and under comparable farming system, soil conditions, topography, etc.
2. Sampling and replications
In order to characterize properly the conditions existing under both the AMP and the control, we suggest that 3 representative plots that should be selected in the field using the AMP and 3 in the control field. The assessment of each indicator/property should be made three times for the plot_AMP and 3 times in the plot_control, resulting in 6 measurements for each indicator (Fig. 1 below).
Soil data requires an accompanying geo-referenced description.
3. Materials needed
- 1 spade – to dig out a 20cm cube of topsoil.
- 1 plastic basin (approx. 35x35x20cm) – to carry the soil (for the drop shatter test for »Soil structure and consistency).
- 1 hard square board (approx. 26x26x1.8cm) – on to which a soil cube is dropped (for the drop shatter test for »Soil structure and consistency).
- 1 heavy-duty plastic sheet (approx. 75x50cm) – on which to spread the soil (after the drop shatter test has been carried out for »Soil structure and consistency).
- 1 copy of the VSA Field Guide printed in colour - to make the photographic comparisons (for »Soil colour)
- Digital camera (use same for all sites). The photos should be taken under same light
conditions in situ (the soil to be photographed should be covered by a white large parasol in
order to diffuse sunlight) and second series of photos have to be taken in the lab (under
same light conditions).
- Wire grid of about 1 cm2 mesh and a wide-mouth bottle (for »Soil slaking test (soil stability))
- Infiltrometer or penetrometer and supporting material (for »Infiltration rate / penetration resistance)
- A palm-sized spectrometer for example a Hach (or generic) 550nm for gauging the change
in colour (the optical density) of the KMnO4 (for analysis of »Labile organic carbon).
The assessment of all proposed indicators should be made in situ except »Labile organic carbon which can be either assessed in the lab or in the field.
The classification ranges of some indicators might still need to be re-evaluated after collecting all the study site data. For this purpose it is necessary to indicate the measured absolute values (e.g. »pH).
If infiltration rate is chosen rather than penetration resistance (»Infiltration rate / penetration resistance) the infiltration experiment should be started before the other investigations. After 20 Minutes, record the volume infiltrated in soil.
In general, the study site researchers should avoid walking on the plot under investigation to prevent any topsoil disturbance (i.e., topsoil compaction).
Texture determination is a prerequisite for the accurate determination of the scoring of »Labile organic carbon and also for the quantitative analysis of the results obtained from the
»Infiltration rate. For this purpose, soils samples (volume of about 200 – 400 cm3) of the investigated plots should be have an additional full texture analysis.
If evaluation is required of »Labile organic carbon of soil sampled at all plots/farms at the end of field work, air dried soil samples (about 50 g of each plot) should be stored in a dry place.