managing soil organic matter - Grains Research & Development ...

Bulk density (BD) = soil mass per knownvolume of soil = g/cm 3Measuring bulk densityThis method works best for moist soils withoutgravel.1. Prepare a flat, undisturbed surface at thedepth you wish to sample.2. Collect a known volume of soil (steel coreor tube or PVC pipe; minimum 40 mmdiameter and 100 mm depth) by pushing orgently hammering core into soil taking carenot to compact it (see Plate 5.2).3. Remove core, brush away any soil on theoutside of tube, check soil is flush with coreends and place the soil from the core into alabelled plastic bag.4. Record date, sample code and location ofsample (if possible using GPS).5. Weigh sample after drying to a constantweight and record soil weight.CalculationsSoil volumeSoil volume (cm 3 ) = 3.14 x r 2 x hh = height of the ring measured with the rulerin cm (e.g. 10 cm)r = radius = half the diameter of the ring in cm(e.g. 7 cm ÷ 2 = 3.5 cm)Soil volume = 3.14 x 3.5 x 3.5 x 10 = 385cm 3Dry soil weightTo calculate the dry weight of the soil:1. Weigh an ovenproof container such as a pietin (record weight in grams = W1).2. Carefully remove all soil from the bag intothe container.3. Dry the soil at 105ºC until a constant weight(usually 24-48 hours). Depending on sizeof the core and soil moisture this may takelonger.4. When dry, weigh the sample (record weightin grams = W2).5. Record the dry soil weight (g) = W2 - W1Bulk density calculationBulk density (g/cm 3 ) = dry soil weight (g)/soilvolume (cm 3 )can be adjusted accurately (see Figure 5.2). In thisexample, a soil with an initial bulk density of 1.2 g/cm 3 and an organic carbon concentration of 1.2per cent (14.4 tonnes organic carbon per hectareto 10 cm) was exposed to farming practices, whichused over a period of 10 years resulted in topsoilcompaction (see Figure 5.2). This increased thebulk density of the topsoil to 1.4 g/cm 3 , but did notchange the percentage of organic carbon in the soil.Without adjusting for the change in bulk density toan equivalent soil mass, a change in organic carbonstock of 2.4 tonnes per hectare would result. If thestocks are adjusted to an equivalent soil weight,then results show no change in organic carbonstocks (see Figure 5.2).THE INFLUENCE OF SOIL ORGANICMATTER ON SOIL WATERPlant residues that cover the soil surface preventthe soil from sealing and crusting. This can resultin better water infiltration and decreased waterlosses associated with run-off. Evaporation is alsodecreased and up to 8 mm of soil water can besaved where more than 80 per cent of the soilsurface is covered with residues compared withbare soil. If this water was available to plants it couldreturn the equivalent of about 120 kg grain perhectare in wheat.CALCULATING HOW MUCH WATERCAN BE STOREDWhile soil organic matter can hold between 2-5times its weight in water, the impact of increasingsoil organic matter on water holding capacitydepends on the mineral composition of the soil, thedepth to which organic matter has increased andthe contribution of organic inputs to the various soilorganic fractions (see Chapter 1).As a general rule, each one percentage increasein soil organic matter increases water holdingcapacity in agricultural soils by an average of 2-4per cent (0.8–8.0 percentage range; Hudson 1994).This is the equivalent of less than two per cent onaverage for a one per cent increase in soil organiccarbon. For a soil that held 200 mm of soil waterthis would be equal to an additional 4 mm of water.However, as most of the soil organic matter will bein the top 10 cm of the soil, reports of increases inwater holding capacity beyond 10 cm are likely tobe an over-estimation.Similarly, changes in water holding capacity shouldbe considered in context to the likely changes in soil41MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE