Simon Leake | Agronomist | 0439 999 173
Gypsum is normally used on dispersible heavy clay soils to ameliorate hard pans by improving the soil structure. This allows roots of plants to move more freely, makes oxygen more available to roots and reduced water logging via better water infiltration into the soil.
Gypsum is also an excellent source of Sulphur with pure gypsum containing 18.6% sulphur. This can be a very convenient way of giving crops a medium-term supply of Sulphur – depending on Sulphur levels in your soil you could spread something like 300 kg/ha of gypsum every 3 years then can save your spreading capacity on spreading Urea after seeding whilst also minimising the acidifying effect of using Sulphate of Ammonia either in high rates prior to seeding or in blends with your urea post emergent. Sounds good to me.
Trouble is whether accessibility limits you? As far as I know Perth, Koorda, Kalannie and Newdegate are the only gypsum supplies in Western Australia. In terms of cartage cost I don’t think that will hold you back (as you will see below) however, capacity for your operation to fit in the extra spreading and trucking is another thing.
So how does it work out economically? I have calculated below by figuring out per unit of S the additional cost of using sulphate of ammonia (SOA) and the lime required to undo its acidifying effect including cartage. With a few assumptions it works out that it’s about double the cost to use gypsum over SOA. On a farm using say 15 units of Sulphur a hectare using SOA would cost you $7.5/ha more!
Assumptions: Working on 15 units/ha of S, same trucking cost for each product at $30/t, cost of lime of $30/t and that 3.15kg of lime required to neutralise acidifying effect of sulphate of ammonia vs urea per unit of N.