The major crops under arable farming include cereals; barley , wheat, maize and oats as well as vegetables; potatoes, squash, onion, peas and beans.
Gypsum to improve soil-structure
Arable land comprises of many different soil types including some heavier soils that suffer structural problems. These soil types would benefit from applications of gypsum especially high-rate applications of gypsum in localised problem areas to help restructure the soil.
Gypsum as a fertiliser
Gypsum can be used to lift calcium levels and improve soil structure without raising soil pH. New Zealand soils are commonly sulphur deficient. There are situations where it is desirable to increase the sulphur content of a soil without increasing the phosphorus content (e.g. with an application of superphosphate). In these circumstances a good case can be made for applications of gypsum as a sulphur (and calcium) fertiliser.
Organic crops are a fast-growing sector where superphosphate is not permitted. Here again, gypsum can be used as a calcium and sulphur fertiliser and should be applied.
Soil management
The soil management objectives of these crops are to increase soil water holding capacity, by improving water infiltration and by deepening the rooting zone.
Soil nutrient sampling and lab testing of the top 15 cm of cultivated soil is usually carried out each year before planting. These provide reasonably accurate information on the fertilisers required to supplement and replenish the soil’s nutrient reserves. Some inaccuracies arise here because of the ‘statistical’ difficulty of sampling a large area of land in which soil quality can be very variable from place to place.
Soil tests (basic soil test profile + exchangeable aluminium) on subsoil samples taken below 60 cm will identify if subsoil acidity is an issue. If pH and calcium levels are low and exchangeable aluminium is high then these will combine to limit root-zone depth and the crop's access to deeper soil water. Where subsoil acidity is of concern, an increase in the topsoil target pH and calcium levels should minimise the amount of acid movement through the soil profile.
A visual inspection of the soil profile will identify if there is compaction below the cultivation depth. Deep ripping is effective to reduce this where the pan can be fractured and it can also facilitate the downward movement of the added calcium through the soil. However, if the underlying causes of subsoil compaction are not addressed, deep ripping will be only a short-term remedy and it is expensive.
While topsoil compaction is improved by cultivation, subsoil compaction is worsened by it. Subsoil compaction will affect crop growth and have long-term effects on soil quality. Gypsum applications raise subsoil calcium levels better than lime due to its greater downward mobility in the soil. Gypsum also improves soil structure without raising its pH.
Lime versus Gypsum application
Normal practice is to apply lime and to cultivate this into the topsoil to achieve appropriate pH and calcium levels based on annual soil tests made prior to planting. Application of nitrogen fertilisers at appropriate rates and timing will minimise soil acidification and also minimise nitrate leaching to the groundwater.
Because the calcium in gypsum is relatively water soluble, applications of gypsum at rates of up to 3,000 Kg/Ha are known to be more effective than lime in moving calcium down through the soil profile to below the cultivation depth. Gypsum will not increase soil pH.
A combined lime and gypsum treatment will facilitate the movement of calcium into the subsoil and will increase soil pH by displacing the sodium and aluminium ions. A 60:40 lime:gypsum mix applied at 5,000kgs/Ha should be effective. Incorporation of the lime and gypsum into the soil profile after application by ploughing or deep ripping will speed their effects.
An application of gypsum will need to take into account the soil type and recent soil lab test results.