Here we define 'market gardening' as the intensive, small-scale growing of a diversity of vegetable and cut-flower species
where the products are usually destined for a local market. Under such production systems, the labour and other management
inputs are usually rather high and it is common for several crops to follow one another in sequence through the year in the
same space with little or no fallow in between. While many of the same species are also grown more extensively
(arable farming), here the economics, the problems and the best management practices are rather different.
The economics of any intensive production system makes attention to detail worthwhile. While vegetable crops have been grown
successfully on a wide range of soils they yield best on a well-drained fertile soil that is not prone to water-logging in
the winter or likely to dry out too quickly in the summer. Much of New Zealand's market gardening is done on the deep and
fertile soils found around Auckland, the Horowhenua, and the Waikato. The application of gypsum in market gardens, especially
in those with less well-drained soils, will improve yield and quality significantly - mainly through its beneficial effects
on soil structure.
Frequent cultivation is known to damage soil structure by breaking down soil aggregates and by exposing unstable soil organic
matter to the environment. With the breakdown of soil aggregates, the soil becomes less aerated and drainage is reduced,
reducing productivity. Soil is also more prone to surface crusting and compaction. While gypsum alone cannot eliminate
this problem, the combined effects of increasing organic matter (composts, ploughing in crop residues etc) and adding
gypsum to stabilise the soil-organic matter aggregates goes a long way to improve and maintain soil structure.
Excessive cultivation can also cause an accumulation of clay low in the soil profile, which can result in poor drainage.
Gypsum can be useful in ameliorating this by flocculating the clay particles together into aggregates. This is especially
the case in soils that are also irrigated as the gypsum is moved through the soil profile to where it is needed much
faster than by natural rainfall alone. Other soils that are generally wet and poorly drained can also benefit from these
properties, such as the gley soils of the Waikato.
Breakdown of its structure makes a soil more prone to erosion. For example, the volcanic loamy clays on the gentle slopes
of the Pukekohe region have been cultivated for over 100 years. This intensive cultivation has caused the loss of up to
1m of topsoil downslope over this time. By stabilising the soil structure through the application of gypsum and organic
matter, erosion can be reduced.
Lastly, the improvement in structure will render the soil more easily worked and should also bring about a general
improvement in plant health by improving root penetration and reducing the chances of disease problems associated with
'wet feet'.
To obtain significant improvements in the structure of clay soils through gypsum application requires quite high
application rates - higher than those indicated for nutritional purposes (see below). Application rates of between
2,000 and 4,000 kg/ha of gypsum are not excessive.
There are also nutritional benefits to be had from gypsum application. Most vegetable crops require between 250 and 500
kg/ha of calcium each year. A substantial part of this can be supplied in the form of gypsum with other
calcium-containing fertilisers accounting for the balance. Most horticultural crops need about 120 kg/ha of sulphur
annually. About one third of this will come from the organic sulphur cycle within the soil, one third from a base
dressing of gypsum (40 kg/ha of sulphur requires 220 kg/ha of gypsum) and the balance from other sulphur containing
fertilisers.
Gypsum can be used in market gardening to:
- Improve soil structure, aeration and drainage
- Reduce soil compaction and cracking
- Reduce surface crusting and improve seedling emergence
- Improve root penetration
- Increase the calcium balance in the soil without changing the pH
- Increase available sulphur in the soil
- Stabilise the organic component of the soil
