A good soil is one which has a high water-holding capacity but drains freely leaving air space. Water and nutrients in such a soil, will be easily available to plants.
Nature delivers us soils with varying physical characteristics – clay, alluvial gravel or silts, volcanic loams or ash, sand, and peat.
Clay soils have very small particles which pack tightly together limiting drainage and air space. The water and nutrients are held tightly making it difficult for plants to take up. On the positive side clay soils hold moisture well and moderate in nutrients except nitrogen, calcium, and phosphorus. Clay soils are the most common in New Zealand, making up almost 80% of our land mass.
Silt soils are usually alluvial flood plains or similar, formed by repeated layers of parent rock which has been broken down and laid by nature with the aid of water or wind. The water and nutrients are usually reasonably freely available, but sometimes the silt is of a clay nature. Silt soils are usually moderately rich in nutrients. The plains of Gisborne, Hawkes Bay, Manawatu, Canterbury, and Southland are typical silt soils.
Volcanic loams formed from ash showers of varying ages which determines soil type, but they are generally free draining. The older volcanic soils have a good water holding capacity and are high in nutrients. Volcanic soils are in central north Island, Waikato, Bay of plenty, Taranaki, with many smaller pockets around small volcanic cones.
Sandy soils are the opposite to clay. They have large particles, very free draining, low water holding capacity, and are low in nutrients because they freely leach out when water drains away.
Peat in many ways is a perfect soil, it is both free draining and has a high water- holding capacity. The water and nutrients are easily available to plants. Peat soils are however very acid, requiring major adjustment and they are low in nutrients compared to other soils. Peat soils are found on the Hauraki plains, with pockets in Cambridge and Southland.
A good soil will be slightly acid (pH 6.2 – 6.8). At this level, the nutrients required by plants are most freely available. Most soils are naturally in this band unless they have been modified by the addition of acidic compost/peat, or are too alkaline from over-use of lime/dolomite.
A good soil will have all the nutrients required for growth in the correct balance.
Fortunately, all soils except peat, can best be improved for plant growth by adding organic matter such as; compost, green-crops, straw, or similar materials. These materials will improve drainage and water holding capacity. They will also make water and nutrients more easily available to plants. Large quantities must be added to make a difference—15-20cm thick and worked into the soil. At least 3 cm should be added each year.
Making home compost is a good thing to do, but the average home garden does not produce enough green waste to make the required amount of compost to improve clay soils. In most cases it will be necessary to buy in trailer or truck loads to provide the necessary level of improvement.
Compost contains nutrients, but not enough to sustain ongoing production of fruit and veges. Compost contributes to feeding plants, but in most soils, additional nutrients are required.
In clay soils which have poor drainage, it will also be necessary to raise the garden or planting area, above the surrounding soil to ensure drainage in the root zone of your garden plants.