MEDIA CONSIDERATION FOR NURSERY CROPS

A review of Chapter 3 will highlight basic information about soil which is central to the production of nursery stock. As in other areas of agricul­tural plant production, the composition of the soil or even the decision to use soil-less media in place of soil, is determined by criteria such as the type of crop being grown, the stage of growth at the time of harvest, the cultural techniques being used, the methods of harvest, and wheth­er the harvested plant will be bare rooted or balled and burlapped.

Nursery soils need a structure that promotes nutrient availability to the plants, and a proper air and water relationship. They must drain well while simultaneously retaining the moisture needed for growth. The soil pH should be between 5.00 and 7.2 for most nursery crops. Ericaceous plants are sometimes an exception, requiring a lower pH for optimum growth.

Soils for field production of woody nursery plants should have these characteristics and also be free of noxious (difficult to eradicate) weeds, insect pests, pathogenic inoculum, or nematodes. The herbicide his­tory of a field should also be known before planting with nursery stock to ensure that a prior crop’s herbicide is not latent in the soil, waiting to damage the new nursery crop. Also, field soil used for nursery crops should be high in organic material. Good soil fertility is also desirable as long as it is not too acidic or basic, which can tie up micronutrients.

Container-grown nursery stock also needs media with the proper­ties already described. In addition, due to space limitations of contain­ers, the media must be heavy enough to anchor the plant and prevent container tipping, have a high cation exchange capacity, provide desired levels of nitrogen throughout the growing season, and retain the mois­ture needed for optimal plant growth.

Porosity of the medium is of great importance in container plant production. Porosity is the size of spaces between the particles that comprise the medium. It determines how much water and air remain in the root environment after the container has been watered and allowed to drain. Large pores hold the air within the medium. smaller pores hold the water within the medium. Shallow containers tend to hold a greater percentage of water following irrigation and drainage than do deeper containers whereas deeper containers usually have good aeration but less water retention capacity.

These differences can be partially compensated for by incorporating materials into the media of shallow containers to improve the aeration porosity. Materials such as sand, perlite, turface (calcined clay), and various wood barks are commonly mixed with fine-textured media for that purpose.

Container media must be readily available in the amounts needed when they are needed. Accordingly, the media must be either commer­cial mixes or easily prepared locally. They must also be inexpensive, easy to handle and affordable to transport, pest free, blendable with other additives, and stable over time, allowing them to be stored without concern for changes in their chemical or physical characteristics. The artificial media described in Chapter 14 are commonly used for nursery crops. The University of California and the Cornell mixes may be used in unaltered forms or with additives to improve aeration porosity. Other container media used include aged and composted hardwood or soft­wood barks. Hardwood barks combined with sand and with ammonium nitrate (for added nitrogen) and elemental sulfur and iron sulfate (to stabilize the pH) is a common mix.