Strawberry soil and water management

Soil requirements for strawberries

Strawberries have no specific requirements in terms of soil type. Fields and tunnels should be well drained and relatively flat with no frost pockets.

Yields are usually slightly higher on heavy soils with a greater water holding capacity. However, light soils warm up faster so that flowering is earlier. In these situations frost protection may be more important.

Light or sandy soils do have advantages in that harvest is earlier, helping the grower catch an early market. These soils also have fewer problems with root diseases.

Soils that have previously grown potatoes or other Solanaceous species should be avoided to reduce the risks of Verticillium wilt and Rhizoctonia solani in the strawberry crop. Strawberries that follow grass can also be subject to greater pest attack from grubs, so planting is commonly delayed for a year, giving the grower a better chance of controlling weeds as well.

Strawberries prefer slightly acid soils with a pH value between 5.5 (light soil) to 6.8 (heavy soil). As a result, liming is not common in strawberry production. In soils with a higher organic matter content strawberries can tolerate a slightly lower pH. When the soil pH drops below 5.5, magnesium, calcium, phosphorus and molybdenum availability drops, so supplies need to be maintained. At pH levels above 7.0, zinc, manganese and iron can become deficient.

Salinity effects on strawberries

Strawberries are very sensitive to salinity.

All irrigation water contains dissolved mineral salts, but the concentration and composition varies. Water with the highest concentrations will cause the greatest salinity issues. Once the salinity of the irrigation water exceeds an ideal EC level of 0.7mS/cm, yield losses will occur and at much higher ECs, yields are significantly reduced.

Salinity symptoms are commonly seen as dry and brown leaf margins, brittle leaves, stunted plant growth, or even dead roots and plants. Poor irrigation management can also lead to a build-up of salts around the roots, so it is important to flush these out of this zone.

The salinity level in the root zone is directly related to water quality, fertilizer rate and the depth of irrigation application.

It is possible to predict potential yield loss using the following equation derived by Maas and Hoffman (1977):

Y = 100 - b (ECe - a)

Where Y is the relative crop yield (%), ECe is the salinity of the saturated soil extract in dS/m, a is the threshold value for maximum, 100% production, and b is the yield loss per unit increase in salinity. For strawberries: a = 0.7, b = 33.3, therefore the equation becomes:

Y = 100 – 33.3 (ECe-0.7)

A high salt content is controlled by leaching using 10-20% more irrigation water than that normally applied. The proportion of water that leaches below the root zone, carrying with it a portion of the accumulated soluble salts is known as leaching factor. The amount and frequency of irrigation should be adjusted to allow sufficient leaching yet at the same time minimizing the risk of excessive soil moisture levels which could cause other problems.

Two types of salt problems exist - those associated with the total salinity and those associated with sodium chloride. Soils may be affected only by salinity or by a combination of both salinity and sodium. Sodium is antagonistic with potassium and calcium. Chloride competes with nitrate. Therefore, levels of Na and Cl in water and fertilisers must be as low as possible.

High concentrations of salts in the soil prevent the plant from absorbing the water, resulting in leaf scorching, wilting and yield loss, and varieties differ in their sensitivity to this salinity.

Substrate and hydroponic systems

Substrate and hydroponic systems have been developed over recent years as a means of improving production consistency, and to ease crop management and harvesting, leading to higher crop yields and better quality.

In addition, these systems help to isolate and minimize issues with disease, which is increasingly difficult to control in soil cultivated crops where soil disinfection is either not permissible or difficult.

Alternative growing systems used in substrate production include the use of inert rock wool, perlite, or volcanic/pumice stone and organic media such as peat, coir and coco peat. Growers also utilize nutrient film techniques (NFT), which are substrate free.

These closed systems are easy to disinfect and the pH and EC are more easily controlled. Drainage water is collected and re-cycled, helping to improve nutrient efficiency.

The optimal pH range is 5.0–6.0. Below this, root tips can be harmed. Any excess ammonium in solution can alter pH to unacceptable levels.

If the pH is lowered some nutrients are more available (e.g. manganese) and potentially toxic.

In contrast though, when the pH in the root zone rises above 7.0, nutrients such as P, Mn, Zn, Cu and Fe, become less available.

The amount of different nitrogen forms is important as ammonium-N alters the pH. Common practice is to use a maximum of 14-20ppm N-NH4 (1.0-1.5mmol NH4) - with the majority of nitrogen supplied as nitrate.

Because of the smaller volume of substrate compared to soil grown crops, high levels of sodium and chloride can cause greater salinity issues in substrate systems.

Maximum levels in water are < 1.5mmol Na/l (35ppm) and < 1.5mmol Cl/l (53ppm). In recirculation systems these maximum levels should not exceed the uptake by the crop.

Crops in a hydroponic system need a full package of nutrients. To reduce work and labor cost, nutrient solutions are prepared in a highly concentrated stock solution which is then diluted.

The stock solution is generally a 10% solution (100x concentrated) and the final solution is diluted to 1.0–3.0g/l depending on the crop and local conditions.

Because of nutrient antagonism and interactions, it is not possible to mix all nutrients in one high concentration stock solution. For example, calcium containing products cannot be dissolved with phosphate or sulphate containing products. Therefore a two tank - A + B tank - system is used. The higher the EC, the higher the nutrient concentration in the solution.

Growers using hydroponic or substrate based systems also need to adjust the final EC of the nutrient solution according to radiation. On cloudy days, water uptake is lower and therefore EC needs to be higher to ensure sufficient nutrient uptake. On sunny days the water uptake is high and therefore the EC should be lowered.

It is important to test the substrate, water source and drip water on a regular basis – normally every week - and adjust it accordingly to ensure an optimal ratio of nutrients are available. Growers should also analyze the pH and EC of the substrate several times a week.